Throttle Body: Description and Operation

THROTTLE BODY AND IDLE AIR CONTROL SYSTEM

Throttle Body Assembly:

The throttle body assembly controls intake air quantities. It consists of:

- Aluminum housing
- Throttle plate
- Throttle Position (TP) sensor
- Idle Air Control Bypass Air (IAC BPA) valve

The idle air control system supplies small quantities of air during engine idle by bypassing air past the throttle body plate. This system consists of the IAC BPA valve made up of the Idle Air Control (IAC) valve and Bypass Air (BPA) valve. The TP sensor is mounted on the LH side of the throttle body. It detects the throttle body plate position and provides the Powertrain Control Module (PCM) with that information.

Idle Control:

During idle, the throttle body plate is almost fully closed. It is therefore necessary for the idle air control system to supply air for combustion during idle. An idle speed adjusting screw is fitted in the IAC BPA to adjust bypass airflow (for idle speed adjustment). During off-idle operation, the throttle body plate position controls the amount of air supplied to the engine. The PCM increases the fuel injection output in proportion to the increased airflow to achieve the appropriate air/fuel ratio.

The IAC BPA valve consists of the BPA valve which functions only during cold engine conditions and the IAC valve, which works throughout the entire engine temperature range. The BPA valve increases idle speed during cold conditions to quickly warm up the engine. Valve operation is controlled by thermowax. During cold engine coolant conditions, the thermowax contracts and the air valve opens. As the engine coolant warms up, the thermowax expands and the valve is closed. Consequently, the amount of air that passes through the valve gradually decreases and the engine speed gradually drops to normal curb idle speed.

To be sure of idle smoothness during all operating conditions, the IAC valve carefully controls the bypass air amount. When the engine is cold, bypass air flows through the BPA valve and through the IAC valve, in order to maintain the factory preset idle speed. During normal engine operation, only the IAC valve controls the air, in order to maintain the factory preset idle speed.