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MY99, 996 six speed.

With A/C switched off engine is a bit rough idle and noticable at inside.

With A/C switched on perfect idle (and slightly higher as without A/C)

No error codes shown on Durametric.

I cleaned the ICV and throttle body but no improvement. Any ideas befor buying a new ICV?

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When you say rough, is the idle wandering? Or is the engine just vibrating a little more than it does at higher RPMs. Does an empty passenger seat shake violently or just a little bit? Make sure the oil is topped off.

I'm not 100% on this and would wait for others to chime in, but if the ICV were bad you would most likely have a wandering idle - if it stays steady I doubt it's the valve. You could also try cleaning the ICV. if the engine RPM raises when you turn on the A/C and stays steady then it is probably doing it's job.

My humble opinon is that slight idle vibration is farily normal for these cars (and have seen threads describing the same thing which people say is normal), but there could be a littony of reasons for engine vibration, if there truly is a problem. I doubt you have these problems though.

Vaccum leak


DMF - does it change when you press the clutch in?

IMS bearing going out causing balance shaft to wobble - doubtful though since I've heard they just "pop"

Worn engine mount

Worn transmisson mount

Worn variocam pads? (not sure about this one, perhaps if severe enough?)

Scored cylinder causing one or more to be out of spec

Worn valve guides causing one or more cylinder to be out of spec

The above problems or others causing low compression causing one or more cylinder to be out of spec

Cracked coil pack or bad spark in a cylinder - likely to throw codes.

A lot of these things will throw codes "if they are bad enough".

Edited by logray
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You're assumtion of my idle problem is correct. It is more a bad vibration, RPM is not bouncing noticably at all.

However passenger seat is shaking a bit without A/C on (750RPM) With A/C on it is on the RPM is at 800.

Reading your link it seems to be normal. Anyways I do suspect engine mounts, will try to replace those first.

Thanks, Rob

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