Fuel System Overview
The fuel 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 the fuel delivery to the engine under all of the operating conditions. The following two types of fuel injection systems deliver the fuel to the engine:
| • | The Central Sequential Port Fuel Injection (Central SFI) |
| • | The Sequential Multiport Port Fuel Injection (SFI) |
The main control sensor is the (Heated) Oxygen Sensor (HO2S). The (H)O2S is located in the exhaust manifold. The (H)O2S 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 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.
The following are the 3 separate classifications of fuel systems:
| • | The Central Sequential Fuel Injection (Central SFI) |
| • | The Sequential Multiport Fuel Injection (MFI) |
| • | The fuel supply system |
Fuel System Application Table
The following is a list of the 2 types of fuel systems with the various applications :
| • | The Central Sequential Multiport fuel injection (Central SFI) for the 4.3L S/T (VIN W) |
| • | The Multiport Fuel Injection (MFI) for the 2.2L S/T (VIN 4) |
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 key is first turned ON, the control module turns on the fuel pump relay for 2 seconds, and the fuel pump builds up pressure to the throttle body. The control module checks the Engine Coolant Temperature (ECT) sensor, the Intake Air Temperature (IAT) sensor, the Throttle Position (TP) sensor, the Manifold Absolute Pressure (MAP) sensor, and the ignition signal. The control module determines the proper air to fuel ratio for starting. This ranges from 1.5:1 at -36°C (-33°F) to 14.6:1, at 94°C (201°F) running temperature.
Clear Flood Mode
If the engine floods, clear the engine by depressing the accelerator pedal down to the floor. The control module then pulses the injector at a 16.5:1 air to fuel ratio. The control module holds this injector rate as long as the throttle stays wide open and the engine is below 600 RPM. If the throttle position becomes less than 65 percent, the control module returns to the starting mode.
Run Mode
The run mode is the mode under which the engine operates most of the time. In this mode, the engine operates in either Open Loop or Closed Loop.
Open Loop
When the engine is first started and it is above 400 RPM, the system goes into the Open Loop operation. In the Open Loop, the control module ignores the signal from the HO2S, and the control module calculates the air to fuel ratio based on the inputs from the Engine Coolant Temperature (ECT) sensor and the Manifold Absolute Pressure (MAP) sensor.
The system stays in an Open Loop until the following conditions are met:
- The HO2S has varying voltage output, showing that it is hot enough to operate properly. This depends on engine temperature.
- The Engine Coolant Temperature (ECT) sensor is above a specified temperature.
- A specific amount of time has elapsed after starting the engine.
A normal functioning system may go into an Open Loop at idle if the Heated Oxygen Sensor (HO2S) temperature drops below the minimum requirement to produce the voltage fluctuation.
Closed Loop
The specific values for the above conditions vary with different engines. The Electronically Erasable Programmable Read Only Memory (EEPROM) stores the values. When these conditions are met, the systems goes into a Closed Loop operation. In a 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 14.6:1.
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 deceleration occurs, the fuel remaining in the intake manifold can cause excessive emissions and backfiring. When the control module observes a fast reduction in the throttle opening and a sharp decrease in the manifold pressure, the control module causes the system to enter the deceleration mode by reducing the amount of fuel delivered to the engine. When deceleration is very fast, the control module cuts OFF the fuel completely for short lengths of time.
Highway Fuel Mode (Semi-Closed Loop)
This mode comes into operation at highway speeds. The purpose of the mode is to improve the fuel economy. For the control module to operate in this mode, the control module first must sense the correct engine coolant temperature, ignition control, canister purge activity, and constant engine speed. During the semi-Closed-Loop operation, there will exist the following items will exist:
| • | Very little long term fuel trim (formerly known as Block Learn) |
| • | Short term fuel trim (formerly known as an Integrator) |
| • | The heated oxygen sensor values reading below 100 millivolts |
Decel En-Leanment
Upon deceleration, the control module senses a high MAP vacuum (low voltage or kPa) to command a leaner air/fuel mixture in order to reduce emissions.
Note that the control module can trigger this condition (decel en-leanment) while the vehicle is not moving.
Decel En-Leanment Operation
The PCM can misdiagnose the Decel-En-leanment mode of operation as a lean condition. The control module runs the system lean on decel, or if the MAP sensor senses a low voltage (high engine vacuum), with the vehicle standing still, the control module leans out the fuel delivery.
If the technician notes, while testing a control module system (with a scan tool) with the transmission in P, that the HO2S reading is low (usually below 100 mV), the long term and short term fuel trims are both around 128 counts, lower the engine speed to 1000 RPM.
