E30 Idle issue at cold-start.
 

I am going to start tackling this in the morning, and will post back with findings but will entertain any opinions. 1990 325i/ 5spd.

Problem:When engine is cool (off for an hour or more) and started, Idle will pull right upto 800rpm, after 20 seconds the idle drops to 500 and stays there. Check engine light comes on until the gas is depressed. Once the gas is depressed, rpms return to 800 and CEL goes out. This continues until the car is at running temp. Once car is up to temp, there is a slight occasional stumble, nothing rhythmic or regular like a particular cyl is causing the issue, more of a random stutter.

The plugs, cap and rotor only have about 3500 miles, wires probably 7K. Fuel filter... god only knows! At first I thought the engine was "hunting" pointing to a vacuum leak, but It's not "sweeping" like a normal leak.

This started happening this morning (temp for last couple days 50-60F) I am going to pull the plugs and check for any differences between them, and check my fuel pressure (and replace filter) in hopes that I am dealing with a fuel pressure problem. I checked the throttle switch, and it does indeed "click" as soon as the throttle is advanced off the stop, so I don't think the idle screw has backed out.

I was running low on gas anyway, so I went ahead and drove around a little extra this evening until my low fuel light came on and put in a half tank of hi-test with detergent... the problem persists :(

Any ideas that I may be overlooking?
Let me know!
Thanks,
-Nick

Is the problem is the CEL? Or is it the slightly low idle?

I'm not much help on E30 engines, but for the CEL, O2 sensor is the only thing that comes to mind.

I'm fairly certain that the CEL is being caused by the low idle. Especially since it goes out as soon as the revs are brought back up to normal. If it is an ignition issue, I suppose there could be excessive fuel not being burned going downstream to the O2 triggering the CEL..

I am getting ready to feed my daughter breakfast and get her to daycare so I can start diving into this. We'll see.

Thanks.

Well, I'd say over half the problem is resolved....
I removed all the plugs, and found no residue or visual difference between them. The gaps had opened up to about .036, so I re-gapped them all. This resolved the subtle "hunting" situation, but the engine rpms still dropped after initial start-up. However, after replacing the fuel filter, that problem is resolved! No more idle down to 500rpm, and no more CEL :)

There is definitely more pull through-out the entire power range, but still an occasional hiccup. It can't be seen on the tach, but is audibly perceived. It seems like maybe one cyl out of every couple hundred revs doesn't fire. I think it more of an annoyance than a performance problem.

I'll see if I can track that down later, it may just be the coil...

-Nick