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Old 12-20-2002, 06:50 PM
sunil190e-1.8's Avatar
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Join Date: Jun 2002
Location: uk
Posts: 236
mystery of oscillating idle speed when warm-c200 merc

the car is a c200 uk spec.-1995

the car idle speed oscillates between 500 to 1500 rpm only when car has warmed up.

what is causing this?
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Old 12-20-2002, 06:52 PM
Join Date: Dec 2001
Location: Southern California, U.S.A.
Posts: 8,508
My car did the exact same thing. Idle oscillated up and down only when the car warmed up.

It was a vacuum leak at the intake manifold seal.

A new seal solved the problem.

Any vacuum leak, though, is false air, and causes the mass air sensor to hunt for the proper idle. It can't find it (because of the vacuum leak) and the idle hunts up and down.
Paul S.

2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".
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Old 12-20-2002, 07:05 PM
sunil190e-1.8's Avatar
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Join Date: Jun 2002
Location: uk
Posts: 236
that was like flash of lightening quick reply!!

now that you mention the intake manifold vacuum leak-i remember diagnosing a similar fault on my brother's bmw 525i -it used to hunt at idle and i diagnosed the site of the leak by listening to the engine-there was a definite hissing of air being sucked in from the intake manifold where part of the seal had fallen out!

thanks for that.
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Old 12-20-2002, 09:58 PM
blackmercedes's Avatar
Just a guy
Join Date: Dec 2000
Location: St. Albert, Alberta, Canada
Posts: 3,492
It is also a sign of a failing MAS. Check for leaks first before spending a few hundred bucks though...
John Shellenberg
1998 C230 "Black Betty" 240K
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