The function of the fuel metering system is to deliver the correct amount of fuel to the engine under all operating conditions. The fuel is delivered to the engine by the individual fuel injectors mounted into the intake manifold near each cylinder.
The 2 main fuel control sensors are the manifold absolute pressure (MAP) sensor, the front heated oxygen sensor (HO2S1), and the rear heated oxygen sensor (HO2S2).
The MAP sensor measures or senses the intake manifold vacuum. Under high fuel demands the MAP sensor reads a low vacuum condition, such as wide open throttle. The engine control module (ECM) uses this information to richen the mixture, thus increasing the fuel injector on-time, to provide the correct amount of fuel. When decelerating, the vacuum increases. This vacuum change is sensed by the MAP sensor and read by the ECM, which then decreases the fuel injector on-time due to the low fuel demand conditions.
HO2S Sensors
The HO2S sensor is located in the exhaust manifold. The HO2S sensor indicates to the ECM the amount of oxygen in the exhaust gas and the ECM changes the air/fuel ratio to the engine by controlling the fuel injectors. The best air/fuel ratio to minimize exhaust emissions is 14.7:1, 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.
The ECM uses voltage inputs from several sensors to determine how much fuel to provide to the engine. The fuel is delivered under one of several conditions, called modes.
Starting Mode
When the ignition is turned ON, the ECM turns the fuel pump relay ON for 2 seconds. The fuel pump then builds fuel pressure. The ECM also checks the engine coolant temperature (ECT) sensor and the throttle position (TP) sensor and determines the proper air/fuel ratio for starting the engine. This ranges from 1.5:1 at -37°C (-35°F) coolant temperature to 14.7:1 at 94°C (201°F) coolant temperature. The ECM controls the amount of fuel delivered in the starting mode by changing how long the fuel injector is turned ON and OFF. This is done by pulsing the fuel injectors for very short times.
Clear Flood Mode
If the engine floods with excessive fuel, it may be cleared by pushing the accelerator pedal down all the way. The ECM will then completely turn OFF the fuel by eliminating any fuel injector signal. The ECM holds this injector rate as long as the throttle stays wide open and the engine is below approximately 400. If the throttle position becomes less than approximately 80 percent, the ECM returns to the starting mode.
Run Mode
The run mode has 2 conditions called open loop and closed loop.
Open Loop
When the engine is first started and it is above 400 RPM, the system goes into Open Loop operation. In Open Loop, the ECM ignores the signal from the HO2S and calculates the air/fuel ratio based on inputs from the ECT sensor and the MAP sensor. The sensor stays in Open Loop until the following conditions are met:
| • | The HO2S sensor has a varying voltage output, showing that it is hot enough to operate properly. |
| • | The ECT sensor is above a specified temperature. |
| • | A specific amount of time has elapsed after starting the engine. |
Closed Loop
The specific values for the above conditions vary with different engines and are stored in the electronically erasable programmable read only memory (EEPROM). When these conditions are met, the system goes into Closed Loop operation. In Closed Loop, the ECM calculates the air/fuel ratio, fuel injector on-time, based on the signal from the oxygen sensor. This allows the air/fuel ratio to stay very close to 14.7:1.
Acceleration Mode
The ECM responds to rapid changes in throttle position and airflow and provides extra fuel.
Deceleration Mode
The ECM responds to changes in throttle position and airflow and reduces the amount of fuel. When deceleration is very fast, the ECM can cut off fuel completely for short periods of time.
Battery Voltage Correction Mode
When battery voltage is low, the ECM can compensate for a weak spark delivered by the ignition module by using the following methods:
| • | Increasing the fuel injector pulse width |
| • | Increasing the idle speed RPM |
| • | Increasing the ignition dwell time |
Fuel Cut-Off Mode
No fuel is delivered by the fuel injectors when the ignition is OFF. This prevents dieseling or engine run-on. Also, the fuel is not delivered if there are no reference pulses received from the central power supply. This prevents flooding.
Fuel Injector
The multiport fuel injection (MFI) assembly is a solenoid-operated device controlled by the ECM. It meters pressurized fuel to a single engine cylinder. The ECM energizes the fuel injector or the solenoid to a normally closed ball or pintle valve. This allows fuel to flow into the top of the injector, past the ball or pintle valve, and through a recessed flow director plate at the injector outlet.
The director plate has 6 machined holes that control the fuel flow, generating a conical spray pattern of finely atomized fuel at the injector tip. Fuel from the tip is directed at the intake valve, causing it to become further atomized and vaporized before entering the combustion chamber. A fuel injector which is stuck partially open will cause a loss of fuel pressure after the engine is shut down. Also, an extended crank time will be noticed on some engines. Dieseling can also occur because some fuel can be delivered to the engine after the ignition is turned OFF.
