The basic function of the air/fuel metering system is to control air/fuel delivery to the engine. The best air/fuel mixture to minimize exhaust emissions is 14.7 to 1, which allows the catalytic converter to operate most efficiently. Fuel is delivered to the engine by individual fuel injectors mounted in the intake manifold near each intake valve.

The main control sensor is the Heated Oxygen Sensor (HO2S), located in the exhaust manifold. The HO2S tells the Powertrain Control Module (PCM) how much oxygen is in the exhaust gas and the PCM changes the air/fuel ratio to the engine by controlling the fuel injector on time. Because of the constant measuring and adjusting of the air/fuel ratio, the fuel injection system is called a Closed Loop system.

The PCM monitors voltages from several sensors to determine the engine's fuel needs. Fuel is delivered under one of several conditions called modes. All modes are controlled by the PCM and are described below.

When the ignition is first turned ON the PCM energizes the fuel pump relay for two seconds, allowing the fuel pump to build up pressure. The PCM then checks the Engine Coolant Temperature (ECT) sensor and the Throttle Position (TP) sensor. During cranking, the PCM checks the crankshaft position signal in order to determine the proper air/fuel ratio for starting.

The PCM controls the amount of fuel delivered in the starting mode by changing how long the fuel injectors are energized. This is done by pulsing the fuel injectors for very short times.

If the engine floods, clear it by pushing the accelerator pedal down all the way and then crank the engine. The PCM then de-energizes the fuel injectors. The PCM holds the fuel injectors de-energized as long as the throttle remains above 80% and the engine speed is below 600-800 RPM. If the throttle position becomes less than 80%, the PCM again begins to pulse the fuel injectors ON and OFF, allowing fuel into the cylinders.

The run mode has two conditions called Open Loop and Closed Loop.

When the engine is first started and engine speed is above 400 RPM, the system is in Open Loop operation. In Open Loop the PCM ignores the signal from the Heated Oxygen Sensor (HO2S), and calculates the air/fuel ratio based on inputs from the TP, ECT, and MAF sensors.

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

1. The HO2S has a varying voltage output showing that it is hot enough to operate properly (this depends on temperature).
2. The ECT has reached a specified temperature.
3. A specific amount of time has elapsed since starting the engine.

The specific values for the above conditions vary with different engines. When these conditions are met, the system enters Closed Loop operation. In Closed Loop the PCM changes fuel injector on-time based on the signal from the HO2S. This allows the air/fuel ratio to stay very close to 14.7:1.

The PCM provides extra fuel when it detects a rapid increase in throttle position and air flow.

The PCM reduces the amount of fuel injected when it detects a decrease in throttle position and air flow. When deceleration is very fast, the PCM may cut off fuel completely for short periods.

When battery voltage is low, the PCM compensates for the weak spark in the following ways:

The PCM has the ability to shut OFF the fuel injectors completely or selectively when certain conditions are met. This fuel shut off mode allows the powertrain to protect itself from damage and also improve its driveability.

The PCM will disable the injectors under the following conditions:

Engine RPM Fuel Cut Off Ranges

The PCM constantly monitors engine operation and estimates conditions that could result in high catalytic converter temperatures. If the PCM determines the catalytic converter may overheat, it causes the system to return to Open Loop operation and enriches the fuel mixture.