IAC

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.

 

It uses a spring loaded tapered plunger that backs out of a hole to allow air behind the throttle plate, speeding the engine up. This is far more precise way to control idle. Idle air control or IAC can also control idle when the air conditioning is being used. In the past a separate solenoid was used to speed up the idle when ac was used. If there is a problem with the IAC air valve the car will usually surge up and down and RPM’s will not steady out. The vehicle may also idle very high or too low if the IAC is stuck in one position. The idle air control will be located on the throttle body and will have an electronic plug running into it. If you remove the IAC you can view the passage where the air will pass through. If the air passage is plugged, carefully clean it out with carb cleaner and make sure that the valve itself will seat and seal itself. If there is a leak on the seat or the gasket area the vehicle will not run correctly. Eventually the valve will wear out and need to be replaced. The other half of the valve is electronic. If it doesn’t move smoothly or is sticking it may need to be replaced. The ECU or ECM controls the movement of the IAC by the feedback given to it from the crank/cam or RPM sensor. This is the most accurate way to control idle speed.