System Description

PURPOSE
The basic function of the air/fuel metering system is to control air/fuel delivery to the engine. Fuel is delivered to tile 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 PCM how much oxygen is in the exhaust gas and tile PCM changes the air/fuel ratio to the engine by controlling the fuel injectors. The best mixture to minimize exhaust emissions is 14.7 to 1 which allows the catalytic converter to operate the most efficiently. Because of the constant measuring and adjusting of the air/fuel ratio, the fuel injection system is called a "Closed Loop" system.

MODES OF OPERATION
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.

Starting Mode
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) and Throttle Position (TP) sensors, and fuel control reference signal to determine the proper air/fuel ratio for starting.

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

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

Run Mode
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 O2S, and calculates the air/fuel ratio based on inputs from the TP, ECT, and Mass Air Flow (MAF) sensors

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

1. The O2S has varying voltage output showing that it is hot enough to operate properly. (This depends on temperature.)
2. The ECT sensor is above a specified temperature.
3. A specific amount of time has elapsed since starting the engine.
4. Engine speed has been greater than 800 RPM since start-up.

The specific values for the above conditions vary with different engines, and are stored in the Programmable Read Only Memory (PROM). When these conditions are met, the system enters "Closed Loop" operation. In "Closed Loop," the PCM calculates the air/fuel ratio (injector on-time) based on the signal from the O2S, allowing the air/fuel ratio to stay very close to 14.7:1.

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

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

Battery Voltage Correction Mode
When battery voltage is low, the PCM will compensate for the weak spark by:

- Increasing the amount of fuel delivered;
- Increasing the idle RPM; and
- Increasing ignition dwell time.

Fuel Cutoff Mode
No fuel is delivered by the injectors when the ignition is "OFF." This prevents dieseling. In addition, the PCM suspends fuel delivery if no reference pulses are detected (engine not running) to prevent engine flooding.

Engine Speed/Vehicle Speed Fuel Disable Mode
The PCM monitors engine speed and turns "OFF" the fuel injectors when engine speed increases above 5600 RPM. The fuel injectors are turned back "ON" when engine speed decreases below 5100 RPM.

The PCM also monitors vehicle speed and turns "OFF" the fuel injectors at a vehicle speed which varies according to axle ratio and tire size and rating.