The throttle body assembly is mounted on the intake manifold plenum, and is used to control air flow into the engine, thereby controlling engine output. The throttle valve within the throttle body is opened by the driver through the accelerator controls. During engine idle, the throttle valve is almost closed, and air flow control is handled by the Idle Air Control (IAC) valve.

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

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.

The non-adjustable TP sensor is mounted on the throttle body assembly opposite the throttle lever. The TP sensor senses the throttle valve angle and relays that information to the VCM. Knowledge of throttle angle is one of the inputs needed by the VCM to generate the required injector control signals (pulses). For further information on replacement, refer to TP Sensor Replacement .

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 fuel delivery to the engine under all 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 HO2S is located in the exhaust manifold. 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 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 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 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.

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 manifold absolute pressure (MAP) sensor.

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 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. 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 14.7:1.

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 the driver releases the accelerator pedal, the air flow into the engine is reduced. The corresponding changes in the throttle position and the manifold air pressure are relayed to the control module, which reduces the injector pulse width, in order to reduce the fuel flow. If the decel 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.

In order to prevent possible engine damage from over-speed, the control module cuts off fuel from the fuel injectors when the engine speed is above approximately 5,500 RPM with the vehicle in any forward gear or reverse. In order to prevent tire damage, the control module also has a fuel cutoff in excess of 108 mph (173 km/h) based on the speed rating of the tires.

Fuel is also cutoff during rapid deceleration. See Deceleration 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, this causes the system to return to the Open Loop operation and enriches the fuel mixture.

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

Fuel System Central SFI

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 and poppet nozzles mounted in the intake manifold near each cylinder.

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 oil pressure 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 oil pressure 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 oil pressure 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

Injectors and Poppet Nozzles

Each fuel injector assembly is a solenoid-operated device, controlled by the VCM. The fuel injector assembly meters the pressurized fuel through a poppet nozzle (5) to a single engine cylinder.

The VCM energizes the injector solenoid, which opens an armature valve (3), allowing fuel to flow past the ball valve and through a fuel tube (1) to the poppet nozzle.

An increase in fuel pressure causes the poppet nozzle ball to open from its seat 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 (4) 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 (2). 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 for V6. Refer to Fuel Injector Balance Test for V8. Refer to Fuel Injector 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 Diagnosis (Diagnosis) for V6. Refer to Fuel System Diagnosis for V8.

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.

Throttle Position (TP) Sensor

The non-adjustable TP sensor is mounted on the throttle body assembly opposite the throttle lever. The TP sensor senses the throttle valve angle and relays that information to the VCM. Knowledge of throttle angle is one of the inputs needed by the VCM to generate the required injector control signals (pulses). For further information on replacement, refer to TP Sensor Replacement .

Fuel System Fuel Supply System

Central SFI Fuel System with Fuel Pressure Gauge