The fuel system is controlled by the VCM located in the engine compartment. The VCM is the control center of the system.

The basic function of the fuel system is to control fuel delivery to the engine under all operating conditions. The following two types of fuel injection systems deliver the fuel to the engine:

The main fuel control sensor is the Heated Oxygen Sensor (HO2S). The HO2S is located in the exhaust manifold. The HO2S monitors the amount of oxygen in the exhaust gas and emits an analog voltage signal which is detected by the control module. The control module changes the air to fuel ratio to the engine by controlling the fuel injectors. Efficient catalytic converter operation requires a 14.7:1 air to fuel ratio.

Several other important engine operation parameters include the following items:

These parameters determine the mode of engine operation.

The following are the 3 separate classifications of fuel systems:

The control module monitors the voltages from several sensors in order to determine how much fuel to give the engine. The fuel is delivered under one of several conditions called modes. The control module controls all of the modes.

When the ignition switch is turned to the ON position, before engaging the starter, the control module energizes the fuel pump relay for 2 seconds allowing the fuel pump to build up pressure. The control module then checks the engine coolant temperature (ECT) sensor and the throttle position (TP) sensor in order to determine the proper air and fuel ratio for starting. The control module controls the amount of fuel delivered in the starting mode by changing how long the injectors are energized. This is done by pulsing the injectors for very short times.

If the engine floods, clear the engine by depressing the accelerator pedal down to the floor. The control module then pulses the injector at a 16.5:1 air to fuel ratio. The control module holds this injector rate as long as the throttle stays wide open and the engine is below 600 RPM. If the throttle position becomes less than 65 percent, the control module returns to the starting mode.

The run mode is the mode under which the engine operates most of the time. In this mode, the engine operates in either Open Loop or Closed Loop.

Open Loop

When the engine is first started and it is above 400 RPM, the system goes into the Open Loop operation. In the Open Loop, the control module ignores the signal from the HO2S, and the control module calculates the air to fuel ratio based on the inputs from the Engine Coolant Temperature (ECT) sensor and Manifold Absolute Pressure (MAP) sensor.

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

1. The HO2S has varying voltage output, showing that it is hot enough to operate properly. This depends on engine temperature.
2. The Engine Coolant Temperature (ECT) sensor is above a specified temperature.
3. A specific amount of time has elapsed after starting the engine.

A normal functioning system may go into an Open Loop at idle if the Heated Oxygen Sensor (HO2S) temperature drops below the minimum requirement to produce the voltage fluctuation.

Closed Loop

The specific values for the above conditions vary with different engines. When these conditions are met, the systems goes into a Closed Loop operation. In a Closed Loop, the control module calculates the air to fuel ratio (injector on-time) based on the signal from the HO2S. This allows the air to fuel ratio to stay very close to 14.7:1.

When the control module senses rapid changes in the throttle position and the manifold pressure, the system enters the acceleration mode. The system provides the extra fuel needed for smooth acceleration.

When the driver releases the accelerator pedal, the air flow into the engine is reduced. The corresponding changes in the throttle position and the manifold air pressure are relayed to the control module, which reduces the injector pulse width, in order to reduce the fuel flow. If the decel is very rapid, or for long periods (such as long closed throttle coast-down), the control module shuts OFF the fuel completely in order to protect the catalytic converter.

In order to prevent possible engine damage from over-speed, the Control Module shuts fuel injector control when the engine speed is above approximately 5500 RPM with the vehicle in any forward gear or reverse.The Control Module also has a fuel cutoff in excess of 108 mph (173 km/h) based on the speed rating of the originallly equipped tires.

Fuel is also Cutoff during rapid deceleration. See Deceleration Mode.

The Control Module constantly monitors engine operation and estimates the conditions that could result in high converter temperatures. If the converter may overheat, the VCM takes appropriate action to cool the exhaust.

When battery voltage is low, the control module can compensate for a weak spark delivered to the distributor by increasing the following items: