Does your car idle rough or the speed cycle up and down. You probably have a bad idle air control valve. The only purpose of the idle air control valve is to keep the engine’s idle smooth. The engine’s computer gives the idle air control valve the feedback it needs to hold a constant rpm. The idle will be around 1000 rpm’s when not in gear and around 800 rpm’s in gear. This iac valve is on a Nissan Maxima and is similar to most idle air control valves.
The idle air control is a control system used on fuel injected cars. The idle air control or IAC valve controls the idle speed of the engine. The IAC valve is controlled by the ECM (computer). The computer gets it’s information from the rpm which is supplied by the distributor, crank or cam sensor. The IAC valve moves in and out to bleed air behind the throttle plate. This air bleeding is the same as opening the throttle plate. The idle speed can be controlled precisely the throttle plate which isn’t controlled as well. Air conditioning will drop the idle speed down and will kill the engine if the IAC doesn’t work well. The idle air control will be located on the side of the throttle body. The IAC can become sticky after time and will cause the the engine to surge, rev or stall the engine. The valve can be removed and cleaned if not too severely worn. Usually the valve needs to be replaced. Make sure the gasket looks alright. Also make sure the idle air control is secured well and doesn’t leak air, or the engine will rev up.
The idle air control is used to adjust idle speed on electronic fuel injected cars. Back before electronic fuel injection was used, carburetors would use a screw to adjust the throttle plates. The more the throttle plates were open the more air and the higher the idle would go. Fuel injected cars use a throttle body to control the air coming into the engine. The idle air control allows air to enter the engine instead of holding the plates open.