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 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
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
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
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
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.
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.
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.