Fuel Delivery System

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 to the engine by individual fuel injectors mounted in the intake manifold near each cylinder.

OPERATION
Fuel is delivered to the engine by individual bottom feed type multiport fuel injectors mounted in the lower intake manifold near each cylinder.
The Powertrain Control Module (PCM) pulses the fuel injectors in pairs. Alternate pairs are pulsed every 180 degrees of crankshaft revolution. This is called Alternating Synchronous Double Fire (ASDF) injection and is described later in this section.
The PCM uses two injector driver circuits, each controlling a pair of injectors. The current in each circuit is allowed to climb to a peak of 4 amps and then is reduced to 1 amp to hold the injector open. This happens very quickly.

The main control sensor is the Oxygen (02) sensor, which is located in the exhaust manifold. This sensor 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 injectors. The best mixture to minimize exhaust emissions is 14.7:1, which allows the catalytic converter to operate most efficiently. Because of the constant measuring and adjusting of the air / fuel ratio, the fuel injection system is called a "Closed Loop" system.

ALTERNATING SYNCHRONOUS DOUBLE FIRE (ASDF) FUEL INJECTION
For better fuel vaporization and improved emission control, fuel is injected at the intake valves while they are in the closed position. To do this, the fuel injectors are alternately pulsed in pairs. Injectors for cylinders 1 and 4 are electrically paired, as are the injectors for cylinders 2 and 3. Each pair of injectors is pulsed twice (double fire), once on the compression stroke and once on the exhaust stroke for each cylinder.

FUEL METERING SYSTEM COMPONENTS
The fuel metering system consists of the following parts:
- Fuel supply components (fuel tank, pump, pipes).
- Fuel pump electrical circuit.
- Intake manifold assembly, consisting of:

- Lower manifold assembly, including:
- MFI fuel injectors.
- Fuel pressure regulator assembly.

- Upper manifold assembly, including:
- Throttle valve.
2 - Idle Air Control (IAC) valve.
- Throttle Position (TP) sensor.

MODES OF OPERATION
The Powertrain Control Module (PCM) monitors information from several sensors to determine fuel requirements based on engine operating conditions. The fuel is delivered under one of several conditions called modes. All modes are controlled by the PCM as described below.

Starting Mode
When the ignition is first turned "ON" (before engaging starter), the PCM energizes the fuel pump relay for a calibrated time (approximately 2 seconds) to allow the fuel pump to build up pressure. The PCM also checks the Engine Coolant Temperature (ECT) and Throttle Position (TP) sensors to determine the proper air/fuel ratio for st2arting. This ranges from approximately 2.55:1 at -40°C (-40°F) to 14.7:1 at 110°C (230°F) engine coolant temperature. The PCM controls the amount of fuel delivered 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, it can be cleared by pushing the accelerator pedal down all the way while cranking the engine. The PCM then de-energizes the fuel injectors and holds them de-energized as long as the throttle stays wide open and the engine speed is below 600 RPM. If the throttle position becomes less than 70%, the PCM returns to the starting mode.

Run Mode
The run mode consists of both "Open Loop" or "Closed Loop" operation.
When the engine is first started and engine speed is above 400 RPM, the system goes into "Open Loop" operation. In "Open Loop," the PCM ignores the signal from the Oxygen Sensor (O2S) and calculates the air/fuel ratio based on inputs from the ECT and Manifold Absolute Pressure (MAP) sensors.

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

1. The Oxygen Sensor 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 after starting the engine.

The specific values for the above conditions vary with different engines and are stored in the Erasable Programmable Read Only Memory (EPROM) portion of the PCM. When these values are met, the system goes into "Closed Loop" operation.

In "Closed Loop," the PCM calculates the air/fuel ratio (injector "ON"-time) based on the signal from the O2S. This allows the air/fuel ratio to stay very close to 14.7:1.

Acceleration Mode
When the driver depresses the accelerator pedal, air flow into the cylinders increases rapidly, while fuel flow tends to lag behind. To prevent possible hesitation, the PCM increases the pulse width to the injectors to provide extra fuel during acceleration. The amount of fuel required is based on throttle position, manifold air pressure and engine speed.

Fuel Cutoff Mode
To prevent possible engine damage from over-speed, the PCM cuts off fuel from the injectors when engine speed is above approximately 6260 RPM. On vehicles equipped with automatic transmission, fuel is cut off at 4000 RPM in park or neutral. The PCM also cuts off fuel to the injectors at vehicle speeds above 108 mph to prevent tire damage. Also see "Rapid Deceleration" under "Deceleration Mode."

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

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

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