Fuel Metering System

PURPOSE
The function of the fuel and air control system is to manage fuel and air delivery to each cylinder to optimize the performance and driveablity of the engine under all driving conditions. Fuel is distributed through the fuel rail to eight injectors inside the intake manifold. Each port fuel injector is located directly above each cylinder's two intake valves. Fuel pressure is controlled by a pressure regulator mounted on the fuel rail. The throttle body regulates air flow from the air cleaner into the intake manifold, which then distributes this air to each cylinder's two intake valves. This allows the driver to control the air flow into the engine, which then controls the power output of the engine.

MODES OF OPERATION
The PCM has several operating modes for fuel control, depending on the information it receives from the sensors.

Starting Mode
When the key is first turned "ON," the PCM will turn "ON" the fuel pump relay for two seconds. The fuel pump runs and builds up pressure in the fuel system. The PCM then monitors the Manifold Absolute Pressure (MAP), Intake Air Temperature (IAT), Engine Coolant Temperature (ECT), Throttle Position (TP) sensors and Ignition Control (IC) module signal to determine the required injector pulse width for starting.

Clear Flood Mode
If the engine is flooded with fuel during starting and will not start, "CLEAR FLOOD MODE" can be manually selected. To select "CLEAR FLOOD MODE," push the accelerator to Wide Open Throttle (WOT). With this signal, the PCM will completely turn "OFF" the injectors and will maintain this as long as it sees a WOT condition with engine speed below 1000 RPM.

Run Mode
The run mode has two conditions: "Open Loop" and "Closed 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 PCM ignores the signals from the oxygen sensors and calculates the required injector pulse width based primarily on inputs from the MAP, IAT and ECT sensors. The system will stay in "Open Loop" until the following conditions are met:

- One oxygen sensor has varying voltage output, showing that it is hot enough to operate properly.
- The ECT sensor indicates above 80°C (176°F).
- A calibrated time delay has elapsed after starting the engine. This delay allows all sensors to stabilize.

When these conditions are satisfied, the PCM goes into "Closed Loop" operation. In "Closed Loop," the PCM adjusts the calculated injector pulse width for each bank of injectors based on the signals from each oxygen sensor.

Acceleration Mode
The PCM monitors changes in TP and MAP sensor signals to determine when the vehicle is being accelerated. The PCM will then increase injector pulse width to provide more fuel for improved driveability.

Deceleration Mode
The PCM monitors changes in TP and MAP sensor signals to determine when the vehicle is being decelerated. The PCM will then decrease injector pulse width or even shut "OFF" injectors for short periods to reduce exhaust emissions.

Battery voltage Correction Mode
When the PCM sees a low battery voltage condition, it can compensate to maintain acceptable vehicle driveability. It does this by:

- Increasing the injector pulse width to maintain the proper amount of fuel delivered.
- Increasing the idle speed to increase generator output.

Load Management Control Mode
When the HVAC controller has a request for rear window defog at idle, the HVAC controller sends a message via UART to the PCM to increase the desired idle speed. The changes to desired idle speed are as follows. At idle in park/neutral desired idle speed will be at least 1040 RPM. When a gear other than park or neutral is selected, the desired idle speed will be at least 704 RPM.

Fuel Shutoff Mode
The PCM has the ability to completely turn all the injectors "OFF" or selectively turn "OFF" some of the injectors when certain conditions are met. These fuel shutoff modes allow the powertrain to protect itself from damage and also improve its driveability.

The PCM will disable all eight injectors under the following conditions:

- Ignition "OFF" (prevents engine run-on).
- Ignition "ON" but no ignition reference signal (prevents flooding or backfiring).
- High engine speed (above red line).
- High vehicle speed (above rated tire speed).
- Extended high speed closed throttle coastdown (reduces emissions and increases engine braking).

The PCM will selectively disable up to four injectors under the following conditions:

- Torque management enabled (transaxle shifts or abusive maneuvers).
- Traction control enabled (in conjunction with front brakes applying).
- Low coolant condition (protects engine from overheating).

FUEL METERING SYSTEM COMPONENTS
The fuel metering system consists of the following parts:

- Fuel supply components (fuel tank, pump, lines).
- Fuel pump electrical circuit.
- Fuel rail assembly, including:
- Fuel injectors.
- Fuel pressure regulator assembly.
- Throttle body assembly, including:
- Idle Speed Control (ISC) actuator.
- Throttle Position (TP) sensor.

FUEL PUMP ELECTRICAL CIRCUIT
When the ignition switch is turned to the "ON" position (before engaging starter), the 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 "OF F" the fuel pump relay, causing the fuel pump to stop.

As a backup system to the fuel pump relay, the fuel pump also can be energized by the fuel pump and engine oil pressure sensor. The sensor has two internal circuits. One operates the oil pressure 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 and engine oil pressure sensor runs the fuel pump.

An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold. The fuel pump and engine oil pressure sensor energizes the fuel pump when oil pressure reaches about 28 kPa (4 psi).