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E28 Bosch Motronic Troubleshooting

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PostPosted: Mon Jan 16, 2012 11:12 pm    Post subject: E28 Bosch Motronic Troubleshooting Reply with quote

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We specialize in BMW fuel injection (L-Jetronic and Motronic) as well as silicone vacuum lines and radiator hoses. If you encounter any sure problems with leaks or fuel injector performance, please contact us and let us know how we can help!

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This article is intended to help owners of cars with Bosch Motronic Fuel injection perform their own service. The article is based on the Original Bosch Motronic system used on the 1987 E28 BMW 535i.

While this article is primarily intended as a fuel injection article, it will include testing information for the ignition and other systems related to the Motronic system. This is because the Motronic system ties into a lot of things, ignition, emission, even the 535i's automatic cabin temperature control. Everything it ties into will be covered.

You will not need any Bosch or BMW testing equipment, this article should allow you to test and troubleshoot with just basic tools and an inexpensive multi meter.

Disclaimer: I have no factory training. Use this information at your own risk. I strongly suggest having a fire extinguisher nearby whenever working on an automotive fuel system. As a matter of fact, just stop reading this right now and go buy a factory BMW shop manual.

How to use this article:

You can use this article in a few ways, first you could follow it through step by step. This would very thoroughly check out your fuel injection and ignitions systems. I recommend this method if you have plenty of time, if your car has more than one problem, or if you just purchased the car and want to get it running just right. If your car has one simple problem you can use this article as a sort of trouble shooting and testing guide for specific components. To make trouble shooting easier I have included the symptoms associated with a defect for each part of the injection system. The sections showing these symptoms can be found quickly by looking for the *Symbol. As a general rule the components that can cause the most problems and/or the biggest problems are towards the beginning of this article. For example, air leaks in the vacuum hoses, lack of power to the fuel injection system, fuel pressure and fuel injectors can all cause a variety of problems, and even prevent the car from running. A faulty idle air control valve will only cause problems at idle so it's covered later in the article.

What cars does this article cover?

The article covers the E28 535i. With a few minor variations the same Motronic system is used on many other BMWs from the 1980's including the BMW 533i, 633i, 733i, 635i, 735i, 528e (Note: some early 533, 633, 733 BMWs use Bosch L-Jetronic. Please refer to my L-Jetronic tech articles for those cars.) Various BMW E30s, the the 325i and 325e use Motronic, however they are set up differently. Most of the component tests in this article are relevant for the Motronic E30s, however the locations of the components will usually be different. I will try to point out the variations in these cars within this article, but there is no way I can nail every variation.

There were many other cars built in the 80's and early 90's using this same basic fuel injection and ignition system including the Alfa Romeo 164, Rolls Royce Silver Spur, and others. The Japanese built clones of this system under license and many Japanese cars from the 90's use this system. Some BMW 635 cars use the Motronic 1.3 system. It's essentially the same as the 1.1 system. In some cases, it still uses the cold start valve/injector, unlike the 1.1 system in the 528e. The big change in the 1.3 system was its ability to store more fault codes, and keep them in memory when the car is shut off. To allow it to do this, the 1.3's ECU (electronic control unit, which is the "Motronic Control Unit", or fuel injection and ignition computer) is hard wired to the battery, so it always has voltage if the battery is charged and connected.

Most BMWs I know of using the Original Motronic system (often called Motronic Basic) have a diagnostic connector like the one shown in the picture below. If your 80's BMW connector has 15 pins, it's Motronic Basic. If it has 20 it's Motronic 1.1.

As far as I know all E28 BMW 535i cars use Motronic Basic. The only E28s I know of that used Motronic 1.1 was the later BMW 528e (build dates from 3-1987 and on). Except where noted, all text and pictures in this article are from Motronic Basic. I will try to cover the 1.1 system in this article, but I don't have a 1.1 car here to take pictures of.

Important Note, the car used in the photos has SILVER Vacuum lines in most locations. They show up in the pictures fairly well. The lines on your car will probably be black.

Pictured below is our 1987 535i. It has a silver silicone hose kit installed. This kit includes engine compartment vacuum lines, windshield washer lines, and two coolant lines. The pieces are pre cut and the kit includes excellent instructions with two good diagrams.

If your BMW has these fuel lines coming out of the passenger's side firewall, check them right away. If they fail fuel can spray onto the exhaust manifold catching the car on fire!

Let's Get Started!

Step 1, Eliminate air leaks. (also called "vacuum leaks". Yes, I am aware that technically vacuum doesn't leak, but leaks in the vacuum system can be called "vacuum leaks" for convenience)

Air leaks are one of the biggest problems on Motronic cars. It's not quite as sensitive to air leaks as the older L-Jetronic system, but it's still sensitive. If you want to see just how air leaks effect the motor, pull the oil dipstick out while the motor is idling. That introduces only a small leak, yet the effect is dramatic. *Air leaks can cause a lot of problems including difficult cold and/or hot starting, loss of power, poor emissions, and general drive ability problems. In an extreme case they can even cause internal engine damage. Motronic will compensate for very minor leaks by adjusting idle speed with the idle stabilization valve. In other words if you have a slight leak you won't see any real change at idle because the system will adjust idle. However it's ability to compensate once the throttle is advanced is marginal at best.

Air leaks cause problems for two reasons. First, the Motronic system measures the volume and density of the air passing through the air flow meter and sends that information to the fuel injection computer also called the ECU. The ECU then calculates fuel requirements based largely on that information. If an air leak allows air to enter the system downstream of the airflow meter then the ECU has no accurate measurement of how much air is entering the engine. The second problem is that various devices including the fuel pressure regulator and evaporative purge valve (both discussed in detail later) as well as many other components require vacuum to function properly. An air leak will not only introduce un metered air into the system it will prevent important components from working properly.

It makes sense to start troubleshooting by looking for air leaks. This is because they are a possible and common cause of nearly every fault, and they are about the easiest and cheapest problems to solve. You don't want to throw hundreds of dollars in parts at a problem only to find out it was an air leak.

