I've been having problems with my idle, especially when cold. Car is a 2000 540i, Dinan S2 (stage 5 software), around 100K miles. The check engine light is on and is giving codes for the intake system/airpump/check valve. Dealer is saying they're 99% sure it's due to a build up in the track between the airpump and the intake manifold. They're referring to a BMW service bulletin that's describing a similar problem. It's a 9+ hour service job, so before I have them do it, I want to get the boards input. Here's all the info:
-Air pump & check valve were replaced with new parts less than 2,000 miles ago, this did not fix the codes the OBD was showing. Idle problem was there before this and after the fix, although not as bad as it currently is.
-Idle hops from normal to 2500+ and then slowly comes down when first starting car, or when coasting to a stop and putting car into neutral. I really only notice it when shifting out of first or second into neutral and coasting to a stop or slowing down.
-When the idle goes up, the MPG gauge goes from normal reading (about center) all the way over to the far left (8MPG/far left) and then when the idle comes back to normal, goes over to the normal reading again (better than center to the left).
-The problem doesn't happen every time, and it happens more frequently when it's below freezing out. Doesn't matter how long the car's been driving (i.e. not just when the engine is cold).
-When at a standstill in neutral, the idle is not smooth. It's not rough like a misfire, but hops around a little (+/- 100 rpm). Sounds like the intake manifold leak I had on my old e31 (the v12's were notorious for it).
-Based on past experience with similar idle problems, I feel like it's an intake leak somewhere. I don't see where a "build-up" in the intake track is going to give these symptoms. I could understand a MAF malfunction, leaking gasket, cracked tubing, loose connection or something else, but "build-up" to me sounds kind of a last guess attempt to fix something.
Welcoming any input (especially from similar experiences)