What function does an "IAC" valve perform in an engine?

The Idle Air Control (IAC) valve plays a crucial role in regulating the engine's idle speed. It is primarily responsible for adjusting the amount of air that bypasses the throttle body, enabling the engine to maintain a smooth and steady idle when the vehicle is stationary or when additional load is placed on the engine, such as when the air conditioning is turned on.

During engine operation, particularly at idle, the IAC valve allows for precise control of the air-fuel mixture that the engine requires. By increasing or decreasing the airflow, the IAC valve helps maintain the desired engine RPM despite changes in load or operating conditions. This function is vital for ensuring optimal engine performance, fuel efficiency, and minimizing emissions.

Although the other choices involve important engine functions, they do not relate directly to the specific role of the IAC valve. Regulating fuel flow is typically managed by the fuel injector and fuel pressure regulator, measuring engine temperature is the function of the coolant temperature sensor, and filtering air is handled by the air filter. Thus, the function of controlling idle speed is the primary and correct role of the IAC valve.