Engine Control Module: Description and Operation

STARTING MODE

When ignition is first turned On, the ECM energizes the fuel pump for two seconds and pressure is built up at the injector unit. The ECM then checks coolant temperature sensor, TPS and crank signal and determines proper air/fuel mixture for starting.

CLEAR FLOOD MODE

A flooded engine should be cleared by fully depressing the accelerator pedal, which causes the ECM to pulse injectors at a rate of 25.5:1. The ECM will maintain this injector rate as long as throttle is wide open and engine RPM is less than 600. When throttle position becomes less than 80%, the ECM returns to the ``starting'' mode.

RUN MODE

The ``run'' mode consists of two separate operating conditions; open loop and closed loop. The system goes into open loop operation when the engine is first started and RPM is above 400. In open loop operation, the ECM disregards oxygen sensor signal and determines air/fuel mixture primarily on inputs from coolant and MAP sensors. The system will remain in open loop operation until the oxygen sensor has varying voltage output and is hot enough to operate, or the coolant sensor is above 158° F and 60 seconds has elapsed after starting engine. When these conditions are met, the system switches to closed loop operation.
In closed loop operation, the ECM calculates the air/fuel mixture based on signals from the oxygen sensor. This allows the air/fuel mixture to stay close to the optimum 14.7:1 ratio.

ACCELERATION MODE

In the ``acceleration'' mode, the ECM senses rapid changes in throttle position and manifold pressure and provides a corresponding amount of added fuel.

DECELERATION MODE

During deceleration, fuel remaining in intake manifold can cause excessive emissions and backfiring. To compensate for this, the ECM senses changes in throttle position and manifold pressure and reduces fuel delivery. When decelerating very rapidly, the ECM will cut off the fuel supply for short periods.

BATTERY VOLTAGE CORRECTION MODE

When battery is low, the ECM compensates to maintain acceptable system performance by increasing injector ``on'' time, idle RPM and ignition dwell time.