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Q: RPM fluctuates

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When I start my truck it's loud like a fan is on before it settles down. No matter if its warmed up or not. When I turn my heater on the RPMs fluctuate as high as 1200 and back to about 500. Just does it back to back non stop as long as heater is on. I'm also able to notice the difference when driving at low speeds. When it fluctuates the car speeds up. When I turn the heater completely off it doesn't do any of that. No fluctuations even when in idle.

My car has 177000 miles.
My car has an automatic transmission.

A: With the age and mileage of your vehicle, i...

With the age and mileage of your vehicle, it would probably benefit from a throttle body service including cleaning the idle air control valve. Over time, oil and carbon build up on the throttle blade so it does not sit perfectly flush in the throttle body bore. This effects the throttle position readings to the ecu.

Also due to carbon build up, the idle air control motor/valve passages will get clogged up, reducing the size of the air passage as well as slowing it’s movement. The idle air valve will let more air into the engine at idle when the heat-A/C is turned on because of the added drag on the engine.

If the idle air valve is clogged with carbon or not moving properly because of it, you can get a low/shaky idle, surging and even stalling when the heat or A/C are turned on.

If you’d like to get this fixed, consider YourMechanic, as one of our mobile technicians can clean your throttle body and service your idle control valve if necessary.

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