Run Mode

Fuel Metering System:

When the engine is first started and rpm is above 400, the system goes into "Open Loop" operation. In "Open Loop," the Electronic Control Module (ECM) ignores the signal from the Oxygen sensor (O2) sensor, and calculates the air/fuel ratio based on inputs from the coolant temperature and Manifold Absolute Temperature (MAT) sensors.

The system stays in "Open Loop" until the following conditions are met:

1. The O2 sensor has varying voltage output, showing that it is hot enough to operate properly. (This depends on temperature)

2. The coolant temperature sensor is above a specified temperature.

3. A specific amount of time has elapsed after starting the engine.

The specific values for the above conditions vary with different engines, and are stored in the memory calibrator (Mem-Cal) portion of the ECM. When these values are met, the system goes into "Closed Loop" operation. In "Closed Loop," the ECM calculates the air/fuel ratio (injector "ON"-time) based on the signal from the O2 sensor. This allows the air/fuel ratio to stay very close to 14.7:1.