Fuel System

DESCRIPTION
The fuel supply system begins with gasoline in the fuel tank. An electric fuel pump, located in the fuel tank with the fuel level gauge sending unit, pumps fuel to the fuel rail through an in-line filter. The pump is designed to supply fuel at a pressure above the pressure needed by the injectors. A pressure regulator located on the fuel rail keeps the fuel available to the injectors at a constant pressure. The unused fuel is returned to the fuel tank by a separate line.

FUEL METERING COMPONENTS
The fuel system is made up of the following components.

- Fuel tank, pump, and lines.
- Fuel pump electrical circuit.
- Fuel Rail Assembly:
- Fuel injectors.
- Fuel pressure regulator.
- Throttle body assembly:
- Idle Air Control (IAC) valve.
- Throttle Position (TP) sensor.

MODES OF OPERATION
The Powertrain Control Module (PCM) uses voltage inputs from several sensors to determine how much fuel to give the engine. 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 starting. This ranges from approximately 0.8:1 at -40°C (-40°F) to 16.8:1 at 104°C (220°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 to the floor. When the throttle position is greater than 75% during cranking, the PCM completely turns "OFF" the fuel. No fuel is delivered from the injectors as long as throttle position is greater than 75% and the engine is not running. If the throttle is released to less than 75%, the PCM returns to the starting mode.

Run Mode
The run mode consists of both "Open Loop" or "Closed Loop" operation.

Open Loop - When the engine is first started and engine speed is above 637 RPM, the system goes into "Open Loop" operation. Also, when the engine is first started and the throttle position is not changed, the system remains in "Open Loop" until the throttle is moved. 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:
- The O2S has a varying voltage output, showing that it is hot enough to operate properly.
- The ECT sensor is above a specified temperature.
- A specific amount of time has elapsed after starting the engine.

Closed Loop - In "Closed Loop" specific values for the above conditions exist for each different engine, and are stored in the Erasable Programable Read Only Memory (EPROM). When these values are met, the system goes into "Closed Loop" operation. 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 14.6:1.


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.


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 to protect the catalytic converter.

Battery Voltage Correction Mode
When battery voltage is low, a Diagnostic Trouble Code (DTC) 53 may be set and the PCM will compensate for the weak spark delivered by the electronic ignition system by:

- Increasing the injector pulse width.
- Increasing the idle RPM.
- Increasing ignition dwell time.

Fuel Cutoff Mode
To prevent possible engine damage from overspeed, the PCM cuts "OFF" fuel from the injectors at a calibrated engine speed, or vehicle speed. On vehicles equipped with automatic transmission, the fuel will be shut "OFF" if the engine exceeds 4000 RPM in "PARK" or "NEUTRAL." Also see rapid deceleration under "Deceleration Mode."

FUEL PUMP CIRCUIT
Start Up
When the ignition switch is turned to the "ON" position (before engaging starter), the Powertrain Control Module (PCM) energizes the fuel pump relay for two seconds, causing the fuel pump to pressurize the fuel system. If the PCM does not receive ignition reference pulses (engine cranking or running) within two seconds, it shuts "OFF" the fuel pump relay, causing the fuel pump to stop.

Backup Operation
As a backup system to the fuel pump relay, the fuel pump also can be energized by the fuel pump switch and engine oil pressure sensor. The sensor has two internal circuits. One operates the oil pressure indicator or gage on the instrument cluster, and the other is a normally open switch which closes when oil pressure reaches about 28 kPa (4 psi). If the fuel pump relay fails, the fuel pump switch and engine oil pressure sensor runs the fuel pump.