Q: Car idle really low, below 250 rpm.

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October 2017 my car doesn't start, so I suspect my alternator or battery is bad. fast forward to Jan 2018, I replace my battery and my car starts. But immediately I notice that it idles extremely low. and stalls out on me. I get a diagnostic report on it and it shows that my Air to fuel sensor is bad. So I replace that. Car still idles low. My mechanic noticed I had a leak in my engine, and fixes the gasket leak. Car still idles low. He then checks the posioning of my throttle, and cleans it and the sensor. Car now will start and idle at 750 rpm, drop down to 500 rpm, and as soon as I start driving drops back dow to under 250 rpm, then stalls. What in the world is wrong with this car please help. Im thinking something related to My IAC valve, but i dont want to spend 275 on that and still have idle problems. I really need concrete assistance please.

My car has 182000 miles.
My car has an automatic transmission.

It sounds like you may have a dirty or failing idle air control valve causing your engine idle to fluctuate. 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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