Engine management system - MEMS (Acceleration enrichment): Description and operation

Acceleration enrichment

When the throttle pedal is depressed the ECM receives a rising voltage from the throttle position sensor and detects a rise in manifold pressure from the manifold absolute pressure sensor. The ECM provides additional fuel by increasing the normal injector pulse width and also provides a small number of extra additional pulses on rapid throttle openings.

Over-run fuel cut-off

The ECM implements over-run fuel cut-off when the engine speed is above 1600 rev/min with engine at normal operating temperature and the throttle position sensor in the closed position, i.e. the vehicle is ’coasting’ with the throttle pedal released. The ECM indexes the idle air control valve open slightly to increase the air flow through the engine to maintain a constant manifold depression to keep emissions low.

Fuel is immediately reinstated if the throttle is opened. If the engine speed drops below 1600 rev/min on over-run, fuel is progressively reinstated.

Over-speed fuel cut-off

To prevent damage at high engine speeds the ECM will implement fuel cut-off at engine speeds above approximately 7000 rev/min; fuel being gradually reinstated as speed falls.

Ignition switch off

When the starter switch is turned off, the ECM will keep the main relay energised for approximately 30 seconds while it drives the idle air control valve to its power down position, ready for the next engine start.

The ECM monitors the engine coolant temperature for a minimum of 2 minutes after the starter switch is turned off.If the coolant temperature rises above a certain limit, the ECM switches on the radiator fans. Once the fans are running the ECM will not power down until either the coolant temperature falls below a certain limit or 8 minutes have elapsed since the starter switch was turned off. If the coolant temperature has not risen above the limit after 2 minutes, the ECM will power down.