Unofficialbmw.com

[wazzu70]

Well my car suddenly died on me a few weeks ago just doing normal every day driving. The car still had plenty of juice in the battery to crank the engine over, it just wasnt catching.

Dale and I went through a laundry list of things to check. I pulled all the plugs and made sure they were sparking (by grounding to the valve cover). I tested each plug wire to make sure there wasnt one bad apple ect. I checked to make sure I was getting fuel and sure enough I am. I took off the supply line to the fuel rail and verified that the pump is outputting plenty of fuel and fuel pressure is at 40ish PSI. Doing a compression check yielded some low low numbers 60,90,30,70,70,70 Yeah the gage was tight but the car was running well before and I cant imagie the compression on all cylinders going that south in an instant.

After verifying this Dale reccomended checking my VR sensor for the correct resistance. I found that my sensor cable was being rubbed on by the alt belt and I replaced the sensor with a new one, Im getting correct varying resistance so its working correctly.

After this I stripped down the front of the engine to verify that the Tbelt didnt slip a tooth or something odd like that. this could be the reason for lack of compression. Nope its still dead on.

Without much else to go on at this point I removed the ECU and checked all the pin inputs per the bentley manual. Everything I could test by myself was within spec. Using a jumper I verified both banks of injectors were firing, fuel pump was working, ect. The only thing I couldnt check from inside the car was the throttle switch. I tested this and I could get continuity at WOT, but nothig at idle. I took the throttle body out and adjusted the throttle switch to try and get a result and I have nothing. This would explain my weird idle I was having before, but the car should still at least fire up.

So with nothing else really left I ask, could no continuity from the throttle switch cause the car to not start? I would think this wouldnt have an effect, it would just idle like crap, but would still at least try and run. Since our throttle switch isnt a "real" TPS that measures angular position, Im counting it out, but maybe thats a bad idea. Hopefully someone with experience can chime in.

The only other thing left is to swap in another ECU to make sure that isnt the problem as its about the only thing left. Im going to borrow one from Dale and see if I can get it to fire up, but want to make sure my throttle switch isnt the problem first.

Im planning on converting the car over to megasquirt, but I need to make sure it will run first so I know any potential problems are stand alone related and not engine related. Thanks for any advice/help ect. I cant believe I cant figure this out.
_________________
-Nick


1984 BMW 325e
1991 BMW 325i
1976 Honda CB400F Super Sport
"The bitterness of poor quality is remembered long after the sweetness of low price is forgotten"

[jlevie]

Lack of an idle signal from the TPS won't prevent the engine from starting or running, but it would affect how well the engine idles.

I think you need to revisit the compression test. The best number you saw is about half of what the numbers should be and are low enough (if real) to keep the engine from starting.
_________________
NASA and BMW Certified instructor
Spec E30 #96 (NASA-SE)

[tncean]

Lack of signal from crankshaft position sensor will no-start, too.

tncean

[jlevie]

True, but then there wouldn't be any spark. The OP states that there is spark, so the CPS isn't likely to be the culprit.
_________________
NASA and BMW Certified instructor
Spec E30 #96 (NASA-SE)

[cymy]

Check that the main harness cable to the ecu is not getting pressure from the glove box door.....seriously.....I chased a problem similar to yours for a month before I figured out the problem. I modified the ecu bracket and moved the ecu away from the glove box door. No problems since...1-1/2 years now.
_________________
o/_ __ _\o
(Oo=00=oO)
[]=****=[]
1991 318i
2000 Ford Ranger Extended Cab
1990 R100GS
1995 R1100GS

[cmcon98]

The low compression might be from the cylinders being flooded with fuel. The rings depend on the presence of oil to seal completely, and fuel will wash the oil away. You might try spraying a little oil in each cylinder, then retesting the compression. Test with all of the plugs out, obviously.

You could also try putting in new plugs and cranking it with the throttle floored, since you're getting a signal from the TPS at WOT. If it starts, you've narrowed it down, at least.

