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Q: Low RPM when on gear causes Vibration when at stopped.

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My Ford Fusion has been having this issues lately. I thought it could be the transmission mounts but it is not it. I replaced those but the vibration is still there. Upon more diagnosis of myself, even though I do not know much of cars, I did realize that my car RPM is lower than usual. Isn't it suppose to be 750 rpm when on park or on gear right? well when its on park, its 750 steady but there are sometimes that it goes down to 500 which shakes the car and goes back up. Which is very unusual. Then when on gear, the RPM goes to 600 which cause more steady vibration to the car. I am not sure if its whats causing it, and I have renting a car for a while and their RPM isnt as low as my current car now. Are there fixes to this? Please tell me in details on how to fix this problem.

My car has 154000 miles.
My car has an automatic transmission.

Hi There, It sounds like you may have a dirty or failing idle air control valve. The idle air control valve monitors the air intake as it is mixed with fuel prior to being injected into the engine at low speeds and at idle. This valve is controlled by the vehicle’s computer and 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 while adjusting the air/fuel ratio constantly to allow this to happen smoothly. 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. I would recommend having an expert from YourMechanic come to your location to diagnose and replace your idle air control valve.

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