Idle/Altitude Compensator (Non Comp. Controlled): Description and Operation

Bypass Air Control (BAC) Valve:

The bypass air control valve (BAC valve), located on the throttle body, is two valves in one. One valve, the idle speed control valve (ISC valve) is computer controlled (see COMPUTERIZED ENGINE CONTROLS). The other valve, the air valve, is a temperature sensitive valve that is open when cold, to allow additional measured air into the engine, bypassing the throttle valves, for cold start fast idle. As the valve is warmed by engine coolant, it closes and the computer takes over control of the idle speed, through signals to the ISC valve.