Circuit Description

PCM controls engine speed with the Idle Air Control (IAC) valve. To increase idle speed, PCM retracts IAC pintle away from its seat, allowing more air to bypass throttle bore. To decrease idle speed, it extends IAC valve pintle towards its seat, reducing bypass air flow. Scan tool reads PCM commands to IAC valve counts. Higher counts indicate more bypass (higher air). Lower counts indicate less air is allowed to bypass (lower idle).