In throttle body and port EFI systems, engine idle speed is controlled by bypassing a certain amount of airflow past the throttle valve in the throttle body housing. Two types of air by-pass systems are used, auxiliary air valves and idle air control (IAC) valves. IAC valve systems are more common.

IAC Valves

The IAC system consists of an electrically controlled stepper motor or actuator that positions the IAC valve in the air by-pass channel around the throttle valve. The IAC valve is part of the throttle body casting. The control computer (ECU) calculates the amount of air needed for smooth idling based on input data such as coolant temperature, engine load, engine speed. and battery voltage. It then signals the actuator to extend or retract the idle air control valve in the air by-pass channel.

If the engine speed is lower than desired, the ECU activates the motor to retract the IAC valve. This opens the channel and diverts more air around the throttle valve. If engine speed is higher than desired, the valve is extended and the by-pass channel is made smaller. Air supply to the engine is reduced and engine speed falls.

During cold starts idle speed can be as high as 2100 rpm to quickly raise the temperature of the catalytic converter for proper control of exhaust emissions. Idle speed that is attained after a cold start is controlled by the ECU. The ECU maintains idle speed for approximately 40 to 50 seconds even if the driver attempts to alter it by kicking the accelerator. After this preprogrammed time interval, depressing the accelerator pedal rotates the throttle position sensor (TPS) and signals the ECU to reduce idle speed.

Auxiliary Air Valve

The major difference between the IAC valve and an auxiliary air valve is that the auxiliary air valve is not controlled by the ECU. But like the IAC system, the auxiliary air valve provides additional air during cold engine starts and warm-up.

The auxiliary air valve consists of an air by-pass channel or hose around the throttle valve, a movable plate or disc, and a heat-sensitive bimetal strip. When the plate opens the channel, extra air bypass the throttle. Opening is controlled by the bimetal strip. As the bimetal heats up, it bends to rotate the movable plate, gradually blocking the opening. When the device is closed, there is no auxiliary airflow.

The bimetal strip is warmed by an electric heating element powered from the run circuit of the ignition switch. This bimetal element is not a switch, but a strip that moves the movable plate directly. The auxiliary air device is independent of the cold start injector. It is not controlled by the ECU but is continuously powered when the ignition key is set to the run position.

When the engine is cold, the passage opens for extra air when the engine starts. When the engine is running and still cold, the passage is open but the heater begins operating to close it gradually. If the engine is warm at start up, the passage is closed and normal air is delivered for idle.