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Q: Idle fluctuates and then stalls.

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Hello!

My car's battery was dying toward the end of last year, so I had the battery replaced at a mechanic's shop. Then, less than a month later, the battery completely died again. I bought a new battery, set up an appointment through YourMechanic, and someone came out to replace my alternator and battery and do a visual inspection of everything else, and my car ran normally for about a month of very infrequent driving until this weekend. Now, all of the sudden, the car has an extremely low, rough idle when stopped, and there's a loud screeching noise coming from the engine area that's loudest when idling and a bit quieter when gas is applied. The visual inspection said that most of my car (including engine, belts, etc) was "excellent" so I'm not sure what the problem could be. Any help would be appreciated!

My car has 66000 miles.
My car has an automatic transmission.

Hi There, It sounds like you may have a belt that needs to be tightened as this will cause a screeching noise when a belt is loose and the engine is running at any pace. This can be fixed usually by tightening the tensioner pulley. The low idle may be caused by a dirty or faulty idle air control valve. The idle air control valve is a small valve on the engine’s intake system that reads the air intake as it comes into the motor. This is controlled by the car’s ECM which uses this information to make adjustments to the air/fuel ratio depending on various inputs such as outside air temperature, intake air temperature, load and various other things. As you accelerate, your car’s engine is receiving a much higher dose of fuel than when at idle and conversely, when you let off the gas pedal, there is a sudden change in this fuel supply as a result of your foot letting off the pedal. When this happens, the job of the idle air control valve is to bring this deceleration down to a slow and smooth idle rather than suddenly cutting off the fuel supply causing the motor to die. When the idle air control valve is not working properly, this cause a disruption in this process resulting in the engine not being able to idle properly. I would recommend having a professional come to your location to diagnose and inspect your vehicle.

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