Fuel Delivery and Air Induction: Description and Operation
DESCRIPTION
The function of the Fuel Metering System is to deliver the correct amount of Fuel to the Engine under all operating conditions. Fuel is delivered to the Engine by individual Fuel Injectors mounted in the Intake Manifold near each Cylinder.
The Main Control Sensor is the Heated Oxygen Sensor, which is located in the front pipe. The Heated Oxygen Sensor indicates to the Engine Control Module the amount of Oxygen in the exhaust gas, and the Engine Control Module 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 to 1, which allows the Three Way 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.
MODES OF OPERATION
The Engine Control Module (ECM) uses voltage inputs from several Sensors to determine how much Fuel to give the Engine. The Fuel is delivered under one of several conditions, called "modes". All the modes are controlled by the ECM.
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 pressure. The ECM also checks the Engine Coolant Temperature (ECT) Sensor and the Throttle Position Sensor, and the 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) ECT. The ECM controls the amount of Fuel delivered in the starting mode by changing how long the Injector is turned "ON" and "OFF". The is done by "Pulsing" the Injectors for very short times.
Clear Flood Mode
If the Engine floods, it may be cleared by pushing the Accelerator Pedal down all the way. The ECM then will fix air/fuel ratio 16.5:1. The ECM holds this Injector rate as long as the Throttle stays wide open. If the Throttle Position becomes less than approximately 70%, the ECM returns to the starting mode.
Run Mode
The run mode has two conditions called "Open Loop" and "Closed Loop"
"Open Loop"
When the Engine is first started, and it is above 500 Revolution Per Minute (rpm), the system goes into "Open Loop" operation. "Open Loop," the ECM ignores the signal from the Heated Oxygen Sensor (HO2S), and calculates the air/fuel ratio based on inputs from the ECT sensor and Manifold Absolute Pressure Sensors. The system stays 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.
"Closed Loop"
The specific values for the above conditions vary with different Engines, and are stored in the ECM. When these conditions are met, the system goes into "Closed Loop" operation. In "Closed Loop", the ECM calculates 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 ECM responds to rapid changes in Throttle position and air flow, and provides extra Fuel.
Deceleration Mode
The ECM responds to changes in Throttle position and air flow, and reduces the amount of Fuel. When deceleration is very fast, the ECM can cut off Fuel completely for short periods.
Battery Positive Voltage Correction Mode
When Battery positive voltage is low, the ECM can compensate for a weak spark delivered by the Ignition Control Module by:
- Increasing Injector pulse width.
- Increasing the idle rpm.
- Increasing Ignition dwell time.
Fuel Cutoff Mode
The Fuel Injectors are shut "OFF" if Engine speed exceeds 6800 rpm. This is to prevent damage to Engine. When Engine speed is below 6800 rpm, the Fuel Injectors are again pulsed at a normal mode.