If the oxygen sensor and the long term fuel trim numbers respond normally at this RPM, this may indicate that the system was fooled into the decel en-leanment mode of operation. If the heated oxygen sensor and the long term fuel trim numbers do not respond at the lower RPM readings, other problems exist with the vehicle.
Battery Voltage Correction Mode
When the battery voltage is low, the control module can compensate for a weak spark delivered to the distributor by increasing the following items:
| • | The injector ON time |
| • | The idle RPM |
| • | The ignition dwell time |
Fuel Cutoff Mode
No fuel is delivered by the injector when the ignition is OFF. This prevents dieseling. Also, the fuel is not delivered if no reference pulses are seen from the Ignition system, which means the engine is not running. The fuel cutoff also occurs during high engine RPM. In order to protect the internal engine components from damage, the control module disables the fuel when the vehicle speed reaches a specified speed.
Important: The engine is at the Operating Temperature 92°C to 104°C (196°F to 222°F)
Engine | Transmission | Gear (Drive/Neutral) | Idle Speed (RPM) | IAC Counts* | Open/Closed Loop** |
|---|---|---|---|---|---|
4.3L Truck (Under 8500 GVW) | Auto Manual | Drive Neutral | 590±25 550±25 | 50-30 50-30 | CL |
2.2L Truck | Auto | -- | -- | 5-50 | CL |
2.2L Truck | Manual | -- | -- | 5-50 | CL |
*On Manual transmission vehicles the scan tool displays the RDL in neutral. *Add 2 counts for the engines with less than 500 miles. Add 2 counts for every 1000 ft. above sea level (4.3L and V8). **Let the engine idle until proper fuel control status (Open/Closed Loop) is reached. | |||||
Fuel System SFI General Description
Fuel System Operation
The SFI system consists of the following parts:
| • | The fuel supply system (fuel tank, pump, pipes) |
| • | The fuel pump electrical circuit |
| • | Intake manifold assembly, including the following items: |
| - | The lower manifold assembly, including the following items: |
| • | The SFI fuel injectors |
| • | The fuel pressure regulator assembly |
| - | The upper manifold assembly, including the following items: |
| • | The throttle valve |
| • | The Idle Air Control (IAC) valve |
| • | The Throttle Position (TP) sensor |
Fuel Supply System
The electric fuel pump is located in the fuel tank with the gage sending unit.
An electric fuel pump pumps the fuel through an in-line fuel filter to the lower manifold assembly fuel passage. The pump provides fuel at a pressure greater than is needed by the injectors.
The pressure regulator keeps fuel available to the fuel injectors at a regulated pressure. The pressure regulator is attached to the end of the lower manifold assembly.
Fuel, in excess of the injector needs, is returned to the fuel tank by a separate return line.
Fuel Pump Electrical Circuit
When the ignition switch is turned to the ON position, the PCM energizes the fuel pump relay for 2 seconds allowing the fuel pump to pressurize the fuel system. If the PCM does not receive any ignition reference pulses (engine cranking or running) within 2 seconds, the PCM de-energizes the fuel pump relay. This causes the fuel pump to stop.
As a backup system to the fuel pump relay, the fuel pump can be energized by the fuel pump and engine oil pressure indicator switch. The switch has the 2 following internal circuits:
- The oil pressure indicator or gage in the instrument cluster
- A normally open switch which closes when the oil pressure reaches about 28 kPa (4 psi)
If the fuel pump relay fails, the fuel pump and engine oil pressure indicator switch runs the fuel pump.
An inoperative fuel pump relay can result in long cranking times. The fuel pump and engine oil pressure indicator switch energizes the fuel pump as soon as the oil pressure reaches about 28 kPa (4 psi).
Intake Manifold Assembly (Upper and Lower Manifold Assemblies)
The intake manifold is made up of the upper (1) and lower (5) manifold assemblies.
The upper manifold assembly is a single casting which includes the following items:
| • | The upper portion of the air chamber (plenum) |
| • | The air inlet (fitted with a throttle valve) |
This assembly replaces the individual throttle body assembly and plenum components.
The throttle valve controls the air flow into the engine, thereby, controlling engine output. The driver opens the throttle valve through the accelerator controls. During engine idle, the throttle valve is almost closed. The air flow control is handled by the Idle Air Control (IAC) valve (2). Various components generate the vacuum signals needed.
The nonadjustable, throttle shaft-driven, Throttle Position (TP) sensor (3) is attached to the upper manifold assembly opposite the throttle cam lever.
The lower manifold is a single casting that includes the intake runners and a longitudinal fuel passage with intersecting injector bores in each runner. This assembly replaces the individual fuel rail and the manifold components used in other systems. The information on the fuel injectors (4) and the pressure regulator (6) are covered separately.
Bottom Feed Type, Sequential Fuel Injectors
