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Date: Thu, 26 Oct 1995 22:52:03 -0400
Subject: adjusting air/ fuel mixture

Steve asked about fuel mixtuer adjustments.

My name is Paul Hahn and I'm a electronics technician who bought a used '84 M635 and had to undo a lot of tampering that some previous owner employed to give his engine greater performance (at the expense of economy and cleanliness). I started with a complete read of the Bosch book on fuel injection and engine management by Charles O. Probst - very educational and worth it's cost many times over.

Most newer BMW's have a single computer chip to control the ignition and the fuel of the engine. The goal of the "motronic" computer , in regards to air/ fuel mixture, is to maintain the optimum ratio of air to fuel so that the car has acceptable power and maintains some standard of economy and cleanliness. Research has shown the proper ratio for a good compromise in all areas to be 14.7 : 1.

Air mass meters - These marvels of modern science input your motronic brain with a lot of info about the air the engine is using. It compensates for volume, density and probably humidity. An electric signal is applied to a special alloy wire that is suspended in the middle of the air flow to the engine and depending upon how much air passes across that wire, it presents an impedence to the flow of the signal and the motronic brain then uses this signal to determine how much fuel is required. The "cooling affect" of the air flow and the resultant change on the impedence of the applied signal compensates for air speed, temperature, density and humidity( in a word = mass). So when you go from sea level to the top of the Rockies, the motronic unit makes all appropriate changes in fuel delivery. Is technology great or what??

Before air mass meters we had air flow meters (on my '84) which are electronic equivalents of the old mechanical types of fuel injection control. They are less comprehensive in their evaluation of the air that the engine is using.

Air flow meters use a wiper that crosses a grid of resistance traces on a printed circuit board. The wiper is attached to an air vane that pivots in an amount proportional to the amount of the air going through the meter. On air flow meters one adjustment is simple, legal and affects only the idle mixture. It is an allen head air bypass that lets unmetered air to the intake manifold. The computer does not "know" about this air and thus does not match it with an appropriate amount of fuel. The more air you allow to bypass, the leaner your idle mixture will become. This has very little affect on your running (off idle) mixture because when you open the throttle, the vane in the air meter opens with the increased flow and measures many times the amount of air that the bypass can handle. This adjustment should only be undertaken after the steps in part two have been completed.

Part two: this may or may not be legal in your state, as it involves the changing of the spring tensioned back pressure of the air flow meter. One must carefully pry off the black plastic cover to the electronic portion of the meter and make the necessary changes in the spring anchor position to give the correct fuel mixture. The spring is used to return the vane to a zero position so more you wind the spring at rest, the less the meter will register and the leaner the fuel mixture. With less spring resistance, the meter will open wider under a given air flow, the motronic unit will inject more fuel and the mixture will be rich. The cover should be replaced to give a water and dust proof seal. Air mass meters have a tamper proof plug which conceals a screw adjustment of some type of compensating bias. In either case the real trick is to somehow measure the exhaust while making adjustments and experimenting til you get the mix right. Obviously, every thing is dependent upon your other engine tune-up conditions being as good as you can get them, so new filters, plugs and maybe a fresh oil bath are in order.

I used the dc voltage output of a new O2 sensor to give me the feedback I needed to adjust the air to fuel ratio. I installed the sensor, clipped a good voltmeter to the output but left it disconnected from my after market Lambda control and and went on a road test ride. The meter I used was a fluke 87 model, which has a recording function to display the average , maximum, minimum, or present reading. While driving under a variety of conditions, I observed the average readout and made small changes to the air flow meter so as to get a reading of .400 volts dc. A too-rich mixture will result in a reading above .500 volts and a too-lean mixture will give a reading below .300 volts dc. The process involved several stops, adjustments and driving tests, but I got good conformity in city and highway driving. The "drivability" of the car went way up after I had corrected the way too rich setting of the former owner. You might have to accept a reading slightly higher or lower than .400 volts dc to get a satisfactory compromise between city and highway.
Paul Hahn
'84 ///M635 CSI (107k with a fresh engine)

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