Hunting down a hunting idle

[jaredmac11]

My M42 seems to have a hunting idle. It used to be when the system was warmed up and it went to a closed loop. I thought this was the IACV, so I replaced it with a used but "working" IACV from Ebay and that seems to have made it worse. The hunting idle starts moments after start-up.

The engine was fully rebuilt. All rubber is new and there are no vacuum leaks.
It does have a Mark D chip.
Ive gone through 3 IACV and Im getting pretty tired of swapping those out (M42 sits almost on the firewall on my E21)

Before I go down the next, more expensive paths, Id like to see if there is any troubleshooting I can do.

I have a few spare crankshaft and camshaft position sensors to try but there's no telling if they are working or not.

I am also using used coils from my COP conversion. I dont know if a hunting idle can be caused by failing coils.


Edit: Idle is back to 'normal' i.e. idle gets rough when it enters a closed loop. Starting to think its the AFM..

My thoughts are the IACV I put on is worse than what I have, but even then the idle is really poor when the engine enters a closed loop.

any other considerations I should make?

[Bimmerfan84]

I put an aftermarket crank sensor in my car and it did exactly what you described, varying idle, rough running in between open and closed loop. Bought the super expensive BMW sensor and haven't had a problem yet. The air flow meters are crazy expensive, so I would suggest finding someone local that has the same car and see if you can briefly swap out and see how it runs. (I bought a rebuild from Oreily's, still $300) You should also check to make sure the fuel pressure is steady with a fuel pressure gauge.

[jaredmac11]

OK I *THINK* its the AFM. Plugged in my E36 AFM and it is running much better. I'll have to take it out for a spin and get it hot but it climbed all the way to operating temp with no issues.