Lets start by looking at the main air intake tube or "rubber boot" that connects the throttle body to the air flow meter. This thing has a lot of potential for leaks. It should be inspected very carefully for cracks. They tend to form on the bottom of the tube so you really need to remove it to properly inspect it. If it has any cracks it should be replaced. If you can't replace it because you can't find the part, or due to economic realities I suggest using "Shoo Goo" from the sporting goods section of Wal Mart. I have also found it in the shoe section of Wal Mart (by the shoe laces and polish). This product is designed for repairing the soles of tennis shoes and will seal up any air leaks on this tube. (don't laugh, Shoo Goo lasts almost forever in this application)

Next start inspecting all the vacuum lines in the engine compartment. All of them are related to the fuel injection system in some way, and a leak in any of them can cause problems. This can be time consuming because a lot of these vacuum lines are hard to see and have to be removed to be inspected effectively. If your lines are 20 years old they will probably have to be replaced. They are almost certainly cracked and brittle with age and it will be very difficult to remove them without cracking them. (Sales pitch alert!!! Our silicone hose kit has all the vacuum lines pre cut for the 535i and includes easy to follow step-by-step instructions and diagrams. It also includes all the windshield washer hoses, and a couple coolant hoses. We use very high quality silicone and the lines should outlast everything else under the hood. It can be found here: all BMW Silicone Vacuum Hose Kits Link )

Step 2, Test the evaporative emission system.

Pictured below is the main intake tube called the "rubber boot" in the BMW manual. The valve attached to it is the Evaporative Purge Valve called the "air valve" in the BMW manual.

Attached to the tube on the forward side is the "air valve". This valve is designed to open when the throttle is closed or nearly closed with the engine running (in other words at high vacuum/low manifold pressure conditions). When it opens it allows vapors from the charcoal canister to enter the motor. Give the valve a good visual inspection to make sure it's not cracked. Check its connection to the intake tube. If it looks like it leaks replace it or use the Shoe Goo technique.

Once you are sure the valve is OK on the outside, we need to make sure the thing actually works. You will need a helper for this. Get the wife or girlfriend to help with this one! First you need to blow on the larger port ( the one with the green tube in the picture ). Air should not go through unless you blow really hard and force it open. Now have your helper suck on the smaller port ( the one with the red tube ). While your helper is sucking, the valve should open and you should easily be able to blow air through the larger port. If it fails either one of these tests you have to replace it.

On the other side of the tube is the Idle Stabilization Valve. This is the cylinder that has an electrical plug on the end near the firewall. This item is explained and tested later in this article. For now just make sure its connection to the tube is leak free.

The part my finger is touching in the picture below is the "Thermo Valve" sometimes called the "evaporative purge valve vacuum switch". It's located near the thermostat at the front of the motor.

The Thermo Valve's entire purpose in life is to shut off the evaporative emission system until the engine is warmed up (meaning a coolant temperature of 110F ( 44C ). It's immersed in coolant so once the coolant temp reaches the specified amount it opens up allowing engine vacuum to reach the evaporative purge valve we just tested, thus allowing fuel vapors to be sucked through the charcoal canister and into the motor. When the coolant temp drops below about 92F ( 34C ) it closes again. *A faulty Thermo Valve can cause irregular running during the warm up period if it's stuck open. If it's stuck closed it will have an adverse effect on emissions.

Testing this thing is really easy. With the engine cold, pull both its vacuum lines off at the other ends (that means not off the thermo valve). Now blow into one line. Air should not go anywhere, the valve should be closed stopping you from blowing any air through it. Put everything back together, start and run the motor until it warms up. Shut the motor off, pull the lines off again and blow through one. The valve should be open and you should be able to blow right through the valve.

Note: If the Thermo Valve and the Evaporative Purge Valve are functioning correctly and your car has a problem only when cold then it's not a problem related to the Evaporative emission system.

Pictured below is the Charcoal Canister. It's located just forward of the windshield washer reservoir.

The Charcoal canister is designed to improve emissions. IT DOES NOT HARM PERFORMANCE! A lot of folks remove these thinking they are gaining something. Leave it in there it truly helps emissions and has no adverse effects. It has two vacuum lines going to it. One comes from the fuel tank, and one goes to the Evaporative Purge Valve.

This device is really simple. Think of it as a fuel vapor filter. Whenever the car is warmed up and the Purge and Thermo valves are working correctly, fuel vapors from the fuel tank get sucked through it and into the intake system by manifold vacuum. That's about it. No moving parts to test or wear out. The only possible fault it can have is a physical damage causing a vacuum leak. Testing it is really simple. Take the two lines off of it so you can blow through them and into the canister. Plug one line with your finger and blow into the other one. If you can't blow into it then it's fine. If you can, it's leaking. Remove it, find the leak and seal it with epoxy. That will make it good as new.

Step 3, the cabin air temperature sensor.

What the heck does this thing have to do with fuel injection? That's a good question. The Motronic system on the 535i has one built in air leak. It's a calibrated leak and the ECU knows about it so it's OK. The leak is at the cabin air temperature sensor. It's important to make sure the sensor is not clogged or this calibrated leak will not allow the correct amount of air in, plus your automatic cabin temperature control won't work!

The cabin air temperature system must have a way of constantly sampling the air so it can adjust cabin temperature up or down. It does this with a sensor located just inboard of the hood release handle inside the cabin. To provide air flow through the sensor without the complexity of moving parts, BMW simply ran a vacuum line from the underside of the intake plenum into the cabin and to this sensor. There is also a very small restrictor orafice in the vacuum line near the sensor. When the engine is running it sucks a small amount of air through the sensor and into the engine. Note: I recently learned about and played with the restricting orifice. It's so minor that it has zero effect on the operation of the engine, or cabin temp sensor. However without it the suction noise through the sensor is more noticeable.

You can check to make sure air is flowing through the sensor by starting the engine and putting you head down near the hood release lever. You should hear air flowing into the sensor. Put your finger over the little intake hole and you should feel a slight amount of suction. It you do, then the sensor is not clogged and the vacuum line is intact. However, the odds are, air won't be moving through it and you will have to remove the sensor and replace its vacuum line.

To remove the sensor you have to remove the lower part of the driver's side dash by removing some screws and pulling the panel free. Replace its vacuum line and make sure its rubber boot seals to its body. The rubber boot will be old so you will probably have to reseal it with silicone and put a small hose clamp on it to keep it in place. Make sure the little foam filter on the intake side is clean and free of debris. Blow through it to make sure it's clear, use your mouth, NOT compressed air, it's delicate.

