Hi,
I just purchased a '91 318is, so I have a vested interest in solving this problem. I haven't had a similar problem with it yet, but I've had this type of problem with other Bosch EFI models such as a Volvo 240. Sometimes the causes are the same. The air mass meter is a particularly weak spot on EFI systems; the vane type meter used on the 318is is better than some, but can still cause problems.

Here's what to check, based on my other Bosch EFI experience:

(1). Is there a spark? Either pull the plug out and ground the end, or attach an inductive timing light to a spark plug wire (preferred); then crank the motor and look for the spark/timing light trigger. DON'T disconnect the spark plug wire from the plug and then crank it--this could blow part of the ignition driver circuit. If there is no spark and the computer senses the fault internally, or if the O2 sensor detects an overly rich exhaust (like what happens when there's no spark), the FI system will probably shut off the fuel pump. Therefore, if you are trying to diagnose the problem by listening for the fuel pump, beware that a problematic spark could manifest itself as a nonoperational fuel pump.

(2). Given that there's spark, check the fuel delivery. It's a bit difficult on the 318is because the fuel injectors are buried beneath that neat-oh aluminum intake manifold. The simplest thing to do is to crank the motor, then remove a spark plug, insert a hose into the cylinder, and sniff for gasoline. This test might be inconclusive. The ideal thing would be to determine that the fuel injectors are receiving an electrical pulse, and that there is fuel pressure. Determining fuel pressure is best done by attaching a pressure guage to an appropriate port in the fuel line; then turn the ignition to "on", or crank the motor, while looking at the guage. I'm not familiar with the particulars of the 318is yet, so you'll need to look in a manual for the best connection point. The low-tech "dangerous" method is to disconnect some part of the fuel line, hook up a hose to it, and put the end of the hose in a bottle. When the ignition is turned on, gasoline should squirt VIOLENTLY into the container (FI systems operate at high pressure). This is really not recommended because it creates an explosion hazard. Given that you have fuel pressure reaching the injectors, you need to determine whether they are being actuated electrically. If you can reach the fuel injector driver wire for the forwardmost injector, a sensitive induction-type ammeter can be used to determine if a current is passing through the wire. DO NOT disconnect the wire from the injector and crank the motor; this may fry a driver circuit. If you have a spare fuel injector hanging around, it's OK to connect the wire to it and crank the motor; you will feel and hear the injector "click" when the electrical pulse causes it to momentarily open.

3). If there's spark and fuel pressure but the injectors are not being electricallyactuated, suspect one or more of the sensors. The main sensors that drive the FI and ignition system (they are integrated in the Bosch "Motronic" system found on the 318is) are: engine RPM/phase (optical or magnetic sensors that determine the position of the crankshaft); throttle position sensor (three state, closed/partial/fully open, I believe); air mass meter (the box that sits above the air filter, with a big, fat electrical connector going to it); and the O2 sensor (looks like a spark plug screwed into the exhaust pipe somewhere).

In my experience, a bad O2 sensor will never cause complete failure of the system; it might cause the car to run rough, smoke somewhat, get bad mileage, etc. However, if you have fuel and no spark, the unburned gasoline vapors flowing past the O2 sensor will cause a "rich" reading, which might then prompt the control unit to shut off the electrical signals to the injectors. If you find that the injectors ar not receiving driver signals, disconnect the O2 sensor and try it again.

A bad crank trigger (optical position sensor) can easily cause complete failure, because it is the main electrical input that times the ignition and injector pulses. Diagnosing it can be difficult; you need to find a test port and look for a signal, or read a computer diagnostic code that indicates failure of the system. If the crank trigger is bad, you will probably have neither spark nor fuel injector signal.

A bad air mass meter can cause starting failure; on my LH-Jetronic system, the proper test for determining air mass meter failure was to disconnect the meter, then try to start the car. Without the meter input, the FI system went into "limp home" mode, providing a basic injector driver pulse that would run the motor sufficiently well to drive home. I'd imagine that it's OK to try to start the 318is with the air mass meter disconnected (nothing _should_ fry, but I take no responsibility), because the system should be designed to accomodate a bad connection (it's not a high-power circuit like the spark plugs and injectors). If you try this and find that the car starts, your air mass meter is probably at fault. It's basically a potentiometer connected to a swinging door on a spring. If the potentiometer gets dirty, it could be sending bad signals to the control unit.

Good luck; let me know what you find.

Mike

*from bimmer.org board*