[meinporsche]

Sometimes when I have the A/C on my car runs fine, then after a little while if I am stopped it will idle at about 800 then run up to 1000 or so and then fall back down. When it is doing this and I take off it seems like it is holding back (just like when idleing (sp)). Could it be that one of my fan switches is bad?

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[Greimann]

This doesn't sound like a fan switch problem. I would speculate that the idle stabilizer valve (ISV) is acting up. TheISV is a small valve that opens and closes rapidly to keep the idle around 840 RPM. If the AC is on, then the stabilizer opens up to keep the RPM up. If the AC kicks off at idle, like it does occasionally, the ISV should instantly close to account for the reduced load. If it doesn't close fast enough then the idle will shoot up when the AC compressor cuts out.



It could be related to something else too, but that is the general area to start looking.

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[meinporsche]

I have been looking on the internet. I have seen them anywhere from 150 to 400. Does that sound about right?

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[Greimann]

I think some more diagnosis is in order before you commit to replacing expensive components.



Do you have any other idle related problems? Can you simulate the idle jump by lightly engaging the clutch with the car in gear. Don't press the gas pedal, just load the engine a bit and then release. If the idle jumps like with the AC engaged, this may narrow it down.

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[rustech]

In my opinion, if it were the ISV, he would experience the symptoms during several different driving situations, not just when the A/C is on. To me, it sounds like the compressor relay is faulty. If the compressor is constantly engaging and disengaging, you would get the symptoms you describe. With the fluctuations, do you here any relay clicking from the engine compartment?

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[meinporsche]

I have a tiptronic. But, there are not other idle problems. It only does this when the A/C is on, and it only does it sometimes. Also, if the A/C button is pushed in and the fan is not on it does not do it. When I turn the fan on to blwo into the car that is when it does it. If I turn the A/C off and back sometimes it goes away. Weird. <img src="/forum/images/smilies/968/rolleyes.gif" class="smilie" alt="" />

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