Auxiliary Air Valve (Idle Speed): Description and Operation

Idle Air Control Bypass Air Valve:

The Idle Air Control Bypass Air (IAC-BPA) consists of the Bypass Air (BPA) valve and the Idle Air Control (IAC) valve.

The BPA valve consists of a thermowax bead and valve, and functions during cold engine operation to increase idle speed. Engine coolant is directed around the thermowax, which opens and closes the valve. During cold weather operation (below 60°C [140°F]) the thermowax is contracted enough to allow the air valve to open. As the engine coolant heats, the thermowax expands. When the coolant reaches temperatures above 60°C (140°F), the thermowax expands and closes the valve.

In order to ensure idle smoothness for all operating conditions, the IAC BPA valve carefully controls the bypass air amount in addition to the bypass air flowing through the air valve. In order to maintain factory set idle speed when the engine is cold, air also flows through the IAC-BPA valve during all modes of engine operations.