Q: Engine idle fluctuates

There are a couple of reasons your engine idle may run high such as a vacuum leak or a dirty or faulty idle air control valve. 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. 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.

A vacuum leak can be one of the most frustrating and difficult problems to diagnose. High idle speed is a common sign and stalling is another. Confirmation of an engine vacuum leak can be a very loud hissing sound coming from the engine area. 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 car’s running condition to determine what may be causing the high idle.