While you are here it makes sense to test this sensor. It's really easy, get out your OHM meter! Under no circumstances should you touch the OHM meter to the wiring harness. That will send electricity to a place that may not be expecting it and could be damaged. All testing with an OHM meter is done on the part itself, not the wiring in the car.

To test the sensor note which terminals connect to the blue and brown wire. Those are the terminals we will be checking. Resistance between these terminals varies with temperature. Use the chart below and check that the resistance values are close to those specified.

104F (40C) = 4700-5400 ohms, 86F (30C) 7500=8400 ohms, 77F (25C) 8599-10500 ohms, 68F (20C) 12000-13300 ohms, 59F (15C) 15100-17200 ohms, 50F (10C) 19400-22100 ohms, 32F (0C) 32400-37700 ohms.

This picture shows me testing the cabin air temperature sensor. The air temperature was about 65F and it registered 12,480 ohms, just about right. I doubt you will ever see one that doesn't test correctly. The issues with these things are a clogged inlet and inadequate vacuum to the sensor.

Step 4, Motronic Powering UP! Electrical power supply and grounds.

Now we will start to go deeper into the Motronic system. *Problems with the system's power supply and or grounds will often cause the engine to quit running and or prevent the car from starting. If your car cranks over but won't start, this is probably the section for you.

Powering up check.

First we need to see if the Motronic system is powering up. *If it doesn't your car won't run. With Motronic Basic we will have to remove a connector from a fuel injector. It's easiest to remove it from the forward most injector. These connectors are normally grey in color. Carefully remove the metal retaining clip with a tiny screwdriver. DO NOT try to force off the connector until you have removed that clip or damage to the connector will result.

Next turn on the ignition and connect your voltmeter to an electrical contact in the connector (either one) and a ground, you should see battery voltage or very close to it. If you don't have a voltmeter you could use a test light, but a voltmeter is better.

DANGER DANGER!!! Make sure your multi meter is NOT selected to Ohms. If it is, it will send current into the Motronic control unit (the ECU). As a general rule, power going into the ECU where it's not supposed to is bad. Damage could result. Make sure it's selected to read voltage. As a general rule, when we are checking something at the wiring harness, it's voltage, when checking an actual component we are checking Ohms.

The picture below shows this test on a Motronic Basic car. The voltmeter's red wire is touching the connector, the black is grounded to the manifold support bracket. The voltmeter shows battery voltage so the check is good!

If you have Motronic 1.1 this test is a little different. Instead of looking for battery voltage at a fuel injector's connector we are going to look for it at the idle air control valve's connector. With the ignition on check for battery voltage between the connector's red/white wire, and a ground.

Now, if the car passed that test, that's great news. We just verified a whole bunch of things work. Now we know that the car's Main Relay is good. The ignition switch works (well, at least the ignition position of that switch), and a whole lot of the wiring is intact. If the car didn't pass this test we need to check the Main Relay

The Main Relay: *

If this thing doesn't work, the car won't run, period. Checking it is easy. On most E28s this relay is located on the inboard side of the fuse box. It's the relay nearest the front of the car. It plugs into a relay socket that's attached to the side of the fuse box. IMPORTANT NOTE: For some reason on 1985-1986 528e cars, the relay is in a different location. On these cars it's on the inboard side of the fuse box, second from the front (where the fuel pump relay is located on the other E28s). ANOTHER IMPORTANT NOTE: The relay sockets can be switched around. There is just enough wiring length for some mechanic to swap the first and second relay socket positions. To make sure it's the Main relay you are looking at, check to see if it has two large (about 4mm diameter) wires going into it from below.

When the ignition switch is turned on this relay should click. You can feel it click if you have a friend work the ignition switch. If it clicks and you didn't have power in the powering up check, the relay is bad. If it doesn't click, the relay is probably bad, but to be sure we will need to verify it's getting power.

To check to make sure it's getting power, use your voltmeter to look for battery voltage between the relay socket's terminal #30 and ground. This terminal is wired directly to the battery so it's always hot. You should see battery voltage there with the ignition on or off. To figure out where the correct terminal is, look at the underside of the relay (they are also labeled on top, but that's usually worn off by now). Look to see where terminal #30 on the relay plugs into the relay's socket, and that's where to check for power.

In the picture below I am verifying we have battery voltage between the main relay's socket terminal #30 and ground. It checks out fine.

If we have voltage there, great! If not it's a certainty we have a wiring problem, probably at the positive terminal on the battery. Simply inspect the heavy wire that connects to the relay socket below terminal #30 and follow it to the battery. It's only about a foot long so this should be easy. Look for a break in the wire, but the problem is probably going to be the connection at the battery, or possibly at the relay socket.

Next, we need to make sure it's getting power. In all 1983 and later model year E28s, the main relay gets its electrical signal to close from the on-board computer when you turn your ignition switch to ON or START. The on-board computer is the thing in the dash that tells you your fuel economy, outside air temperature, etc. This is probably the only really stupid thing in the E28's otherwise really good electrical system. I can't believe they supply power to a critical relay through that piece of junk. Anyway you need check for battery voltage between terminal 86 and ground WITH THE IGNITION ON. Power goes to the on-board computer through the ignition switch. Inside the on-board computer is a little relay that closes when the ignition switch is in the ON, or START positions. The on-board computer also has internal fuses, but those are NOT involved in the operation of the Motronic system.

Here I am checking out voltage between terminal 86 and ground. It checks out fine. Notice voltage is slightly lower than the value seen at terminal 30. This is normal and is due to the route the voltage has to take to get to this terminal.

If we don't have voltage at terminal 86 with the ignition on, we probably have a problem with the ignition switch. If it's not the switch, it's the ECU or a wiring issue.

If we have voltage at both terminals 30 and 86 and the car did not pass the "powering up check" it's a certainty that the Main Relay is bad and should be replaced.

I strongly suggest carrying a spare main relay in the car, as well as a selection of relays and fuses if you use your E28 a lot.


