Q: Q: High idle problem.

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The car seems to sound rough when I come to a stop. And all my lights are on in the inside (check engine light tracking lights and abs lights) it all started when I went out of town for a week and my battery was dead and when I jumped it off all lights came on before the trip I had a thermostat water pump and radiator replaced the car still seemed to run hot occasionally until I replaced the radiator cap now it doesn't run hot but it idles high when I come to a complete stop and I'm thinking my o2 sensors have went bad. Haven't gotten a tune up in about 12mths

My car has 191633 miles.
My car has an automatic transmission.

High engine idle can result from things like vacuum leaks, a dirty or faulty idle air control valve or potentially an improperly adjusted throttle stop. The idle air control valve reads the air intake as it is mixed with fuel prior to being injected into the engine at low speeds and at idle. Since this valve is controlled by the vehicle’s computer, it will adjust idle speed based upon other measurements such as engine temperature, intake air temperature and electrical system load or voltage. This is also an important function when starting the motor as it allows the motor to run and idle on it’s own once the motor fires. When you accelerate, the engine RPM increases, and as you let off the gas, the RPM slowly returns to the normal idling speed with the help of the idle air control valve making the transition from a higher RPM back down to idle speed. When the engine RPM drops below the normal range of about ~800 RPM, this often times will cause the engine to stall indicating a dirty or faulty idle air control valve. When a vacuum leak is present in an engine, this causes an excessive imbalance in the air/fuel ratio which creates a loss of power and causes the motor to run at a much higher RPM as a result of the increased oxygen present in the fuel charge being injected into the motor. I would recommend having an expert from YourMechanic come to your location to diagnose your vacuum leak as these can be very difficult and time consuming to trace.

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