Idle Speed Control

There is a narrow range of ignition points for all engine conditions which give an acceptable compromise between power output and emission control.

The ignition mapping contained within the ECM memory keeps the ignition timing within this narrow band. The ignition timing is used to control engine idle speed in conjunction with the IACV stepper motor.

As the MEMS 3 system does not have a knock sensor, ignition timing advance is controlled using different mapping at high engine and intake air temperatures in order to avoid detonation (pinking).

Idle Speed Control

The ECM regulates the engine speed at idling. The ECM uses two methods of idle speed control: l Ignition timing adjustment l IACV stepper motor.

When the engine idle speed fluctuates and there are no additional loads on the engine, the ECM will vary the ignition timing and the IACV to regulate the idle speed. This allows very rapid correction of out of tolerance idle speeds. When an additional load is placed on the engine, such as when the power steering is turned on full lock, the ECM uses the IACV stepper motor to control the idle speed to specification.

The idle speed is determined from the CKP sensor, but there are also inputs to the ECM from the following: l Alternator l A/C system Cooling fan status l A/C compressor on/off status.

If the ECM receives information from the above inputs that an extra load is being placed on the engine, it can immediately compensate and avoid engine poor idle or stall conditions.

The IACV stepper motor is mounted on the inlet manifold and controls a throttle valve air bypass port. To increase the idle speed, the stepper motor allows more air to bypass the throttle and enter the cylinders. To decrease the idle speed, the stepper motor allows less air to enter the cylinders.

The stepper motor is a bi-polar type which consists of two windings controlled by Pulse Width Modulated (PWM) signals from the ECM.

The position of the stepper motor is always referenced on power down of the ECM, this may take from three to five seconds. The stepper motor is also used to reduce manifold vacuum during deceleration to control emissions.

Evaporative Emission (EVAP) Control System

The hydrocarbon vapour given off by petrol is harmful to health and the environment. Legislation limits the amount of hydrocarbons (HC) which can be emitted to atmosphere by a motor vehicle.

To meet the limits imposed, a charcoal canister is fitted to the fuel system to absorb fuel vapour from the tank when the vehicle is not in use. The charcoal canister has a finite capacity and therefore needs to be purged when the vehicle is driven.

This is achieved by drawing the fuel vapours out of the canister and into the cylinders of the engine. The HC vapours are converted into carbon dioxide (CO2) and water (H2O) by the combustion process and catalytic converter.