Fuel Delivery and Air Induction: Description and Operation
The fuel metering system consists of the following components:
^ Fuel Supply Components (fuel tank, pump, 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
Basic System Operation:
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.
Modes of Operation:
The PCM uses voltage inputs from several sensors to determine how much fuel to give the engine. The fuel is delivered under several conditions, called "Modes." All modes are controlled by the PCM, and are described below.
Starting Mode
When the ignition is first turned ON, the PCM turns ON the fuel pump relay for two seconds and the fuel pump builds up system pressure. The PCM first checks speed density, then switches to the Mass Air Flow (MAF) sensor. Engine Coolant Temperature sensor (ECT), Throttle Position (TP) and Manifold Absolute Pressure (MAP) sensors are used to determine the proper air/fuel ration for starting. The PCM controls the amount of fuel delivered in the starting mode by changing how long the injectors are turned ON and OFF. This is done by pulsing the injectors for very short times.
Clear Flood Mode
If the engine floods, clear it by pushing the accelerator pedal all the way down. The PCM then pulses the injectors at an air/fuel ratio of 20:1. The PCM holds this injector ratio as long as the throttle remains above 80% and engine speed remains less than 300 rpm. If the throttle position becomes less than 80%, the PCM returns to the starting mode.
Run Mode
The run mode has two conditions called Open Loop and Closed Loop. When the engine is first started, and the rpm is above 400 rpm, the system goes into Open Loop operation. In Open Loop, the PCM will ignore the signal from the oxygen sensors, and calculate the air/fuel ratio based on inputs from the ECT, MAP, MAF and TP sensors. The system will stay in open Loop until the following conditions are met:
1. The oxygen sensors have 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 EEPROM. When the conditions are met, the system goes into Closed Loop operation. In Closed Loop, the PCM will calculate the air/fuel ratio (injector on time) based on the signal from various sensors, but mainly 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 passible 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, coolant temperature, manifold air pressure, mass air flow 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, manifold air pressure and mass air flow 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 the PCM shuts "OFF" fuel completely to protect the three-way catalytic converter.
Battery Voltage Correction Mode
When battery voltage is low, the PCM can compensate for the weak spark delivered by the distributor by:
^ Increasing the amount of fuel delivered.
^ Increasing idle rpm.
^ Increasing ignition dwell time.
Fuel Cutoff Mode
To prevent possible engine damage from overspeed, the PCM cuts off fuel from the injectors when engine speed is above 5800 rpm with the vehicle in any forward gear position, or in reverse.