Throttle Body System Overview

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 air flow is the positive crankcase ventilation (PCV) system. The combined idle air flow (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 air flow to the engine through the idle air passage and the PCV valve. The IAC valve assembly provides additional air when commanded by the 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.