Fuel Delivery System

DESCRIPTION
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 each cylinder by two injectors (primary and secondary), located in separate intake ports.

There are two Heated Oxygen Sensors (HO2S), one located in each exhaust manifold, that sense the amount of oxygen in the exhaust gas from each cylinder bank. This information is used by the Engine Control Module (ECM) to determine the amount of injector on time for correct fuel delivery. The best mixture to minimize exhaust emissions is 14.7:1 which allows the three-way catalytic converters to operate the most efficiently. Because of the constant measuring and adjusting of the air/fuel ratio, the Sequential Multiport Fuel Injection (SFI) system is called a "Closed Loop" system.

MODES OF OPERATION
The ECM looks at voltages 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 ECM and are described below.

Starting Mode
When the ignition switch is turned to the "ON" position (before engaging starter), the ECM checks the Engine Coolant Temperature (ECT) and Throttle Position (TP) sensors to determine the length of time to operate the secondary fuel pump, and to determine the proper air/fuel ratio for starting. The ECM energizes both fuel pump relays for a minimum of two seconds, causing both fuel pumps to operate and build up system pressure quickly. Air/fuel ratios for starting range from 1.5:1 at -36°C (-33°F) to 14.7:1 at 94°C (201°F) coolant temperature.
Fuel delivery in the starting mode is through the primary injectors only. The ECM controls the amount of fuel delivered by changing the length of time the injectors are energized or pulsed. During starting, all eight primary injectors are pulsed simultaneously.

Clear Flood Mode
If the engine floods, it can be cleared by pushing the accelerator pedal to the floor. When throttle position is greater than 80% during cranking, the ECM shortens the injector pulse width to achieve an air/fuel ratio of 20:1. The ECM holds this injector rate as long as the throttle stays wide open, and the engine speed is below 600 RPM. If the throttle position is less than 80%, the ECM returns to the starting mode.

Run Mode
When the engine is first started, and engine speed is above 500 RPM, the ECM checks the Crankshaft Position (CKP) sensor and the Camshaft Position (CMP) sensor signals to initiate timed Sequential Multiport Fuel Injection (SFI) pulses. CMP sensor input is used to synchronize fuel injection pulses with intake valve opening. If the ECM does not detect a CMP sensor signal above 500 RPM, or if it detects extra CMP pulses, it sets a DTC 31 and initiates Sequential Multiport Fuel Injection (SFI) based on the ignition reference signal (from the CKP sensor) only.

Once the engine is running above 500 RPM, the fuel metering system goes into "Open Loop" operation. In "Open Loop," the ECM ignores the signals from the Heated Oxygen Sensors (HO2S) and calculates the air/fuel ratio based on inputs from the Engine Coolant Temperature (ECT) and Manifold Absolute Pressure (MAP) sensors.

The system stays in "Open Loop" until:

1. The heated oxygen sensors have varying voltage output, showing that they are hot enough to operate properly, approximately 315°C (600°F).
2. The ECT sensor is above about 40°C (104°F).
3. A specific amount of time has elapsed after starting the engine. The length of time depends on coolant temperature at engine start-up.

The specific values for the above 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 ECM calculates the air/fuel ratio (injector on-time) based on the signal from various sensors, but the primary input is from the oxygen sensors. This allows the air/fuel ratio to stay very close to 14.7:1.

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

Power Modes
The driver can select two engine power modes, "FULL" or "NORMAL" power, with a console mounted key switch. This switch is wired into the ECM, and allows the driver to determine the engine output by controlling secondary air inlet valves and secondary injectors. When the key switch is in the "FULL" power position, the "FULL" light is illuminated on the console.

In the "NORMAL" power position, the ECM does not allow the secondary air inlet valves to be opened, or allow the secondary injectors to be energized. In the "FULL" power position, the ECM enables secondary operation only if no Diagnostic Trouble Codes (DTCs) are stored and other criteria are met such as engine oil temperature, throttle position, and engine speed. If all the conditions are met, the ECM energizes the secondary air inlet solenoid valve (causing the valves to open), allowing the secondary injectors to provide the additional fuel required for full engine power operation. When the secondary injectors are energized, total fuel flow to each cylinder is divided equally between the primary and secondary injectors.

