Ignition or injector cause of intermittent stall?

[Gene in NC]

Ignition or injector cause of intermittent stall?

'89 525m 180k runs OK from cold. Given a few minutes to hot soak, at traffic lights or parked, it begins intermittent very short stalls. Seems to quit dead and then refire. Can see tach drop, especially in third, for example. Does not sputter like ignition cross firing, just quits dead.

Have another of these pigs that likes to give codes for injector trouble.

So, how to tell difference between injectors not being pulsed and ignition not firing. The current trouble exists despite new wires, rotor, cap, and swap of known good airflow box, coil, ecm, fuel and main relays, etc.

How to rig an ignition tell tale to the windshield or inside the car? Something like the little neon device that is inserted between wire and spark plug.

Already have fuel pressure gauge rigged to windshield. FP looks OK.

[genphreak]

Sonds like an intermitent voltage drop to me... I'd jump +12V to the ECU power pin, and drive like that. I can't tell you what intermittent ECU shutdown drives like, but if not that, I'd take that +12V wire and run a few other things. Also, try driving with the 02 disconnected. Could be MAF, so once warm disconnect and drive around to see if it does it. (will drive very slowly and badly, so take a lot of care doing this). One can jumper a lot of things, but if u jumper the AFM, only use 5V. Not all things are 12V

Typical culprits are body electronics modules (under the rear seat) and fusible links (tho they often look and test ok).

And a left of field suggestion.. Is there a dodgy car alarm in there trying to disable the ignition?

[Tiger]

Crank Position Sensor

[Gene in NC]

OK, we have run the following tests, all dynamic:

Fuel pressure- gauge rigged to windshield

O2 sensor voltage w/in spec

12v power drop or interruption. Test point at ECM.

Ignition primary and secondary with oscilloscope.

All tests run with car at full operating temp and enough time that mini stalls or bumps are readily seen on tach and felt (seat of pants). No indication from any test of failure or out of spec for that function. No ignition break.

That seems to leave injection- interruption or as my buddy continues to suspect O2 sensor perhaps sending signal so far out of range that combustion fails from over rich or the ECM does something strange. Interesting shot in the dark but ignition continues w/o interruption. No feeling of lean, firing back through intake, etc.

So what the hell is going on with this '89 525m, 185k, O2 sensor claimed replaced at 171k by PO.

[bmwrp8]

sounds exactly what happened to me 2 years ago. i'll go with tiger with the crank position sensor.

[Gene in NC]

TIGER and BMWRP8 were right. Thanks much!

Swapped out RPS and running clean. Still don't understand how failure could be intermittent, but won't argue with success.

[Tiger]

Glad you got it solved. BMW CPS has a notorious mysterious symptoms... you can test it fine but it doesn't work fine... They seems to die between 80 to 120K miles...

[Ross]

Definately can be intermittant. I've had one "die" that after a hard whack was resurrected for the drive home.