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Q: Why a vibration is felt when the AC is on during stops on street lights?

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When the AC is on and I make a stop on a street light, I feel a vibration that seems to come from the engine. The vibration in that situation is not steady. It comes like the waves in the ocean. it can stop for a little while and do it again. When I driving the car I don't feel the vibration. I appreciate your help.

My car has 25000 miles.
My car has an automatic transmission.

A: Hello, thanks for writing in. This vibratio...

Hello, thanks for writing in. This vibration is usually the result of low idle. When loads such as the AC system or a steering wheel turn are placed on the engine, the idle needs to rise to offset this. If it does not, then the engine will begin to run rough. Most of the time this is caused by a dirty throttle body. When the throttle becomes dirty the computer has a hard time trying to maintain an idle. It can also be caused by a worn engine mount so I check those also to try and isolate the problem. I would recommend having an expert from YourMechanic come to your home or office to confirm a diagnosis on the rough idling issue, and make or suggest the necessary repairs or replacements.

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