Whenever Wide Open Throttle (WOT) is commanded and the secondary air inlet valves are enabled, the ECM continues to monitor heated oxygen sensor outputs. If a lean condition (low HO2S voltage) exists for more than two seconds, a DTC 55 is set, and secondary air inlet/secondary injector operation is disabled.

Fuel Cutoff Mode
To prevent possible engine damage from over-speed, the ECM cuts off fuel from all injectors at approximately 7100 RPM. If a DTC 61 is stored and the ECM detects vacuum at the vacuum sensor in the "NORMAL" mode, secondary air inlet/secondary injector operation is disabled, and fuel cutoff occurs at approximately 3000 RPM. Also, see "Rapid Deceleration" in "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 ECM, which reduces the injector pulse width to reduce fuel flow. If the deceleration is very rapid, or for long periods (such as a long closed throttle coast-down), the ECM shuts "OFF" fuel completely to protect the three-way catalytic converters.

Battery Voltage Correction Mode
When battery voltage is low, the ECM can compensate by:

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

FUEL SUPPLY COMPONENTS
The fuel supply is stored in the fuel tank. Two electric fuel pumps (primary and secondary) are used to meet the fuel flow and pressure requirements of the injectors. (Refer to "Fuel Pump Electrical Circuit" for explanation of fuel pump operation.) Both fuel pumps are attached to a single fuel gage sending unit and share common fuel feed and return pipes. Fuel is pumped through an in-line fuel filter to the fuel rail assembly. The pressure regulator, part of the fuel rail assembly, maintains the correct fuel pressure at the injectors.

FUEL METERING COMPONENTS
The fuel metering system is made up of the following parts:

- Fuel supply components (fuel tank, pumps, pipes).
- Fuel pump electrical circuit.
- Fuel rail assembly, including:
- Fuel injectors.
- Fuel pressure regulator assembly.
- Throttle body assembly, including:
- Idle Air Control (IAC) valve.
- Throttle Position (TP) sensor.
- Secondary air inlet valves.

FUEL PUMP ELECTRICAL CIRCUIT
The fuel pump electrical circuit includes two ECM controlled fuel pump relays (one for each fuel pump).

The primary pump is in operation whenever the engine is running. The secondary pump operates on demand, based on information from the Engine Coolant Temperature (ECT) sensor, Manifold Absolute Pressure (MAP), engine speed (RPM) and air flow, and during periods of engine operation when a DTC 14 or 15 has been stored by the ECM.

When the ignition switch is turned to the "ON" position (before engaging starter), the ECM checks engine coolant temperature to determine the length of time to operate the secondary fuel pump. The ECM energizes both fuel pump relays for a minimum of two seconds, causing both fuel pumps to operate and build up system pressure quickly. If the ECM does not receive ignition reference pulses (engine cranking or running) within two seconds, it shuts "OFF" both fuel pump relays, causing both pumps to stop. If, after the engine is started and running, and coolant temperature is less than 80°C (176°F), the secondary fuel pump relay will be held energized until coolant temperature reaches 80°C (176°F), at which point it is turned "OFF" If, after starting, coolant temperature is above 80°C (176°F), the secondary fuel pump relay will be turned "OFF" immediately after the initial two second period.

The secondary fuel pump relay (and fuel pump) is also energized when the engine is operating in the backup fuel mode as a result of a DTC 14 or 15 being stored by the ECM.

SECONDARY SFI CONTROL MODULES
Battery voltage to power the secondary injectors is controlled by two ECM controlled solid-state modules (one for a group of four injectors). If the ECM receives the proper signals to enable secondary injector operation, it energizes the secondary SFI control modules to power the injectors. The ground circuits for the secondary injectors are through the ECM primary injector drivers. Refer to the description of power modes above.