Auxiliary Air Valve (Idle Speed): Description and Operation

Fig. 3 Bypass air control valve:

The bypass air control (BAC) valve is located on the throttle body and controls idle smoothness by regulating the throttle plate bypass air, Fig. 3. This valve consists of the air bypass valve which functions only during cold engine conditions and the idle speed control solenoid valve which works throughout the entire engine speed range. The air bypass valve functions by sensing the engine coolant temperature supplied to the valve while the idle speed control solenoid is controlled by an ECA signal.
The bypass air control valve allows air to pass around the throttle plate to control, cold engine fast idle, no touch start, dashpot, hot engine idle speed and engine idle load correction.