Fuel Metering System
DESCRIPTIONThe basic function of the air/fuel metering system is to control air/fuel delivery to the engine.
Fuel is delivered to the engine by individual fuel injectors mounted in the intake manifold near each intake valve.
The main control sensor is the Heated Oxygen Sensor (H02S) located in the exhaust manifold. The HO2S tells the PCM how much oxygen is in the exhaust gas and the PCM changes the air/fuel ratio to the engine by controlling the fuel injectors. The best mixture to minimize exhaust emissions is 14.7 to 1 which allows the catalytic converter to operate most efficiently. Because of the constant measuring and adjusting of the air/fuel ratio, the fuel injection system is called a "Closed Loop" system.
COMPONENTS
The fuel metering system consists of the following components:
- Fuel injectors.
- Throttle body.
- Fuel rail.
- Fuel pressure regulator.
- Powertrain Control Module (PCM).
- Crankshaft Position Sensor (CKP).
- Camshaft Position Sensor (CPS).
- Idle Air Control (IAC) valve.
- Fuel pump.
- Fuel pump relay.
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 monitors voltages from several sensors to determine the engine's fuel needs. Fuel is delivered under one of 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 energizes the fuel pump relay for two seconds allowing the fuel pump to build up pressure. The PCM then checks the Engine Coolant Temperature sensor (ECT), Throttle Position (TP) sensor, and crankshaft position signal, and determines the proper air/fuel ratio for starting. The PCM controls the amount of fuel delivered in the starting mode by changing how long the injectors are energized. 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% and the engine speed is below 800 rpm. If the throttle position becomes less than 80%, the PCM again begins to pulse the injectors "ON" and "OFF" allowing fuel into the cylinders.
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 Heated Oxygen Sensor (HO2S), and calculate the air/fuel ratio based on inputs from the ECT, Mass Air Flow (MAF) and TP sensors. The system will stay in open Loop until the following conditions are met:
1. The HO2S has a varying voltage output, showing that it is hot enough to operate properly. (This depends on temperature).
2. The ECT has reached a specified temperature.
3. A specific amount of time has elapsed after starting the engine.
4. Engine speed has been greater than 850 rpm since start up.
The specific values for the above conditions vary with different engines, and are stored in the Programable Read Only Memory (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 the HO2S. 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 reduces the amount of fuel injected when it detects a decrease in throttle position and air flow. 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 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.
THE PCM WILL DISABLE THE INJECTORS UNDER THE FOLLOWING CONDITIONS
- Ignition OFF (prevents engine run-on).
- Ignition ON but no ignition reference signal (prevents flooding or backfiring).
- High engine speed (above red line).
- High vehicle speed (above rated tire speed).
- Extended high speed closed throttle coastdown (reduce emissions and increases engine braking).
ENGINE RPM FUEL CUT OFF RANGES
Important: These RPM cut off ranges may vary according to PCM calibration, axle ratio, tire size, and tire rating.
- Park and neutral 5000 RPM.
- Reverse 4000 RPM.
- Drive 4000 RPM.
CATALYTIC CONVERTER PROTECTION MODE
The PCM constantly monitors engine operation and estimates conditions that could result in high catalytic converter temperatures. If the PCM determines the catalytic converter may overheat, it causes the system to return to Open Loop operation and enriches the fuel mixture.