Fuel system - MPi (page 3 of 7)

Throttle position sensor

The throttle position (TP) sensor is a potentiometer attached to the throttle housing and is directly coupled to the throttle disc. The TP sensor is non-adjustable. Closed throttle is detected by the TP sensor which initiates idle speed control via the idle air control valve.

The ECM supplies the TP sensor with a 5 volt supply and an earth path. The TP sensor returns a signal proportional to throttle disc position.

Throttle disc movement causes voltage across the TP sensor to vary. The ECM calculates the rate of change of the voltage signal in positive (acceleration) or negative (deceleration) directions. From this the ECM can determine the required engine speed, rate of acceleration or rate of deceleration and apply acceleration enrichment, deceleration fuel metering or over-run fuel cut-off.

Idle air control valve

The idle air control (IAC) valve is mounted on the inlet manifold and controlled by the ECM. The IAC valve opens a pintle valve situated in an air passage in the throttle housing, allowing air to bypass the throttle disc and flow straight into the inlet manifold.

By changing the amount the IAC valve is open the ECM can control engine idle speed and cold start air flow requirements by adjusting the flow of air in the passage.

During cold starting the ECM indexes the IAC valve open slightly to provide a level of fast idle, dependent on engine coolant temperature. As the engine warms, fast idle is gradually decreased until normal operating temperature is reached. The position of the IAC valve can be checked using TestBook and should be within the range of 20 to 40 steps when the engine is running. If it is identified as being outside this range it can be adjusted to within range using TestBook. This ensures that the IAC valve is at the optimum position within its range for providing further movement to compensate for changes in engine load or temperature in accordance with signals from the ECM.

NOTE: The position of the throttle disc is preset during manufacture and the throttle position setting screw MUST NOT be adjusted.