What makes the idle air control valve go in and out?

The Idle Air Control (IAC) valve moves in and out (or, more accurately, its internal valve plate moves) in response to signals from the Engine Control Unit (ECU). The ECU controls this movement to regulate the amount of air bypassing the throttle plate when the engine is idling.

Here's the breakdown:

* Engine Speed: The ECU constantly monitors the engine's RPM. If the RPM drops too low, the ECU signals the IAC valve to open further, allowing more air into the engine to increase the idle speed. Conversely, if the RPM is too high, the ECU signals the IAC valve to close, reducing the amount of air and lowering the idle speed.

* Engine Temperature: The ECU considers engine temperature. A cold engine typically requires a higher idle speed to warm up faster. The IAC valve will open wider during cold starts to achieve this higher idle.

* Other Factors: The ECU might also consider other factors such as air pressure, coolant temperature, and load on the engine when adjusting the IAC valve's position. This ensures smooth and stable idling under various conditions.

In short, it's not a self-actuating device. The ECU's commands based on various engine parameters are what causes the IAC valve to move its internal components, controlling the airflow to maintain the correct idle speed.