Fuel Delivery and Air Induction: Description and Operation
The fuel metering system consists of the following components:
^ Fuel injectors
^ Throttle body
^ Fuel rail
^ Fuel pressure regulator
^ PCM
^ Crankshaft position sensor
^ Camshaft position sensor
^ Idle Air Control (IAC) valve
^ Fuel pump
^ Fuel pump relay
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 checks Engine Coolant Temperature sensor (ECT), Throttle Position (TP), and Crankshaft Position signal, 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 de-energizes the injectors. The PCM holds the injectors de-energized as long as the throttle remains above 80%. 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, 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.
4. Engine speed has been greater than 800 rpm since start up.
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 PCM 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 PCM looks at rapid changes in throttle position and air flow, and provides extra fuel.
Deceleration Mode
The PCM looks at changes in throttle position and air/flow to reduce the amount of fuel. When the deceleration is very fast, the PCM may shut off fuel completely for short periods.
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
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.
Engine Speed/Vehicle Speed Fuel Disable Mode
The PCM monitors engine speed and turns "OFF" the fuel injectors when engine speed increases above 5600 rpm. The injectors are turned back "ON" when engine speed decreases below 5100 rpm. The PCM also monitors vehicle speed and turns "OFF" the fuel injectors at a vehicle speed which varies according to axle ratio, tire size and rating.