*Every single possible running problem you car might have can be caused by poor grounds. This includes difficult or impossible starting, poor running, high emissions, low power, high fuel consumption and more. Thankfully, unlike some other cars I work on, the E28's grounds are easy to get at, and are rarely bad. Simply locate them, clean them up and tighten them. In the pictures below I will show some of the typical ground locations on a 535i.

This is the main grounding point of the entire electrical system. It's located just outboard of the battery on the inner fender.

This grounding strap is located on the passenger's side firewall. It's easy to miss.

The grounds in the picture below are located on the head. These are often neglected.

There is one other big grounding strap connected the the passenger's side motor mount. Don't forget to check this one.

Step 5, Testing the Fuel Pumps and pressure regulator.

The 535i and all the other E28s sold in the U.S. actually have two fuel pumps. They have a low pressure "transfer" pump located in the fuel tank, and a relatively high pressure "main" fuel pump located under the car. The purpose of the transfer pump is to insure a positive supply of fuel to the main pump. The car will normally run with an inoperative transfer pump, however *an inoperative transfer pump may cause difficult starting when the car is hot, or when the fuel tank quantity is low. It may also cause hesitation under certain conditions. If your car runs well until you are low on fuel, the problem is probably a bad transfer pump.

The main fuel pump is a critical component. *When they fail, they normally fail completely and the car simply won't run. If the main fuel pump works, but is faulty, it can cause difficult or impossible starting and can cause a large loss of horsepower.

The pumps normally only run when the engine is cranking over, or running. Of course it's hard to hear them over the noise of the engine. They do run for a moment with the engine off when you first turn on the ignition switch, but it's so brief you can't really hear them.

A quick way to check to see if the main fuel pump is working is to hold the return line exiting the fuel pressure regulator (it's the rubber hose) while a friend cranks the engine over. If the pumps and regulator are working, you will feel the fuel going through that hose. If you don't feel any fuel going through, it's time to check the power supply.

Checking the fuel pumps power supply.

As with most electrical components, the first thing to check is the fuse. The fuel pumps share one fuse. It's a 16 amp fuse located in the main fuse box in position #1. This is the location on the inboard side of the box all the way towards the front of the car. In other words, it's located in the front inboard corner position. If this fuse is blown, or has bad contacts, the pumps, and thus the engine will not run. Check the fuse and the contacts.

Next, we should check the fuel pump relay. On the 535i this relay is located just aft of the Main Relay we checked earlier. Make sure it's not the main relay, as the positions of the relay sockets can be swapped around as I explained in "The Main Relay" section of this article.

On the 535i with the ignition switch off, there should be approximately battery voltage between the fuel pump relay socket's terminal #30 and ground. In the unusual case of the 1982 528e, the ignition must be on for this test (ignition switch in run, or start positions).

This picture shows a good voltage test on the fuel pump relay socket's terminal #30.

If you don't get voltage at this terminal, look at the wire going into the terminal under the socket. That's the most likely source of the problem. It's a red wire, and it goes to the battery, sometime it disappears into a wiring bundle first, making it a pain to trace.

If you have good voltage at terminal #30 we need to move on and test the fuel pumps to make sure they run. There are a few ways to do this, but I suggest the following. I use a jumper wire (ideally one with a 16 amp inline fuse) and connect the fuel pump socket's terminals 30 and 87. This will turn on both pumps. If this works, and the pumps turn on, but didn't run earlier, you have a bad fuel pump relay.

If the pumps don't run with the jumper wire, you have either a bad pump(s) or bad connection(s). The bad connection will almost certainly be at the pump itself.

Fuel Pressure:

At this point, we know the fuel pumps run. However just because they run doesn't mean we have proper fuel pressure. A faulty main pump can work, but deliver low fuel pressure. A faulty fuel pressure regulator can cause pressure to be either too high or too low.

Incorrect fuel pressure can cause just about any possible problem including: *difficult hot or cold starting, poor idle, lack of power, poor fuel economy, high emissions, stalling, starting and stalling or any combination of these.

Let's take a break from testing things for a moment to discuss fuel pressure. This may be a little confusing so you may have to read it twice. I know I would. Proper fuel pressure is very important on the E28s. The Motronic ECU maintains control of the fuel air mixture by measuring the engine's air intake and then injecting in the proper amount of fuel. It controls fuel delivery exclusively by controlling the amount of time the fuel injectors are open (except during starting, which will be covered later). If fuel pressure is too high or too low the ECU won't know it, and at idle or full throttle it will make no attempt to compensate. Under these conditions if fuel pressure is too high the injectors will allow too much fuel to spray in because the ECU won't know to reduce the amount of time they are open. If it's too low, we have the opposite problem. The Motronic system can compensate for small variations in fuel pressure during normal driving (i.e. not during starting, warm up, idle, or high power operations). It does this via inputs from the O2 sensor. We will cover that later.

Before we get into testing fuel pressure, let's go over how much fuel pressure these cars should have. With the engine off, and the pumps running the 535i should have 43.5psi +/- .9psi. For you Metric guys, that's 3 bar, +/- .6 bar. Under the same conditions the 533i and 528e should have 36.3psi +/-.7 psi. If you love the Metric system that's 2.5 bar +/-.06 bar.

Of course it has to get more complicated that that! Notice those numbers are with the engine off. The actual values with the engine running MUST vary. Follow along to understand why. The pressure regulator is not designed to hold the pressure at a fixed value. It IS designed to hold the pressure differential between the fuel pressure at the inlet side of the injectors, and the air pressure in the manifold constant. Think of it this way. If our system held fuel pressure at a constant 36 psi and we had 37 psi of air pressure in the intake manifold when the fuel injectors opened, fuel would not flow into the manifold. Instead air would flow into the fuel system. Of course on a stock E28 that can't happen because manifold pressure in these cars will rarely exceed 15psi. The point is that if fuel pressure was constant, then variations in manifold pressure would effect the amount of fuel injected for a given injector duration period.

