BMW 3-Series E46 (1998 - 2005) - (98-05) 318i - fluctuating revs

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Fri 18 Jan 2008 08:52
(98-05) 318i - fluctuating revs - pswiz
All,

Recently bought a BMW 318i with 46k miles.

Its been fine but recently noticed when it warms up it seems to alternate between 1000 revs and 1500 revs. This is not very noticeable when driving but in traffic its quite noticeable.

Petrol economy guage also seems to fluctuate when this happens

When I get home if I leave the car to cool for an hour and then re-start it seems to idle fine.

Any ideas ?

{header amended to make less vague}

Edited by Dynamic Dave on 18/01/2008 at 10:05

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Fri 18 Jan 2008 11:48
(98-05) 318i - fluctuating revs - Screwloose

Could well be an air leak into the inlet manifold. Breather hoses and split breather-valve diaphragms are quite common.
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Mon 21 Jan 2008 09:11
(98-05) 318i - fluctuating revs - Car hobbiest
I agree with the last respondent....worth a check. I have a 2003 Merc C180k which developed a similar problem, the cause of which was a leaking vacuum pipe in a difficult to see, inaccessable place. Get a can of Q20 or similar spray (flammable) and spray it in likely areas around the engine when the problem is occurring....this quickly shows up trouble spots in the induction side....the engine will either slow down or further increase revs. Also check the vacuum line to the brake booster. Good luck !
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