1991 735i won't run

[Headshrinker]

I am working on a 1991 735i automatic trans car with 200,000 miles on it. It slowly began to degrade in performance until it will no longer even start. I changed plugs (severely carbon fowled and wet with fuel), hooked up a fuel gage (reads 40-45 lbs while cranking and holds pressure after shutoff), checked to ensure spark (check good), tested plug wires (all between 5k ohm and 7k ohm, and hooked up an injector tester to verify injector pulsing (also good). I happen to have a complete injection system recently striped from another running 1991 735i that I am in the process of installing on my 1980 euro 635CSi to help control it’s T4 intercooled cartech turbo, so I’ve tried swapping out the ignition computer as well as the air flow meter, neither produced any effect.

Now I’m stumped. Certainly if the mixture is off or it is in limp mode (open loop) it should run for a while until the new plugs fowl. If any one out there has run into a similar problem please let me know.

Thanks

[RickyJ]

The idle control valve could be open. Try clamping off the hose.

[winfred]

if it's dumping fuel check the temp sensor, if it's shorted or wide open that can screw with it, the icv doesn't have anything to do with the performance of the injection system, it's possible the air flow meter could be screwing up

[Headshrinker]

Thanks for the suggestions. The icv seems to be functioning, plus I've tried opening the throttle while cranking and still no luck. Since I just put in new plugs I'm pretty sure it would start even if it is running rich. I did try swapping out the air flow meter with a known good one and it did not help. Is it possible that it is running so rich that it can't start even with fresh plugs?

[winfred]

possible, what's the spark look like big and healthy or small and crappy? (hold coil wire about 1/4" from a good ground, hopefully a better ground then you while watching as it's cranked) a weak spark will make it run rich also


Thanks for the suggestions. The icv seems to be functioning, plus I've tried opening the throttle while cranking and still no luck. Since I just put in new plugs I'm pretty sure it would start even if it is running rich. I did try swapping out the air flow meter with a known good one and it did not help. Is it possible that it is running so rich that it can't start even with fresh plugs?

[Headshrinker]

The spark did seem a bit weak when I hooked up a test sparkplug and cranked it, but I figured if it is firing then it would be enough. I suppose I should check the coil next. Let me know if you have any other Ideas.

[Paul in NZ]

rotor and or dizzie cap?

[Headshrinker]

Cap and Rotor are nearly new and perfect (I removed them to verify anyway). I did notice the injector pulse and plug spark seem irregular, so I'm thinking my next test will be of the reference sensor. I have a new sensor for a 95 525i laying aournd and other than the wire being a about 6" longer, it appears to be identical to the sensor for the big 6. I'll try that tonight if I can. After that, I'm out of ideas again so I hope this is it.

Lisle

[Headshrinker]

To update my 735i troubleshooting, I just tried replacing the crank position sensor with a known good one, and that also got me now where. The only other possible source of irregular spark and injector firing I can think of is the inductive pickup on the number one spark plug wire. Anyone have any suggestions on how to check this? If it has failed, it is only an intermittant failure as the system does spark and the injectors do fire, only not smoothly enough for the engine to run. Wires for these cars are an expensive item to replace without knowing they are bad.

[Paul in NZ]

post up on the 5 series forum there are wizards there on the m30