Manifold pressure in these cars varies from about 7psi to about 15psi depending on throttle position, rpm, etc. In order to keep the fuel pressure differential constant the pressure regulator raises and lowers fuel pressure in response to changes in manifold pressure. With the engine idling we should see 42 psi on a 535i, and about 34psi on a 533i or 528e (Note, the actual number may vary. The official range is 40.6-46.4 on the 535i and 33.4-39.0 on the 533i/528e). What's important here is that the value is lower with the engine idling than it was with the engine off. If you remove the vacuum line from the fuel pressure regulator it will think the engine is at or near full throttle and fuel pressure should increase to about the value normally seen with the engine off. What's important here is that it does increase when you remove the vacuum line.

Important Note For Turbo and Supercharged applications: If you have one of the ultra rare turbo BMWs with this system (i.e. E23 745i) fuel pressure must increase directly with boost. For example at 5psi of boost, fuel pressure should go up 5psi. If you are doing a turbo or supercharged conversion on your E28, E24, or other Motronic BMW this is still true. On that subject Motronic with various inexpensive modifications is well suited to these projects up to about 10 pounds of boost. However that's another article for another time. If you are supercharging or turbo charging an E28 and need help, feel free to contact me. In regards to forced induction I can provide good tech help for Bosch L-Jetronic, Early Motronic, SDS brand fuel injection, water and water/methanol injection, and supercharger selection. I can not provide any tech help for any other fuel injection systems.

Testing the Fuel Pressure. Under Construction

Now that we understand how fuel pressure should react, and we know what values to look for, let's test the main pump and regulator for proper pressure.

In the picture below, my hand is holding the high pressure hose that delivers fuel under pressure to the fuel rail. From there it goes to the fuel injectors.

There are many fuel lines under the hood of an E28. However most of them are low pressure hoses. We need to take a fuel pressure measurement from a high pressure hose. On all 533i, and 535i cars we can splice into the high pressure line at the back of the fuel rail near the firewall. The same is true of the 528e with Motronic 1.1 injection. If you have a 528e with Motronic Basic splice into the fuel line going to the cold start injector (often called a cold start valve). Make sure you are splicing into the the hose that actually connects to the back of the fuel rail. Many of the other hoses under the hood are low pressure.

This next picture shows a fuel pressure tester and a "T" type fitting used to splice into a high pressure line.

High pressure fuel line is expensive so I didn't want to cut into mine for the sake of a photo. However, from the two pictures above, it should be pretty easy to see how and where to take the pressure reading.

Once you have your fuel pressure tester connected. Get your fire extinguisher ready. With the engine off energize the fuel pump via the fused jumper wire described in the "Checking the fuel pumps power supply" section of this article. Watch carefully for leaks. Once you are sure everything is OK, note the fuel pressure. Make sure it's within the parameters described earlier. Start the engine and make sure the fuel pressure is within parameters at idle. It should be lower than it was with the engine off, because the manifold has vacuum while idling. Blip the throttle a few times and make sure the fuel pressure goes up and down with throttle application. Note: fuel pressure does NOT vary with RPM, it varies with manifold pressure/vacuum. It's possible to have the engine at 2000rpm with very little throttle, in which case it would have quite a bit of vacuum and relatively low fuel pressure. Conversely at full throttle and 2000rpm it would have almost no vacuum, so fuel pressure will be relatively high.

Ok, you have tested fuel pressure and it's too high or too low, what next?

If fuel pressure is too high under all conditions, you need to replace your fuel pressure regulator. If it's too high ONLY under vacuum (engine at idle) it's very likely the vacuum line on the fuel pressure regulator isn't air tight. Replace the vacuum line. There is a very small chance that the problem could be caused by a blockage in the fuel return system. The rubber fuel line on the fuel pressure regulator is a return line. There are actually four rubber return lines under the hood of a typical 535i. If in doubt replace them all. (Shameless commercial alert! At Hi Performance Store, Inc. we sell a fuel line kit for the 535i, it includes the correct sizes of pre-cut high and low pressure hoses and easy to follow step-by-step instructions with a diagram) BMW Fuel Line Kit Link

If the fuel pressure is too low it could be either a bad fuel pressure regulator or a bad fuel pump. It's pretty easy to figure out which one. Simply perform an engine off fuel pressure check as described above. Slowly pinch off the fuel pressure regulator's return line, if the pressure doesn't rise you have a bad pump. If it does rise you have a bad regulator. CAUTION FIRE HAZARD. If your fuel lines are old, they may and often do split when you pinch them off. This causes fuel to suddenly spray everywhere at high pressure causing a serious fire hazard, and possible injury. If in doubt replace the fuel lines BEFORE you perform this test. Be aware that if you fully pinch off the line a properly working pump will raise the fuel pressure to about 70psi! If your fuel system is in good shape, this isn't a problem, but it can burst old hoses.

The Fuel Pressure Regulator is located near the front of the engine at the forward end of the fuel rail (535i)

If you don't have a 535i you will have to find your fuel pressure regulator. They all look pretty much the same so the picture above should help. On Motronic cars they are almost always located on the fuel rail and have one vacuum line, and one fuel line attached to them.

Step 6, Fuel Injector on and off car checks.

If you have been following this article from the start, your car is now free of vacuum leaks, the injection system powers up properly, the grounds are good and it has good fuel pressure. Next we need to look at the fuel injectors. *Fuel injectors are another major component of the injection system, and they can cause a wide range of problems including difficult hot or cold starting, irregular idle, low power, high emission, poor fuel economy, stalling, and the starts and stalls syndrome. The injectors work by opening and closing at a rate and duration specified by the Motronic ECU. When open, fuel under pressure sprays in. The injectors can develop external leaks, they can stick open, stick closed, they can stick a little bit so they don't open and close at the correct rate, they can leak internally, and most common they can be partially clogged so they don't flow the correct amount of fuel.

A word of caution. Each injector has it's own fuel filter. The filter's inlet screen is so fine the holes are not visible to the naked eye! DO NOT put fuel system cleaner or fuel injector cleaner in your E28's fuel tank. All this will do is wash debris that's been building up for all these years downstream into these tiny filters. At least it will tend to clog all the filters equally, and this may actually improve running if it's masking another fault, however partially clogged injectors will not deliver enough fuel at full throttle and if knocking occurs as a result you could have serious engine damage. Trust me on this one, don't use these products on older cars. Well, I guess there is one exception to this. If you are going to pull and have your injectors cleaned anyway, it wouldn't hurt to use these products right before you did that. That would clean out the fuel system and since you are having the injector filters replaced there won't be any harm.

