Q: At light or slowing down, car has irregular idling rev

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There is an irregular idling rev when I am stopped at a red light or when I slow down. Sometimes the engine will independently go off. The sound of the engine fluctuates from low to high and also to the level of almost going off. I recently had the car serviced by a local MB dealer, but they are not able to find the problem. In your professional expertise, what is causing this and how can I get it corrected?

Hello. Most of the time when this issue occurs, it is due to a faulty idle air control valve. This valve is responsible for letting some air from the air cleaner bypass the throttle body so that the engine can breathe without the accelerator pedal being depressed.

The idle air control valve also controls the air fuel mixture at idle and if the valve has a fault, this could cause the engine revving to be inconsistent. I would recommend having another mechanic check your idle air control valve for excessive carbon build up or faulty electrical components. If either one of these are found, the idle air control valve should be replaced with a new one. If you would like to have this diagnosed, a certified professional from YourMechanic can come to your car’s location to pinpoint the cause of the rough idle issue.

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