Fuel Delivery and Air Induction: Description and Operation
Fuel Supply System Diagram:
The fuel metering system consists of the following components:
1. Fuel pump electrical circuit.
2. Fuel supply components, including:
a. Fuel lines and pipes.
b. Fuel pump.
c. Fuel tank.
3. Fuel rail assembly, including:
a. Fuel injectors.
b. Fuel pressure regulator.
4. Throttle body assembly, including:
a. Idle air control valve.
b. Throttle position 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 then checks the coolant temperature sensor, throttle position sensor (TPS), manifold air temperature (MAT) sensor, and determines the proper air/fuel ratio for starting. This ranges from 1.5:1 at -36°C (-33°F) to 14.7:1 at 94°C (201°F) running temperature. 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 pulses the injectors at an air/fuel ratio of 20:1. The ECM holds the injector rate as long as the throttle remains wide open, and the engine rpm is below 600. If the throttle position becomes less than 80%, 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 sensor, and calculate the air/fuel ratio based on inputs from the coolant and MAP sensors. The system will stay in open Loop until the following conditions are met:
1. The oxygen sensor has a varying voltage output, showing that it is hot enough to operate properly. (This depends on temperature).
2. The coolant sensor is above a specified temperature, about 40°C (104°F|.
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 mem-cal. When the conditions are met, the system goes into Closed Loop operation. In Closed Loop, the ECM will calculate the air/fuel ratio (injector time on) 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:
No fuel is delivered by the injector when the ignition is OFF. This prevents dieseling. Also, fuel is not delivered if no reference pulses are seen from the distributor, which means the engine is not running. This prevents flooding.