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Q: Seems to be an occasional misfire that is somehow related to the AC compressor

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If the AC is off when I start the car, I have no problems. Sometimes right after starting the car if I switch the compressor on it will cause the car to run rough for a bit and then it just stalls, but will start up again without a hitch, maybe running rough for a hundred feet or so. I have recently replaced the compressor myself and it has been in working order for several weeks. If the AC is not on the typically I do not have this problem. The problem feels like a misfire but the odd part is that it is related to whether the AC is on or not. Other than that the car runs great. With the AC on when the car has been running for a while it does not kill it but rather causes it to idle a bit rough. Any advice would be wonderful. Also if there is a way to diagnose a coil pack without going to a shop this would be wonderful. I have some tools but not all the fancy ones. Thank you in advance! -Alex

My car has 171000 miles.
My car has an automatic transmission.

A: This is normal when the AC is on. The air c...

This is normal when the AC is on. The air conditioning compressor requires torque from the belt system on the engine to power it. When the AC is off, the compressor clutch does not activate, meaning the AC is not straining the engine. When the AC is on, the compressor clutch activates, causing the drive belt on the engine to turn the compressor. The Idle Control Actuator is what controls the engine idle. When the AC kicks in, the engine will drop some RPM, and the actuator opens to allow more air into the engine. When you are at speed, the throttle is open from the accelerator pedal under your foot. the Idle control actuator is not active at this time. What you describe is normal.

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