Fuel Delivery System
PURPOSEThe basic function of the fuel metering system is to control fuel delivery to the engine. Fuel is delivered to the engine by individual fuel injectors mounted in the intake manifold near each cylinder.
OPERATION
The main control sensor is the Heated Oxygen Sensor (H02S) which is located in the exhaust manifold. The oxygen sensor tells the Powertrain Control Module (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 the 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:
^ PCM
^ Fuel tank
^ Fuel pump
^ Fuel pump relay
^ Fuel lines
^ Fuel pump electrical circuit
^ Fuel injector rail
^ Fuel injectors
^ Pressure regulator
^ Throttle body assembly, including:
^ Idle Air Control (IAC) valve
^ Throttle Position Sensor (TPS)
MODES OF OPERATION
The PCM looks at voltages from several sensors to determine how much fuel should be delivered to the engine. The fuel is delivered under one of several conditions, called "modes." All the 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 pressure. The PCM then checks the Engine Coolant Temperature (ECT) sensor, Throttle Position (TP) sensor, and fuel control signal to determine 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 turned ON and OFF This is done by "pulsing" the injectors for very short times.
Clear Flood Mode
If the engine floods, it can be cleared by pushing the accelerator pedal down all the way. The PCM then completely turns OFF the fuel. No fuel is delivered from the injectors as long as the throttle stays wide open, and the engine speed is below 600 RPM. If the throttle position becomes less than 65%, 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 RPM is above 400 RPM, the system is in "Open Loop" operation. In "Open Loop," the PCM will ignore the signal from the 02S, and calculate the air/fuel ratio based on inputs from the TP sensor, ECT sensor and MAF sensor.
The system will stay in "Open Loop" until the following conditions are met:
1. The HO2S has 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 over 1200 RPM for at least 10 seconds.
The specific values for the above conditions are stored in the PROM. When these 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 H02S. 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 possible 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, manifold air pressure, 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 and manifold air pressure 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 (such as long closed throttle coast-down), the PCM shuts OFF fuel completely to protect the catalytic converter.
Engine Speed/Vehicle Speed Fuel Disable Mode
The PCM monitors engine speed and shuts the fuel injectors OFF 5600 RPM. Fuel is turned back ON when engine speed decreases to below 3500 RPM.
The PCM also monitors vehicle speed and shuts fuel OFF at a speed which varies according to specific vehicle axle ratio and tire size and rating. When battery voltage is low, the PCM will compensate for the weak spark by:
^ Increasing the amount of fuel delivered
^ Increasing the idle RPM
^ Increasing ignition dwell time
Fuel Cutoff Mode
No fuel is delivered by the injector when the ignition is ON) This prevents dieseling. Also, fuel is not delivered if no reference pulses are seen (engine not running.) This prevents flooding.
Battery Voltage Correction Mode
When battery voltage is low, the PCM will compensate for the weak spark by:
^ Increasing the amount of fuel delivered.
^ Increasing the idle RPM.
^ Increasing ignition dwell time.
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. When this occurs, the Malfunction Indicator Lamp (MIL) flashes.