Idle speed control (Engine management system - V8)

Idle speed control

The ECM regulates the engine speed at idling. The ECM uses the idle air control valve (IACV) to compensate for the idle speed drop that occurs when the engine is placed under greater load than usual. When the throttle is in the rest position i.e. it has not been pressed, the majority of intake air that the engine consumes comes from the idle air control valve.

IACV control idle speed Conditions in which the ECM operates the IACV control idle speed is as follows: l If any automatic transmission gears other than P or N are selected. l If air conditioning is switched on. l If cooling fans are switched on. l Any electrical loads activated by the driver.

Function The idle air control valve utilises two coils that use opposing pulse width modulated (PWM) signals to control the position of a rotary valve. If one of the circuits that supplies the PWM signal fails, the ECM closes down the remaining signal preventing the idle air control valve from working at its maximum/ minimum setting. If this should occur, the idle air control valve assumes a default idle position at which the engine idle speed is raised to 1200 rev/min with no load placed on the engine.

Evaporative emission control

Due to increasing legislation, all new vehicles must be able to limit evaporative emissions (fuel vapour) from the fuel tank.

The ECM controls the emission control system using the following components: l EVAP canister. l Purge valve. l Canister vent solenoid (CVS) valve – (NAS vehicles with vacuum type EVAP system leak detection capability only) l Fuel tank pressure sensor – (NAS vehicles with vacuum type EVAP system leak detection capability only) l Fuel leak detection pump – (NAS vehicles with positive pressure type EVAP system leak detection capability only) l Interconnecting pipe work.

Refer to Emissions section for operating conditions of evaporative emission systems. EMISSION CONTROL - V8, DESCRIPTION AND OPERATION, Evaporative emission control operation.

On-Board Diagnostics (OBD) - North American Specification vehicles only

The ECM monitors performance of the engine for misfires, catalyst efficiency, exhaust leaks and evaporative control loss. If a fault occurs, the ECM stores the relevant fault code and warns the driver of component failure by illuminating the Malfunction Indicator Light in the instrument pack.

On vehicles fitted with automatic gearbox, the ECM combines with the Electronic Automatic Transmission (EAT) ECU to provide the OBD strategy.

Conditions If the OBD function of the ECM flags a fault during its operation, it falls into one of the following categories: l min = minimum value of the signal exceeded. l max = maximum value of the signal exceeded. l signal = signal not present. l plaus = an implausible condition has been diagnosed.