Fuel Delivery and Air Induction: Description and Operation

Closed Loop System Diagram:

Fuel Supply System Diagram:

BASIC SYSTEM OPERATION
The basic function of the fuel control system is to control fuel delivery to the engine. Fuel is delivered to each cylinder by two injectors (primary and secondary), located in separate intake ports.

The main control sensors are the Oxygen (O2) sensors, located in both the right and left exhaust manifold, that sense the amount of oxygen in the exhaust gas from each cylinder bank. This information is used by the Electronic 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 catalytic converter to operate the most efficiently. 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 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 the modes are controlled by the ECM and are described below.

^ STARTING MODE
When the ignition switch is turned to the "ON" position (before engaging the starter), the ECM checks the Coolant Temperature Sensor (CTS) and Throttle Position Sensor (TPS), to determine the length of time to operate the secondary fuel pump, and determine the proper air/fuel ratio for starting. The ECM turns "ON" both fuel pump relays for a minimum of two seconds, causing both fuel pumps to operate and build up 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 turned "ON" 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 the 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 rpm is below 600. If the throttle position is less than 80%, the ECM returns to the starting mode.

^ RUN MODE
When the engine is first started, and the engine speed is above 500 rpm, the ECM checks the Crank Sensor and Cam Sensor signals to initiate timed sequential fuel injection pulses. Cam Sensor input is used to synchronize fuel injection pulses with intake valve opening. If the ECM does not detect a Cam Sensor signal above 500 rpm, or if it detects extra Cam pulses, it sets a Code 31 and initiates sequential fuel injection based on the ignition reference signal (from the Crank 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 Oxygen (O2) sensors, and calculates the air/fuel ratio based on inputs from the Coolant Temperature Sensor (CTS) and Manifold Absolute Pressure (MAP) sensors.

The system stays in "Open Loop" until:
1. The O2 sensors have varying voltage output, showing that they are hot enough to operate properly, approximately 315°C (600°F).
2. The coolant 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 MEM-CAL. 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 signals from the various sensors, but the primary input is from the O2 sensors. This allows the air/fuel ratio to stay very close to 14.7:1.

^ 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 port throttle 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 port throttle valves to be opened, or allow the secondary injectors to be turned "ON." In the "FULL" power position, the ECM enables secondary operation only if no trouble codes 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 consecutively turns "ON": the secondary port throttle valve solenoid (causing the valves to open), then the secondary injectors to provide the additional fuel required for full engine power operation. When the secondary injectors are "ON," 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 port throttle valves are enabled, the ECM continues to monitor oxygen sensor outputs. If a lean condition (low O2 sensor voltage) exists for more than two seconds, a Code 55 is set, and secondary port throttle/secondary injector operation is disabled.

^ FUEL CUT-OFF MODE
To prevent possible engine damage from overspeed, the ECM cuts off fuel from all injectors at approximately 7100 rpm. If a Code 61 is stored and the ECM detects vacuum at the vacuum sensor in the "NORMAL" mode, secondary port throttle/secondary injector operation is disabled, and fuel cut-off occurs at approximately 3000 rpm.

^ 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 flow. If the deceleration is very rapid, or for long periods of time (such as a long closed throttle coast-down), the ECM shuts the fuel completely "OFF," to protect the catalytic converter.

^ BATTERY VOLTAGE CORRECTION MODE
When battery voltage is low, the ECM can compensate by:
- Increasing the amount of fuel delivered (increased base injector pulse width).
- Increasing idle rpm.
- Increasing ignition dwell time.