The bad news is it's not possible to fully test the fuel injectors with them on the car. The good news is there are a couple checks we can do with them on the car, and these checks are pretty good at finding some of the more serious possible problems.

First, we need to check the injectors' electrical impedance. Once again you will need to get out your OHM meter. We do this test first because if the injector fails this test, it's junk and can not be fixed. Start by locating the injectors. There will be one per cylinder, meaning on all E28s sold in the U.S. there are six main fuel injectors (there is also a 7th injector called a "cold start valve" which we will cover later). The injectors are easy to find, they are all attached to the fuel rail so look for the fuel pressure regulator which is attached to the front end of the fuel rail. Follow the rail back until you find all six injectors. Each injector has an electrical connector on it.

In this picture, my finger is touching cylinder #2's fuel injector. The injector's connector has been removed for clarity.

Remove the injector's electrical connector. Be careful, it's held on with a tiny metal retaining clip. This metal clip must be removed before you pull the connector off or damage to the connector will result. Use a really small screwdriver to pry the clip away. Don't drop it! Well, you probably will drop one at some point, so have a magnet nearby. Once the clip is off you can see the injector has a two terminal electrical connection. We need to measure the resistance between these two terminals. It should fall within the following values. For the 528e with Motronic 1.1, and all 535i cars resistance should be between 14.5 and 17.5 ohms. If you have a 528e with Motronic Basic, or a 633i the values should be between 2 and 3 ohms. Test for these values with your OHM meter, be sure to test for resistance between the terminals on the injector. DO NOT touch the ohm meter to the wiring harness' connectors. If an injector fails this test, it's junk and must be replaced. Also, don't reinstall those metal clips yet. You will need those to be off the the next test.

This BMW 535i injector passes its Ohms test!

Here is a Bosch BMW E30 injector. It's very similar to most E28 injectors. On the left is a removed injector inlet filter. The filter's screen holes are too small to be seen with the naked eye! If you think fuel system cleaner will flush debris through that, you are wrong! All it will do is clog the inlet screens with 20 years worth of debris from upstream in the fuel system.

The second check we can perform is something called a "balance check". This is the same check the dealers perform. It's pretty easy, but it does have some serious flaws. First of all, it only compares the injectors against each other. So if they are all partially clogged it won't help us. Second, if your engine and ignition system are not in like new condition the results will be unreliable. Of course since dealers tend to work on newer cars they love this test. The cars they usually deal with are newer and thus don't usually have clogged injector issues or worn out engines or ignition systems. Now, if your car has one or two really bad injectors this test will find the problem, so it is a helpful test.

To perform a balance check you will need to be able to easily pull off each injector's electrical connector. That means before you start this test you need to remove each little metal clip securing the injector's connector. Leave the connector on the injector, without the clip, start the engine and let it fully warm up. Once it's fully warmed up and idling very steadily, remove each injector's connection one at a time and note the RPM drop. Now in a perfect world the cylinder with the LOWEST RPM drop is the one with the worst injector. In other words, if the RPM does not drop at all, then it stands to reason that fuel injector is not delivering any fuel. I can't stress enough the flaw in this test is the lack of a perfect world situation on a 20 year old car. It could be the lowest RPM drop is on a cylinder with a really good injector but poor compression, or a weak spark. Heck it could just be time for a new cap, rotor, and spark plugs. This test has one other BIG flaw. It only tests the injector when it's under low demand. It's not unusual for an injector to work perfectly at idle but poorly under high demand conditions (i.e., full power, high rpm), which is they way I drive my BMW.

In order to really thoroughly test fuel injectors I am afraid they must be off the car and tested on a machine that can measure their output within the operating ranges of fuel pressure, duration, and RPM.

I offer fuel injector flow checking and overhaul service on the latest type of machine for BMW owners. I have a fuel injector servicing shop. I have the latest flow checking and injector cleaning equipment. Details are at . We overhaul BMW injectors with the correct parts!

If you would like your injectors overhauled I can ultrasonically clean them, replace the filters, pintle caps, and all seals for a very good price. I very thoroughly check them all all the RPM levels and pulse widths your injectors will experience on the car. Most shops only check them at idle. I also warranty the injectors I service for 12 months. I have found that when injectors are serviced by other shops they often start to leak about 6 months after the service. This is because the internal seals are delicate and can be easily damaged by the fluids many shops use to clean and flow check. The correct fluids to test and clean these early injectors are $20 a quart so many shops don't bother to use them. I do, that's why I can warranty the injectors I service against external leakage, or any other problems.

After a full overhaul, old BMW injectors usually work like new and our warranted for 12 months.

Here is a set of BMW E30 325i injectors from a running car. You can see that relative to injector 3, they are all a low. Cylinders 1 and 4 were so low it's a miracle the engine didn't blow a hole in a piston.

What the picture above doesn't show is that all the injectors were actually low. I just ran the check until the first injector hit 100ccs so the percentage of variation would be easy to see in the picture. After my injector overhaul service this set of injectors was usable again.

The same set of injectors after level 3 service. The picture speaks for itself.

The only way to have a known good set of injectors is to have them flow checked at a variety of rpm and duration levels, and at the correct fuel pressure values. Obviously it's important that the person testing them knows what these values should be. It's also important that the injectors be cleaned and flow checked with the correct types of fluids. (Obviously we can't use gasoline or we would blow ourselves up.) The correct fluid for BMW injectors is about $20 a quart. That's so expensive a lot of injector shops just don't use it. Using the wrong fluid tends to damage the injector's internal seals resulting in external leakage about 6 months later. They tend to develop these leaks between the injector's metal body and plastic cap. At my shop we use the correct fluids, that's one of the reasons we can warranty the injectors we service. Also, the correct fluid has to have the same specific weight of gravity as gasoline or the flow checking results will be meaningless compared to original specs.

New injectors: New Bosch injectors are expensive, but they are high quality. There are various other brands that can substitute for the 535's Bosch injectors. These other injectors that will physically fit and are so close in flow rate that they will work. The only drawback is that the Bosch injectors seem to be more precise. The Bosch injectors are usually within 3% of each other, the other brands tend to be within 5% of each other. For serious performance applications I like to flow check a couple dozen injectors and pick the six that are closest to each other. This normally allows me to get injectors within 1%.

