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There are a few things that can make the idle fluctuate. Normally the idle will drop, then rise slightly, and then return to steady when the compressor cycles on and off, or when the cooling fans turn on and off as well. The added draw on the engine causes it to compensate for the load. A problematic cause would be a faulty idle compensator. This piece activates when the AC is turned on and raises the engine rpm to compensate for the added load of the compressor. Newer vehicles are controlled by the ECM and will adjust the IAC valve as needed. Either way you will notice a momentary change in engine RPM. Anything more than momentary such as surging will need to be checked out and most likely involves the throttle body.

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17y ago

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