Audi S3 - Idle issue

roll_with_it

Wammer
Wammer
Nov 16, 2005
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Middlesbrough, Cleve
Very strange one this ...

Had car MOTd yesterday. No issues.

On my way home whilst at the lights, the engine started revving by itself to approx 1.5K - 2K rpm, then lowering and repeating, accompanied by a quiet but high pitched whistling sound, possibly a belt??

When driving away it goes back to normal, just happened on idle.

This morning, fine for 40 minutes driving, until I was almost at work when it happened briefly.

Spoke to an Audi specialist last night and its a new one on him.

Getting it diagnosed later today. Just wondered if anyone has an idea of what it could be??

I've been advised that nothing in the MOT could have cause such a thing (by both the MOT garage and the Audi specialist).

 

julian2002

Wammer
Wammer
Dec 15, 2005
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Lakeside Wixams
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Julian
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Vacuum leak would be my guess.

Not sure if they have an electronically controlled idle control valve but if it does the motor can go bad and cause this kind of thing.

 

diceman

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May 29, 2009
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Vacuum leak would be my guess. Not sure if they have an electronically controlled idle control valve but if it does the motor can go bad and cause this kind of thing.
My thinking as well. It could be a pipe come off for the idle speed control valve (I would expect anything form that era to have an electronic one!) or a general vacuum leak for the Manifold absolute pressure (MAP) sensor.

 

diceman

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May 29, 2009
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Positive crank case breather can become blocked with oil sludge and as such teh engine tends to breathe through the vacuum hoses which are connected to the ISC & MAP sensors, the oily sludge then makes its way into them and starts to gum them up. Normally a good clean out of the ISC & PCV & hoses sorts it all sometimes with an ECU reset to relearn ISC idle settings. ECU reset varies on vehicles but on the stuff I worked on it involved removing the battery lead!

 

karlinamillion

Wammer
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Mar 9, 2007
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Londinium
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Karl
Did the MOT guys leave it idling for ages? Quite a few cars don't like this. Sometimes a good thrash may sort it.

Does sound a bit like the aux air control valve (leans the mixture out at idle for lower emissions), whistling could be the air being drawn in at higher revs when it is normally shut.

 

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