What would cause coils to stop trying to fire while starting?

[pklym]

Still haven't solved my no-start problem explained here: http://www.r3vlimited.com/board/showthread.php?t=384330

We have determined the coils fire in the right order as do the injectors BUT after a few cycles of trying to start, the coils quit firing completely. It seems that the car wants to start but it gives up before it gets there.

There are more details in the other thread, but notably, we just replaced the coils and it seemed to help but not enough to get it going. Have tested and replaced camshaft sensor and crankshaft sensor, tried two supposedly good ECU's. Fuel pump runs fine.

Literally don't know what to do short of replacing the engine at his point, almost all the sensors have been replaced.

[e30time]

I don't know if you have check the AFM, but the first 1991 318 I bought I got for next to nothing. The guy said it wouldn't run, and I was going to do a swap anyway so didn't much care. Got it back to the shop and put a good battery in it, checked a few things, and it cranked but no start. Had fuel but no spark. I can't remember why I checked, but in this case the I found the "barn door" inside the air flow meter to be seized in the closed position. I forced it half open and turned the key, and voilà, it was running. Maybe there is another way for the AFM to cause a hard fail.

[pklym]

The AFM ohms out fine. We'll try to take a reading of it while cranking to see if it does anything funky.

The car should start without the AFM plugged in too, right?

[e30time]

I wasn't sure, so I went out and gave it a try... it started with the AFM unplugged.

[pklym]

I wasn't sure, so I went out and gave it a try... it started with the AFM unplugged.

Thanks for checking that for me. We'll probably go ahead and leave it unplugged until we figure this thing out then.

[pklym]

Gurgle gurgle pop.

We replaced the crank pully wheel and the car definitely is thinking it wants to start more than it did before, but alas, it won't.

It will seem to catch then die, or catch and backfire. Lotso' backfiring and burping and odd noises as it tried to get going.

It acts like it wants to start on wasted spark mode, but as soon as it is ready to go into firing order it stops firing. WHY!?!?!?!?!

The headers are not hooked up to full exhaust at the moment but I don't think lack of back pressure would cause a complete no start, do you?

Probably looking to get ANOTHER brand new CPS to try that.

[pklym]

Great but frustrating news!!

The car is alive. It runs. The problem appears to be fixed.
I had been coming across an old, apparently discontinued, part number for the crankshaft position sensor. It is NLA. But I found one on ebay, bought it, and now the car runs. This has been almost six months chasing this no-start and throwing two new, supposedly correct, crank sensors at it. So incredibly frustrating, but thank goodness its alive.

The part number that works on my engine, with a 91 318i ecu, is 12141721968
The one that didn't work was 12141721504

[AndrewBird]

Both should work just fine. The newer part number supersedes the old one, but is an identical part (functionally).

My guess is there is another problem that you accidentally fixed while swapping out the sensor and didn't realize it. Fingers crossed it doesn't come back!

[pklym]

Both should work just fine. The newer part number supersedes the old one, but is an identical part (functionally).

My guess is there is another problem that you accidentally fixed while swapping out the sensor and didn't realize it. Fingers crossed it doesn't come back!

I mean, my rationale mind agrees with you, but it did what it did. I suppose its possible both other (new) CPS were faulty despite ohming ok (there are lots of reports of bad new CPS on this forum) but I thought I should document my experience just the same, in case it helps anyone down the line.