* Throttle Position Sensor (TPS): This sensor tells the PCM (Powertrain Control Module) the position of the throttle plate. At idle, the TPS indicates a nearly closed throttle. The PCM uses this information, among other things, to maintain the correct idle speed.
* Powertrain Control Module (PCM) / Engine Control Module (ECM): The PCM/ECM is the "brain" of the engine. It receives input from various sensors (TPS, IAC, MAP, etc.) and uses this information to adjust the idle speed to maintain a consistent engine RPM. It does this by controlling the amount of air and fuel entering the engine.
* Idle Air Control (IAC) Valve: This valve regulates the amount of air bypassing the throttle plate. At idle, when the throttle plate is nearly closed, the IAC valve opens to allow the necessary amount of air for the engine to run smoothly. The PCM controls the IAC valve to adjust the idle speed based on various factors.
* Various other sensors: The PCM also considers information from other sensors to fine-tune the idle, including:
* Mass Airflow Sensor (MAF): Measures the amount of air entering the engine.
* Manifold Absolute Pressure (MAP) Sensor: Measures the pressure in the intake manifold.
* Coolant Temperature Sensor: Influences idle speed, often raising it when the engine is cold to help it warm up faster.
* Vehicle Speed Sensor (VSS): Helps the PCM determine if the vehicle is stationary or moving, influencing idle characteristics (e.g., slightly higher idle in Park or Neutral).
In short, while the IAC valve directly controls the air flow at idle, the PCM is the overall master, using input from many sensors to determine the ideal idle speed and adjust the IAC accordingly. A problem with any of these components, especially the PCM or IAC valve, can lead to an erratic or incorrect idle speed.
