The basic function of the air/fuel metering system is to control the air/fuel delivery to the engine. The best air/fuel mixture to minimize exhaust emissions is 14.7:1, which allows the cleanest burn for internal combustion engines. 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 powertrain control module (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 2 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 and camshaft position signals in order to determine the proper injector synchronization

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

The run mode has 2 conditions called Open Loop and Closed Loop.

When the engine is first started and engine speed is more than a predetermined RPM, the system begins 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 ECT sensor, the MAF sensor, the MAP sensor, and the TP sensor.

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

1. Both HO2S have a varying voltage output, showing that the sensors are hot enough to operate properly.
2. The ECT is more than 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, and are stored in the electrically erasable programmable read-only memory (EEPROM). The system begins Closed Loop operation after reaching these values. In Closed Loop, the PCM calculates the air/fuel ratio based on the signal from various sensors, but mainly the HO2S. This allows the air/fuel ratio to stay very close to the 14.7:1 mixture.

When the accelerator is depressed, air flow into the cylinders increases rapidly and the PCM increases the pulse width to the injectors to increase the fuel flow. The PCM determines the amount of fuel required based on the throttle position, the engine coolant temperature, the manifold air pressure, the mass air flow, and the engine speed. The system enters Open Loop.

Indicates that the PCM has detected conditions appropriate to operate in Deceleration Fuel mode. The PCM will command the Deceleration Fuel mode when a sudden decrease in the throttle position has been detected while the vehicle is traveling at more than 25 mph. While in Deceleration Fuel mode, the PCM will decrease the amount of fuel delivered by entering Open Loop and decreasing the injector pulse width.

When battery voltage is low, the PCM will compensate for the weak spark by performing the following operations:

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 to improve driveability.

The PCM will disable the injectors under the following conditions:

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, the PCM causes the system to return to Open Loop operation and enriches the fuel mixture.