Throttle Body System Overview: Notes

NOTE: This overview is for applications without electronic throttle control (ETC). For ETC applications, refer to TORQUE-BASED ELECTRONIC THROTTLE CONTROL (ETC) .

NOTE: The traditional idle air adjust procedure and the throttle return screw are no longer used on vehicles with on board diagnostics (OBD).

The throttle body system meters air to the engine during idle, part throttle, and wide open throttle (WOT) conditions. The throttle body system consists of an idle air control (IAC) valve assembly, an idle air orifice, single or dual bores with butterfly valve throttle plates, and a throttle position (TP) sensor. One other source of idle airflow is the positive crankcase ventilation (PCV) system. The combined idle airflow (from idle air orifice IAC flow and PCV flow) is measured by the MAF sensor on all applications.

During idle, the throttle body assembly provides a set amount of airflow to the engine through the idle air passage and the PCV valve. The IAC valve assembly provides additional air when commanded by the powertrain control module (PCM) to maintain the correct engine idle speed under varying conditions. The IAC valve assembly mounts directly to the intake manifold assembly in most applications. Idle speed is controlled by the PCM and cannot be adjusted.

Throttle rotation is controlled by a cam/cable linkage to slow the initial opening rate of the throttle plate. The TP sensor monitors the throttle position and provides a signal to the PCM. Some throttle body applications provide an air supply channel upstream of the throttle plate to provide fresh air to the PCV or IAC systems. Other throttle body applications provide individual vacuum taps downstream of the throttle plate for PCV return, exhaust gas recirculation (EGR), evaporative emission (EVAP), and miscellaneous control signals.