Good Luck!
_________________
Cirrusblau '88 325iC 5-speed
Cosmosschwartz '99 M3 'vert 5-speed

[bearing01]

You can try the new ECM. That would be a fast & easy remove/replace to check that cause off your list.

You said you got spark and fuel. I wouldn't overlook the obvious, which could be those low compression numbers. If those are real, then they are low enough to not start the car. To double check those numbers you should hook a vacuum gauge to the manifold (like tee it into the FPR vac line) and while cranking the engine to start you should see at least 4 or 5 inches of vacuum. If no, then those numbers can be real. Redo the compression test again. If the gauge is working fine and those numbers are real then either it's a plugged exhaust, valves not opening or a warped head (blown head gasket?). The only time I've seen low compression numbers like that on all cylinders is after a overheat and the cylinder head is warped... where you got no seal between adjacent cylinders. Another sanity check is to stick your finger in the spark plug hole while you hand turn the crank with a wrench. On the compression stroke you should feel just enough pressure to want to blow your finger out of the hole. Before pulling the head I would (a) make sure the timing belt didn't slip and then (b) pull the exhaust just before the catalytic converter. See if the cylinder pressure increases with the cat-back exhaust removed.

Before pulling the head it also wouldn't hurt to hook up a timing light to cylinder #1 plug wire and see where your ignition timing is with respect to the harmonic balancer mark. Obviously it should be somewhere near TDC. If it's totally out of wack then maybe your problem is electrical. Also, just because a plug fires when it's out of the cylinder doesn't mean the coil is powerful enough to fire the plug under pressure (cyl pressure should be around 150psi). The timing light flash will verify you got spark with the plug inside the cylinder.

Wet spark plugs (smells like fuel) should tell you you've got fuel.

[bearing01]

Stupid question.... but you did have the throttle blocked wide open when you ran the compression test right?

[Quincy56]

Otis did have a no start condition caused by the TPS (or it was running so bad it couldn't really go more than a few feet)

I found out the car runs OK with it disconnected.
_________________

92 325ic - Isabelle
91 318i - Oscar - RIP
89 325i - Otis - RIP

[wazzu70]

wow lots of great feedback here. Thanks guys!

jlevie; I agree that the low compression could be the cause of it not running. I have a hard time believing though that it would go from running decently to nadda in a matter of a second and have the compression drop on all cylinders that drasticly. I still dont have an explanation for the low compression though.

tncean; you are correct that wihtout the ECU knowing where TDC is, the car will not run because it has no idea when to fire the plugs! My tests confirmed though that the crank sensor is working correctly though.

cymy; good call about the ECU connection. I thought of that too and since I had to get access to the pigtail the ECU is just sitting in my glove bax right now. I tried firing it up after all the pin values turned out to be in spec with no dice.

cmcon98; Good thought about the fuel lowering compression. I did put some oil in the spark plug hole and ran another test and I got about 130 compression. Next time I work on the car ill put a little oil in all cylinders, get decent values for compression (if possible) and try to light the car up again. Maybe that was the problem. I was hoping to get the car running and warmed up and then performing another compression test to see if the values raised....I still have a hard time believing all the values can be that low all of a sudden. They couldnt have been that low previously or the enging would not run.

bearing01; no I didnt block the throttle plate open....I ll do that next time and see if it helps! That was an overlook on my part. you also bring up some really good points Im going to check over. I did verify that the timing belt did not slip and is dead on, so we can rule that out. I wll get a vac gage and T into the FPR line to see how much vacuum I am getting to see if my compression numbers are legit. Good though about the cat converter too. I didnt think of that. Its an old cat and the honeycomb could be breaking down and causing excess backpressure. Ill remove that and see if it helps. Good call. the car didnt overheat or anything, so yeah, Im at a loss for how the numbers are so low too. Since I got a good number after putting oil in the cyl and cranking, I have to assume the head (and gasket) are sealing decently. Also a good call on verifying Im getting spark in the cylinder (assuming it ever gets up past 60ish psi).