Step 7, Coolant Temperature Sensor!

The coolant temp sensor is one of my favorite components to test. Probably because it's so easy, and yet, it's such an important device. This device reports the coolant temperature to the Motronic ECU. The ECU uses that information to adjust mixture for engine temperature. It richens the mixture when cold.

If you are a veteran of working on earlier L-Jetronic systems you probably remember how a bad connection at this sensor would instantly shut off the engine. Well it seems somebody at Bosch decided that wasn't such a good thing because they improved it for the Motronic system. On the E28s if this sensor is disconnected or if the the connection is poor, the engine will still run. It may not start, and it will run poorly, but at least it will run so it's no longer a device that can leave you stranded on I-10, 20 miles outside of Lordsburg, New Mexico.

The coolant temperature sensor is located at the front of the engine near the thermostat housing.

If your coolant temp sensor is bad, it can cause a lot of problems, although most likely, your car will still run. These problems include * poor fuel economy, lack of power, difficult or impossible cold, or hot starting, irregular idle, and rough running. Depending on how bad the sensor is, these problems can range from minor to severe.

Testing the coolant temperature sensor is easy. Once again, get out your ohm meter. If you want to be really thorough you will need to test the sensor when the engine is cold, partially warmed up, and fully warmed up. Simply remove the sensor's electrical connector. Once again we have that dang metal clip that must be removed first. Here are the resistance values we should see on a good sensor:

On a 528e with Motronic Basic, or a 533i if the coolant temperature is 12F-16F (-9C to -11C) resistance between the two terminals should be 7000-11600 ohms. At a coolant temperature of 66F-70F (19C-21C) it should be 2100-2900 ohms. When the engine is fully warmed up the coolant temperature should be 174F-178F (79C-81C) and you should have 270-400 ohms of resistance.

If you have a 528e with Motronic 1.1 or a 535i you are looking for a slightly different set of values. With a coolant temperature of 12-16F (-9C to -11C) resistance between the two terminals should be 8200-10500 ohms. At a coolant temperature of 66F-70F (19C-21C) it should be 2200-2700 ohms. When the engine is fully warmed up the coolant temperature should be 174F-178F (79C-81C) and you should have 300-360 ohms of resistance.

This test is simple. Measure resistance between the two terminals on the coolant temp sensor. If the sensor is faulty, it should be replaced.

Step 8, Speed Sensor and Reference Sensor!

So, you are cruising along on interstate 10 in your Motronic Basic 528e, 533i, or 535i secure in the knowledge that Robert Bosch improved the Motronic system so that a coolant temp sensor won't leave you stranded like it did with the old L-Jetronic system in your E12 528i. Now is when you find out he added two new sensors that will leave your stranded. *Failure of either of these will stop your car dead, right NOW! Well, that's a little bit of an exaggeration, but not much. Please read on.

The Motronic Basic 528e, 533i, or 535i all have two sensors mounted on the bell housing. The first is called the speed sensor. As you may have guessed it reports engine RPM to the Motronic ECU. Failure of this sensor may or may not totally disable your car. It's supposed to limp along poorly constantly missing if this sensor fails. Reality may be different.

The reference sensor is more critical. Without it the car will not run.

These are both mounted on the driver's side of the bellhousing. The speed sensor is mounted above the reference sensor. Caution: the two sensors are actually identical, so it's possible to connect them incorrectly. If the speed sensor is plugged into the connector for the reference sensor, and vice versa, the car won't run. The speed sensor mounts above the reference sensor and connects to the BLACK plug. The reference sensor connects to the GREY plug.

Both of these plugs have the Bosch style connectors on them with the dreaded metal clip. It's probable that the connectors will have been damaged at some point in the cars life. Mechanics love to test these sensors, but they really don't like messing with the metal clips so they tend to force and break things. If the connector is damaged DO NOT try to cut off the connector and solder on a new one. You won't be able to do it. You will have to find another way to secure the connector (tape, zip ties, etc.), or replace the entire sensor.

The Speed and Reference sensors connect to the Motronic wiring harness at the crossover bracket. Watch out, these two connectors can be reversed so make sure the upper sensor connects to the black connector.

Thankfully it's very easy to test these sensors. Once again, get out your Ohm meter. Remove one of the connectors. Note that this time there are three electrical terminals present. That means three tests per sensor. The terminals are numbered 1, 2, and 3 from left to right as viewed from the front of the car. If your car's crossover bracket is missing orient the sensor so that the single tab is up, and the two tabs are down. Now the terminal on the left is number 1.

Measure resistance between terminals 1 and 2. You should have 864-1056 ohms. Between terminals 1 and 3 you should have 100,000 ohms minimum. Between 2 and 3, again, 100,000 ohms minimum. Now Bosch wants you to take this reading with the outside air temperature of 77F (25C). I find it doesn't make much difference, but if it's really hot or cold out and the sensor is a little out of spec, I probably wouldn't worry about it.

Now test the other sensor. The values we are looking for are the same because these two sensors are EXACTLY the same part number.

In the picture below I am testing the reference sensor between terminals 1 and 2. The sensor passes!

It's a really good idea to carry a spare sensor with you if you are taking a long trip. Since they are both the same you really only need to keep one with your spare fuses and relays.

Sharp observers will notice a position sensor mounted on the front of the engine of a Motronic Basic 528e, 533i, or 535i. This sensor has NOTHING to do with the running of the engine. It's not linked to the Motronic system in any way. It's used to hook up some device the dealers have.

If you have a 528e with Motronic 1.1, your car does not have the two reference sensors on the bellhousing. Instead it has a "pulse sensor" mounted at the front of the engine. Test the resistance of this sensor and look for an ohm reading from 486-594. You also need to verify that the tip of the sensor is within .7mm-1.3mm from the toothed wheel.

Step 9, Air Flow Sensor (AFS)

The Air Flow Sensor is a very important part of the Motronic system. *It's another component that can cause a large variety of problems, including the starts and stalls syndrome, irregular idle, lack of power, and high emissions. In some rare situations it can cause high fuel consumption.

