'86 325e Idle Problem Solved (long)! (archive)

My car was having this strange idle problem: only when the car was very warm and being restarted (like when you run into the store for a coke and come back out) my idle would oscillate for about 30 secs before smoothing out at 700rpm. Turning on the A/C would smooth it out immediately.

So I opened up the Bentley and ran every test in there and EVERYTHING checked out ok. I was about to bite the bullet and order a new ICV/ICM combo when I sat down and mapped everything out to see if there was anything I was missing.

I then remembered two other salient points: the car was running a little rich (I installed an air/fuel ratio gauge recently so I could tell AND my mileage was a little off) and I have one of those adjustable BMP pressure regulators. I checked the pressure and it seemed in spec but then I removed the little vacuum hose from the regulator to the manifold and pressure rose about 5psi. Hmmm...maybe the pressure spec I was using was supposed to "regulating", i.e. with the vacuum open. So I lowered the regulator to the right spec and guess what...

IDLE ROCK SOLID and STOICH AIR/FUEL MIXTURE. YES!!!! I was running fuel pressure too high, hence the richness and idle woe. After about 30 secs the ECM was probably adjusting for the mixture and smoothing things out...since I wasn't that far over.

What's the point of the story?

1. Always check the obvious first before the non-obvious
2. Add the FPR to a list of possible culprits of idle problems, and careful messing with those adjustable ones!
3. Get an air/fuel ratio gauge!

Sorry for the length of the post!

MATT

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