The central sequential fuel injection (Central SFI) system is controlled by the control module located in the engine compartment. The control module is the control center of the system. The basic function of the fuel system is to control fuel delivery to the engine under all operating conditions.
The main control sensor is the heated oxygen sensor (HO2S). The HO2S is located in the exhaust system. The HO2S tells the control module the amount of oxygen in the exhaust gas. The control module changes the air to fuel ratio to the engine by controlling the fuel injector. Efficient catalytic converter operation requires a 14.7:1 air to fuel ratio. Because of the constant measuring and adjusting of the air to fuel ratio, the fuel injection system is called a Closed Loop system.
Several other important engine operation parameters include the following items:
| • | The engine speed |
| • | The manifold pressure |
| • | The engine coolant temperature |
| • | The throttle position |
These parameters determine the mode of engine operation.
Modes of Operation
The control module monitors the voltages from several sensors in order to determine how much fuel to give the engine. The fuel is delivered under one of several conditions called modes. The control module controls all of the modes.
Starting Mode
When the ignition switch is turned to the ON position, before engaging the starter, the control module energizes the fuel pump relay for 2 seconds allowing the fuel pump to build up pressure. The control module then checks the engine coolant temperature (ECT) sensor and the throttle position (TP) sensor in order to determine the proper air and fuel ratio for starting. The control module 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 the engine by depressing the accelerator pedal down to the floor and then crank the engine. The control module reduces the injector pulse width in order to increase the air to fuel ratio. The control module holds this injector rate as long as the throttle stays wide open and the engine speed is below a predetermined RPM. If the throttle is not held wide open, the control module 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 engine speed is above a predetermined RPM, the system goes into Open Loop operation. In Open Loop, the control module ignores the signal from the HO2S. The control module calculates the air to fuel ratio based on the inputs from the engine coolant temperature (ECT) sensor and manifold absolute pressure (MAP) sensor. The system stays in Open Loop until the following conditions are met:
- Both HO2S have varying voltage output, showing that they are hot enough to operate properly. This depends on engine temperature.
- The ECT sensor is above a specified temperature.
- A specific amount of time has elapsed after starting the engine.
The specific values for the above conditions vary with different engines. When these conditions are met, the system enters Closed Loop operation. In Closed Loop, the control module calculates the air to fuel ratio (injector on-time) based on the signal from the HO2S. This allows the air to fuel ratio to stay very close to 14.7:1.
A normal functioning system may go into Open Loop at idle if the heated oxygen sensor (HO2S) temperature drops below the minimum requirement to produce the voltage fluctuation.
Acceleration Mode
When the control module senses rapid changes in the throttle position and the manifold pressure, the system enters the acceleration mode. The system provides the extra fuel needed for smooth acceleration.
Deceleration Mode
When the driver releases the accelerator pedal, the air flow into the engine is reduced. The corresponding changes in the throttle position and manifold air pressure are relayed to the control module. The control module reduces the injector pulse width in order to reduce fuel flow. If the deceleration is very rapid, or for long periods, such as long closed throttle coast-down, the control module shuts off the fuel completely in order to protect the catalytic converter.
Fuel Cutoff Mode
The control module cuts off fuel from the fuel injectors when certain conditions are met. This fuel shut off mode allows the powertrain to protect itself from damage and also improve its driveability.
The control module disables 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 coast down -- reduces emissions and increases engine braking |
Converter Protection Mode
The control module constantly monitors engine operation and estimates the conditions that could result in high converter temperatures. If the control module determines the converter may overheat, it causes the system to return to Open Loop operation and enriches the fuel mixture.
Battery Voltage Correction Mode
When battery voltage is low, the control module compensates for the weak spark delivered by the ignition system by increasing the following items:
| • | The injector ON time |
| • | The idle RPM |
| • | The ignition dwell time |
