Read many, but wish to offer more info. Rough idle assistance please

[E34M50B20]

Greetings All.

N42 in 2002 E46/3. Car only has 33k. If sitting on driveway with engine idling, the engine may sit there and behave at 900 or so. It may jump back and forth from 600 to 900. Then it may behave again then suddenly raise the revs to 1700 and back down. The rev raising never goes above 1700, in fact it cuts off dead at 1700 and reverts to either 600-900 or stable 900. The car drives perfectly. Good pull, some hesitation from 1000 onwards. (Not really sure how it should feel, I am so used to my 530i). Engine may stall after lift off at lights.

No fault codes. Have removed and cleaned MAF and throttle body with correct products. Throttle response very improved but idle fault remains unchanged. With all trims removed spraying GT45 about the pipes and hoses does nothing.

Further help would be much appreciated.

Thank you

[JayhawkM3]

I would bet my favorite milking lass that there is a vacuum leak somewhere...Upper Manifold gasket would be preferred but hard to say...keep spraying?

[MIKYZZ4]

I agree, and also suspect an intake leak. Check all rubber intake boots, and all engine vacuum hoses for cracks.
An intake smoke test, performed with a professional smoke machine, is a much more reliable way of confirming the presence, and location of intake leaks.

[Mless5]

Gut feeling says it is fuel related.

[JayhawkM3]

Did you get a chance to get into this yet?

[E34M50B20]

I never did answer this thread. The reason for the rough idle was of course an air leak. The oil filler cap is as we all know a sort of two part affair. There was a crack in it. I cleaned and superglued it and all was well. Now have a new cap assembly. Never had this trouble on my Triumph Spitfire. Saying that the M54 doesnt have the stupid cap either.

[MIKYZZ4]

Thanks for the update, better late than never, lol.