Quincy56; I cant even get the car to catch like its even trying to start. Even getting a cough of exhaust would be good at this point!

Well Ill keep everyone up to date on what I find. I wont be working on the car this weekend, I need a break.

Thanks for all the help!
_________________
-Nick


1984 BMW 325e
1991 BMW 325i
1976 Honda CB400F Super Sport
"The bitterness of poor quality is remembered long after the sweetness of low price is forgotten"

[bearing01]

If the throttle is closed during the compression test then the numbers you get will be around 1/3 of what they would be with throttle blocked open. That means you probably get somewhere around 150-200 psi if you redo it again with throttle open.

However, you still have one cylinder that's low (30psi). Maybe just a bad measurement. Still, wouldn't hurt to run the test again. Remember, the test should be ran when the engine is warmed up. Your engine will be cold so the numbers will be slightly lower.

[wazzu70]

Cool, hopefully I can get some more respectable numbers to at least rule out head warpage, head gasket ect. There isnt any coolant/oil mix so I dont think the gasket would be the culprit, however, somewtimes its not a total blowout and just a small leak that causes problems.

If the compression goes up to a reasonable amount after running the test again, and switching the ECU does not work Im really going to be at a loss for whats wrong!

Hopefully I can at least get it going though.
_________________
-Nick


1984 BMW 325e
1991 BMW 325i
1976 Honda CB400F Super Sport
"The bitterness of poor quality is remembered long after the sweetness of low price is forgotten"

[wazzu70]

Well I borrowed some spare parts for tesing purposes like an ECU and an AFM ect. (thanks Dale). I put in the ECU first with no dice. Then I took off my AFM and tested it and it seemed ok but in about the middle of the range the resistance dipped down and then went back up. I tested the spare AFM I borrowed and it was constant throught the range so I swapped that in, still no luck. I did another compression test (throttle open this time ) and got similar results. Then I thought, hey, I can get decent compression adding some oil in the cylinders, Ill put some oil in all 6 and then try to start it.

The engine did start and run (roughly) for about 5 seconds and then wound down suddenly like before. I checked compression again and it was back down to what it was before adding the oil. This gave a strong conclusion that the rings were to blame for this problem. I then used Dales handy leak down tester and listened around for where the air was escaping. I got a little in the valve cover from the well used valve guides, which was to be expected. I got nothing out of the throttle body or the exhaust, and no bubbles in the coolant tank. I opened up the oil dipstick and there was air blowing out of it at a pretty good rate. The air was going past the rings like there wasnt any.

Im going to get another used engine and swap it in most likely, then rebuild my M20 on the side.

Just thought I would shed some light on what the problem was.
_________________
-Nick


1984 BMW 325e
1991 BMW 325i
1976 Honda CB400F Super Sport
"The bitterness of poor quality is remembered long after the sweetness of low price is forgotten"

[bearing01]

Your leak down test, did it suggest one bad cylinder or all? Seems a little odd for suddenly all piston rings don't hold compression. Maybe one bad cylinder, but all?

What's the chances you had a knock / detonation problem and you blew holes in your pistons? All of them? Or broke your rings on all cylinders?

Let us know what happened when once you pull the head.

[wazzu70]

I tested two different cylinders with the same results at random so I figured that was good enough. for me. Either way the engine is not getting fixed without a rebuild so I figured it was worthless to keep pushing for more explanation. Ill post up pictures or whatever of the engine when I open it up. Im curious to know exactly what went wrong as well.

As far as the detonation, I guess anything is possible although I couldnt hear/feel any knocking, that does not mean there was not any. the motor has about 250k on it and does not look like it was maintained well at all so I guess its possible they all kinda gave up the ghost at the same time? I guess they could wear fairly evenly until a point where the rings cant expand enough to hold a decent compression. Im not exactly sure.
_________________
-Nick


1984 BMW 325e
1991 BMW 325i
1976 Honda CB400F Super Sport
"The bitterness of poor quality is remembered long after the sweetness of low price is forgotten"