This device does two things. First it measures the volume of air flowing into the engine. Second it measures the temperature of the air. It measures the volume with a spring loaded door. Air entering the engine has to pass through this door, the more air that goes through, the farther it pushes the door open. As the door opens it actuates a potentiometer which varies a voltage signal to the ECU. The Motronic ECU uses this airflow and temperature data from the AFS to calculate the weight of the air entering the engine. The ECU then combines this information with data from other sensors and determines how much fuel to inject for various operating conditions.

The Air Flow Sensor is a very reliable device. They seem to last a really long time, like 30 years and 300,000 miles. They were replaced in later fuel injection systems with Hot Wire Mass Air Flow Sensors. The hot wire sensors are a newer design, and have no moving parts, yet they have never achieved the reliability and longevity of the older flapper type sensors. It's quite common to hear people claim that the AFS is a HUGE restriction in the intake system. I am quite certain these people never actually measured just how much of a restriction it is. It's not the horrible blockage that some people think. I will discuss the AFS's effects on airflow and power in the performance section of this article.

In the picture below I am holding the AFS's electrical connector. This connector plugs in to the AFS low on the firewall side of the device. It has a push to release metal clip, that pushes up from the bottom. (why didn't they use this style clip on everything?)

Ok, it's time to break out the voltmeter. DANGER DANGER!!! Make sure your multi meter is NOT selected to Ohms. If it is, it will send current into the Motronic control unit (the ECU). As a general rule, power going into the ECU where it's not supposed to is bad. Damage could result. Make sure it's selected to read voltage. As a general rule, when we are checking something at the wiring harness, it's voltage, when checking an actual component we are checking ohms.

It's time to test for voltage supply to the AFS. Look at the connector, it has five sockets. For the sake of simplicity, I am going to number them 1-5 from left to right. With the metal retaining clip down, like in the picture below the one on the far left is blank. The next four have electrical terminals that connect to the AFS.

With the ignition on you should see the following voltage readings on your BMW E28 535i. Terminal 1, blank, nothing to test, Terminal 2, Zero volts, Terminal 3, 4.0-5.0 volts, Terminal 4, 2.0-4.0 MILLIVOLTS! I am not kidding, they supplied this terminal in Millivolts. Hopefully your voltmeter can read that low. Terminal 5, 4.0-5.0 volts.

In the picture below I am testing for voltage at terminal 3 with the ignition on. (this picture looks like I am touching terminal 2, but I am not.)

Hopefully your car passed that voltage supply test. If so, do not put the connector back on yet, we have more to test. If it did not pass the voltage supply test your problem is most likely in the ECU or the wiring harness. Double check steps 4 and 5 to make sure the Motronic system is powering up and is grounded. If you still have a problem getting power to the AFS go to step 15.

If your car passed that test, it's time to test the AFS's internal door, and potentiometer. Testing the door is simple. Remove the hard plastic elbow that connects the AFS to the air filter box. Reach inside with your fingers and simply push the door. It should move freely without any strange noises, or binding. Assuming it does, do not put the elbow back on yet, move on to the electrical test.

Locate the electrical plug on the AFS, the one where the connector plugs in. We are going to test for ohms on the AFS itself. There are four terminals on the AFS. For the sake of this article I am numbering them 1,2,3 and 4. Terminal 1 is the outboard terminal, nearest the fender, 4 is inboard nearest the engine. Test for ohms at terminals 1 and 2. You should see about 550 ohms. Now test between 1 and 3. Here you should see about 80 ohms with the door closed. Start to open the door and ohms should increase. They should increase to about 1200, and then as the door opens farther, start dropping. (note: It's so sensitive it's actually very difficult to get it to read 1200.)The OHM reading with the door fully open should be about 600. The last test here will be between terminals 2 and three. Look for about 600 ohms with the door closed. The reading should increase to about 1000 as the door opens and drop to about 100 with the door fully open.


I am testing the Air Flow Sensor for ohms between terminals 1 and 3. With the AFS's door partially open I am getting 886 ohms. With the door open a little more it went to about 1200. This sensor is acceptable.

That's all for the airflow sensor, at least until the performance section of this article.

Step 10, Idle Air Stabilizer Valve / Idle Air Control Valve

*If your car has a problem with its idle speed, there is a good chance your idle air stabilizer valve needs some love. This is a somewhat troublesome device is sometimes called an idle air control valve. Thankfully, it can only cause serious problems at idle. It can't harm performance, fuel economy or emissions (except at idle) or much of anything else. It is not a critical item. It's possible for it to cause minor irregularities during light throttle operations but under no conditions can it cause any problems at full throttle.

Back in the days of carburetors and early fuel injections systems, if we wanted to change the idle speed of an engine we simply adjusted the throttle stop. If we needed to raise rpm at idle we turned a screw to hold the throttle open more at idle and vice versa. Most early systems has some sort of mechanism to hold the throttle open farther when the engine was cold for a fast idle. Other than that, they had no automatic control. If you turned on the A.C. idle speed would drop. If you heavily loaded the electrical system or drove up to a higher altitude, idle speed would drop. Of course this wasn't a big deal, if the drop was unacceptable, a quick turn of the screwdriver, and all was well again.

Fast forward into the 1980's and everything changes. First of all, consumers don't like to see their idle speed change, so an automatic idle control system was needed so the engine, once warmed up would always idle at about the same speed. More importantly were the ever increasing emission issues and they didn't want people messing with the injection system's settings.

Bosch addressed these issues by installing a fixed throttle plate, which is not easily adjusted. When the throttle of a Motronic car is closed, very little air can pass through it, far too little to allow the car to idle. In fact, the throttle essentially closes all the way, with only the slightest gap present to prevent it from jamming closed. Of course with the throttle closed, they needed to come up with a device that would allow enough air past the throttle so the engine would idle.

This device they came up with is the Idle Air Stabilizer Valve. It's an electronically operated valve. It opens to allow air to flow from the intake downstream of the air filter directly into the intake plenum, bypassing the throttle. When the engine needs to idle faster, like when it's cold, it lets more air in and vice versa.

There are two types, a two wire, and a three wire. The car used in this article is a 53
Helping BMW's that are hurtin' for a squirtin'
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