Q: Will not idle correctly.

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I have a brand new engine, the truck set up for over a year. Now it will not idle correctly. Some times fine, most of the time not. Throttle body?

My car has 165000 miles.
My car has an automatic transmission.

It sounds like you may have a fuel supply issue. I would recommend starting with checking fuel pressure in the system to be sure that the fuel pump and the fuel pressure regulator are both working properly and getting the correct amount of fuel to the injectors. You may also want to check the idle air control valve to be sure it is not dirty and has proper connectivity. 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.

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