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the car runs normal when it's off. And when I run the A/C while in park, the engine's power fluctuates up and down almost like it's going to stall. What does this sound like? Just replace the waterpump, and the car has almost 180,000 miles on it so I don't want to spend more than the car is really worth, but right now I also can't afford a new one either.

 
Rep Power: 245

Your car's computer is signaled that your AC is turned on because the compressor will add load to the engine. The computer will compensate for this by increasing the idle speed. The rough or wandering idle that you're experiencing could be from a number of things. It could also be rough because you might have a busted motor mount. The one on the lower drivers side near the crankshaft pulley is notorious for failing. A good thing to check is your IACV valve which controls your idle. They tend to clog and can be removed and cleaned out with brake cleaner (do not use card cleaner). It may also be a good idea to check your ignition timing and set your base idle.

 
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its normal, mine does that too. all the extra work/ your fans kicking on as well