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 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:

These parameters determine the mode of engine operation.

The following are the 3 separate classifications of fuel systems:

The following is a list of the 2 types of fuel systems with the various applications :

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.

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.

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.

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:

1. The HO2S has varying voltage output, showing that it is hot enough to operate properly. This depends on engine temperature.
2. The Engine Coolant Temperature (ECT) sensor is above a specified temperature.
3. 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.

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.

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.

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:

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.

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.

When the battery voltage is low, the control module can compensate for a weak spark delivered to the distributor by increasing the following items:

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)

Fuel System Fuel Metering System Component

The fuel metering system consists of the following parts:

The fuel supply is stored in the fuel tank. An electric fuel pump, located in the fuel tank with the gauge sending unit, pumps fuel through an in-line fuel filter to the fuel meter body assembly.

The pump provides fuel at a pressure greater than is needed by the injectors. The fuel pressure regulator, part of the fuel meter body assembly, keeps the fuel to the injectors at a regulated pressure. The unused fuel is returned to the fuel tank via a separate line.

When the ignition switch is turned to the ON position (before engaging the starter), the VCM energizes the fuel pump relay for 2 seconds causing the fuel pump to pressurize the fuel system. If the VCM does not receive the ignition reference pulses (engine cranking or running) within 2 seconds, the control module shuts OFF the fuel pump relay, causing the fuel pump to stop.

As a backup system to the fuel pump relay, the fuel pump and engine oil pressure indicator switch can energize the fuel pump. The switch has 2 internal circuits. One circuit operates the oil pressure indicator or gage in the instrument cluster. The other circuit is 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 the engine oil pressure indicator switch runs the fuel pump.

An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold. The fuel pump and the engine oil pressure indicator switch energizes the fuel pump as soon as oil pressure reaches about 28 kPa (4 psi).

(1)	Fuel Pressure Regulator Assembly
(2)	Fuel Meter Body
(3)	Fuel Line
(4)	Fuel Injector Assembly
(5)	Poppet Nozzle
(6)	Fuel Pressure Regulator Assembly Retainer

The fuel meter body assembly is mounted to the lower portion of the intake manifold. The assembly performs the following functions:

•	Allows for an even distribution of fuel to the injectors

•	Integrates the fuel pressure regulator into the fuel metering system

SFI Fuel Injectors and Poppet Nozzles

Each SFI fuel injector assembly is a solenoid-operated device, controlled by the VCM. The SFI fuel injector assembly meters the pressurized fuel through a poppet nozzle to a single engine cylinder.

The VCM energizes the injector solenoid, which opens an armature valve (1), allowing fuel to flow past the ball valve and through a fuel tube to the poppet nozzle.

An increase in fuel pressure causes the poppet nozzle ball to open from its seat (2) against the extension spring force. This allows the fuel to flow from the nozzle (at approximately 280 kPa (40 psi)).

De-energizing the injector solenoid closes the armature. De-energizing also reduces the fuel pressure acting on the poppet nozzle ball. The extension spring closes the ball to the seat. The extension spring also checks the pressure between the ball and seat and the injector armature and fuel tube shutoff.

An injector poppet nozzle that is stuck partly open would cause a loss of pressure after the engine shut down. Consequently, the driver would notice long cranking times on some engines. Dieseling could also occur because the fuel injector could deliver some fuel to the engine after the driver turns the ignition to OFF. These components are diagnosed in The Injector Balance Test and The Injector Coil Test. Refer to Fuel Injector Balance Test and Fuel Injector Solenoid Coil Test .

Fuel Pressure Regulator Assembly

The fuel pressure regulator (1) is a diaphragm-operated cartridge relief valve with the fuel pump pressure on one side and the regulator spring pressure and intake manifold vacuum on the other. A retainer (2) holds the fuel pressure regulator.

The regulator's function is to maintain a constant pressure differential across the injectors at all times. The pressure regulator compensates for engine load by increasing the fuel pressure as engine vacuum drops.

With the ignition ON leaving the engine off (zero vacuum), the fuel pressure at the pressure test connection should be 415-455 kPa (60-66 psi). If the pressure is too low, poor performance could result. If the pressure is too high, excessive odor may result. The Fuel System Diagnosis has information on diagnosing fuel pressure conditions. Refer to Fuel System Pressure Test .

Throttle Body Assembly

The throttle body assembly is a downdraft design. The throttle body is mounted on the intake manifold plenum. The VCM uses the throttle body in order to control the air flow into the engine, thereby, controlling the engine output.

The throttle valve within the throttle body is opened by the driver through the accelerator controls. During the engine idle, the throttle valve is almost closed, and the iIdle air control (IAC) valve handles the air flow control.

The throttle body also provides the location for mounting the throttle position (TP) sensor. The throttle body also senses changes in the engine vacuum due to the throttle valve position. The vacuum ports are located at, above, or below the throttle valve in order to generate the vacuum signals that are needed by the various components.

Idle Air Control (IAC) Valve Assembly

The purpose of the IAC valve assembly is to control the engine idle speed while preventing engine stalls due to changes in the engine load.

The IAC valve, mounted in the throttle body assembly, controls the bypass air around the throttle valve. By moving a conical valve known as a pintle IN toward the seat (in order to decrease the air flow), or OUT away from the seat (in order to increase the air flow), a controlled amount of air moves around the throttle valve.

If the engine speed is too low, more air is bypassed around the throttle valve in order to increase the RPM. If the engine speed is too high, less air is bypassed around the throttle valve in order to decrease the RPM.

The VCM moves the IAC valve in small steps, called counts which can be measured by using a scan tool connected to the Data Link Connector (DLC).

During idle, the proper position of the IAC valve is calculated by the VCM. This position is based on the battery voltage, the engine coolant temperature, the engine load, and the engine RPM. If the RPM drops below specification and the throttle valve is closed, the VCM senses a near stall condition, and then the VCM calculates a new valve position in order to prevent stalling.

If the IAC valve is disconnected and reconnected while the engine is running, the resulting idle RPM may be wrong. This will require the resetting of the IAC valve.

After running the engine, the IAC valve will reset when the ignition is turned OFF. The IAC valve should only be disconnected or connected with the ignition OFF.

If the VCM is without battery power for any reason, the programmed position of the IAC valve pintle is lost. The control module replaces the lost position with a default value. In order to return the IAC valve pintle to the correct position, see the Idle Learn Procedure.

The IAC valve affects the idle characteristics of the vehicle. A fully retracted valve allows too much air into the manifold causing a high idle speed. A valve which is stuck closed allows too little air in the manifold, causing a low idle speed. If the valve is stuck part way open, the idle may be rough, and the idle will not respond to the engine load changes.

Fuel System Fuel Supply System

Central SFI Fuel System with Fuel Pressure Gauge