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 ECM 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 ECM, and are described below.
Starting Mode
When the ignition is first turned ON, the ECM turns ON the fuel pump relay for two seconds and the fuel pump builds up system pressure. The ECM checks Engine Coolant Temperature sensor (ECT) and Throttle Position (TP), to determine the proper air/fuel ration for starting. The ECM 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 ECM then de-energizes the injectors. The ECM holds the injectors de-energized as long as the throttle remains above 62% and engine speed remains less than 600 rpm. If the throttle position becomes less than 62%, the ECM 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 ECM will ignore the signal from the oxygen sensors, and calculate the air/fuel ratio based on inputs from the ECT and IAT 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 PROM. When the conditions are met, the system goes into Closed Loop operation. In Closed Loop, the ECM will calculate the air/fuel ratio (injector on time) based on the signal from various sensors, but mainly the oxygen sensor. This allows the air/fuel ratio to stay very close to 14.7:1.
Acceleration Mode
The ECM looks at rapid changes in throttle position and air flow, and provides extra fuel.
Deceleration Mode
The ECM looks at changes in throttle position and air/flow to reduce the amount of fuel. When the deceleration is very fast, the ECM may shut off fuel completely for short periods.
Battery Voltage Correction Mode
When battery voltage is low, the ECM 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 ECM cuts off fuel from the injectors when engine speed is above 6000 rpm with the vehicle in any forward gear position, or in reverse. The ECM also cuts-off fuel to the injectors at vehicle speeds above 108 mph to prevent tire damage.
Catalytic Converter Protection Mode
The ECM constantly monitors engine operation, and estimates conditions that could result in high catalytic converter temperatures. If the ECM determines the converter may overheat, it causes the system to return to "Open Loop" operation and enriches the fuel mixture.