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Engine Reving Up on 2004 Pontiac Grand Am

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When I turn on the A/C and the compressor engages, the engine will over rev to about 2000rpm - it is normally around 900rpm. When the a/c is turned off, the engine will go back to around 750rpm
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Engine timing usally gets advanced when the AC is turned on so in that sense the engine may run faster.
I suspect you have a stuck Idle Air Control Valve,it is what controls normal Idle speed.
You can try to remove it and spray with WD40, they build up carbon or get cooked from engine heat,,,,