System Description

PURPOSE
The function of the fuel metering system is to deliver the correct amount of fuel to the engine under all operating conditions. Fuel is delivered by the Throttle Body Fuel Injection (TBI) unit which is controlled by the Powertrain Control Module (PCM).

The main control sensor is the Oxygen Sensor (02S) located in the exhaust manifold. The 02S tells the PCM the amount of oxygen in the exhaust gas, and the PCM changes the air/fuel ratio to the engine by controlling the fuel injectors. A 14.7:1 air/fuel ratio is required for efficient catalytic converter operation. 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 control module monitors voltages from several sensors to determine how much fuel to give the engine. The fuel is delivered under several conditions, called "modes." All modes are controlled by the control module, and are described below.

Starting Mode
When the ignition is first turned "ON," the control module turns "ON" the fuel pump relay for two seconds, and the fuel pump builds pressure up to the throttle body injection unit. The control module checks the Engine Coolant Temperature (ECT) sensor, Throttle Position (TP) sensor, Manifold Absolute Pressure (MAP) sensor, and the Ignition signal; then, determines the proper air/fuel ratio for starting. This ranges from 1.5:1 at -36°C (-33°F) to 14.7:1 at 94°C (201°F) running temperature.

The control module controls the amount of fuel delivered in the starting mode by changing how long the injector is turned "ON" and "OFF." This is done by pulsing the injector for very short times.

Clear Flood Mode
If the engine floods, it may be cleared by pushing the accelerator pedal down all the way. The control module then pulses the injector at a 16.5:1 air fuel ratio. The control module holds this injector rate as long as the throttle stays wide open, and the engine speed is below approximately 600 rpm. If the throttle position becomes less than approximately 65%, the control module returns to the starting mode.

Run Mode
The run mode is the mode under which the engine operates most of the time. The run mode has two conditions, called OPEN LOOP and CLOSED LOOP.

Open Loop - When the engine is first started, and engine speed is above 400 rpm, the system goes into OPEN LOOP operation. In OPEN LOOP, the control module ignores the signal from the Oxygen sensor, and calculates the air/fuel ratio based on inputs from the ECT sensor and MAP sensor.

The system will stay in OPEN LOOP until the following conditions are met:
- The oxygen sensor has a varying voltage output, showing that it is hot enough to operate properly. (This depends on engine temperature)
- The coolant temperature is above a specified temperature.
- A specified amount of time has elapsed after starting the engine. normal operating system may go into OPEN LOOP at an idle if the O2 sensor temperature drops below the minimum requirement to produce voltage fluctuation.
- The throttle position is above idle.

Closed Loop - The specified operating conditions vary with different engines, and are stored in the Programmable Read Only Memory (PROM). When these conditions are met, the system goes into CLOSED LOOP operation. In CLOSED LOOP, the control module calculates the air/fuel ratio (injector on-time) based on the signal from the oxygen sensor. This allows the air/fuel ratio to stay very close to 14.7:1.

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

Deceleration Mode
When deceleration occurs, the fuel remaining in the intake manifold can cause excessive emissions and backfiring. When the control module observes a fast reduction in the throttle opening and a sharp decrease in manifold pressure, it reduces the amount of fuel delivered. When deceleration is very fast, the control module cuts off fuel completely for short periods.

Battery Voltage Correction Mode
When battery voltage is low, the control module can compensate for the weak spark delivered to the distributor by:
- Increasing the injector pulse width.
- Increasing the idle RPM.

Fuel Cut-off Mode
No fuel is delivered by the injector when the ignition is "OFF." This prevents dieseling. Also, fuel is not delivered if no reference pulses are seen from the distributor, which means the engine is not running.