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 pump relay is mounted in the underhood electrical center located in the engine compartment.

This includes the fuel injector poppet assembly, fuel pressure regulator, fuel pump and fuel pump relay. The fuel system wiring schematic diagram is covered in Fuel Pump Circuit Diagnosis.

If a problem occurs in the fuel metering system, it usually results in either a rich or lean exhaust condition. This condition is sensed by the HO2S. This condition causes the VCM to change the fuel calculation (injector pulse width). The change made to the fuel calculation is indicated by a change in the short and Long Term FT values which can be monitored by a scan tool. A momentary change to the fuel calculation is indicated by the short term fuel trim value, while a prolonged change is indicated by the long term fuel trim value. Average fuel trim values will measure around 128. The averages may vary slightly from engine to engine.

Important: When using a scan tool to observe fuel trim values, remember that if the system is in control, no action is required unless a driveability symptom is present.

Listed below are examples of lean and rich HO2S signals with the system in control and out of control.

If both fuel trim values are fixed well above 128, see DTC P0131 for items which can cause a lean system. Refer to DTC DTC P0131 HO2S Circuit Low Voltage Bank 1 Sensor 1 .

If the fuel trim values are fixed well below 128, see DTC P0132 for items which can cause the system to run rich. Refer to DTC P0132 HO2S Circuit High Voltage Bank 1 Sensor 1 .

If a driveability symptom exists, refer to the particular symptom in Symptoms, for additional items to check.

Fuel delivery is controlled by the control module system.

The diagnosis of fuel control starts with Engine Cranks But Will Not Run. This table will test the fuel system to determine if there is a problem. Refer to Engine Cranks but Does Not Run .

Testing of the fuel injector circuit is located in the Fuel Injector Circuit Diagnosis table.

A fuel injector which does not open may cause a no-start condition. An injector which is stuck partially open could cause loss of pressure after sitting, resulting in extended crank times on some engines. Also, dieseling could occur because some fuel could be delivered to the engine after the key is turned OFF.

If the pressure regulator supplies pressure which is too low, poor performance could result. If the pressure is too high, exhaust odor may result.

The diagnosis of idle air control (IAC) can be found in Idle Air Control (IAC) System Diagnosis .

If the IAC valve is disconnected or connected when the engine is running, the idle RPM may be wrong. The IAC valve may be reset by turning the ignition switch ON for 10 seconds, and then OFF for 5 seconds.

The IAC valve affects the idle speed of the engine as well as throttle follow-up to compensation for sudden throttle closing. If it is open fully too much air will be allowed in the manifold and idle speed will be high. If it is stuck closed, too little air will be allowed in the manifold, and idle speed will be too low. If it is stuck part way open, the idle may be rough, and will not respond to engine load changes.

The relay has a terminal to test the fuel pump operation which is a separate terminal located near the fusible link cluster. By applying voltage at this terminal, it can be determined if the fuel pump will operate. This terminal will also prime the fuel line to the fuel injection unit.

An inoperative fuel pump will cause a no start condition. A fuel pump which does not provide enough pressure can result in poor performance.

The engine coolant temperature sensor is a thermistor (a resistor which changes value based on temperature) mounted in the engine coolant stream. Low coolant temperature produces a high resistance (100,000 ohms at -40°C (-40°F) while high temperature causes low resistance (70 ohms at 130°C (266°F).

The VCM supplies a 5 volt signal to the engine coolant temperature sensor through a resistor in the VCM and measures the voltage. The voltage will be high when the engine is cold, and low when the engine is hot. By measuring the voltage, the VCM calculates the engine coolant temperature. Engine coolant temperature affects most systems the VCM controls.

The scan tool displays engine coolant temperature in degrees. After engine start-up, the temperature should rise steadily to about 90°C (194°F) then stabilize when thermostat opens. If the engine has not been run for several hours (overnight), the engine coolant temperature and intake air temperature displays should be close to each other. A fault in the engine coolant sensor circuit should set DTC P0117 or DTC P0118.

The intake air temperature (IAT) sensor is a thermistor which changes value based on the temperature of air entering the engine. Low temperature produces a high resistance (100,000 ohms at -40°C (-40°F), while high temperature causes low resistance (70 ohms at 130°C (266°F). The VCM supplies a 5 volt signal to the sensor through a resistor in the VCM and measures the voltage. The voltage will be high when the incoming air is cold, and low when the air is hot. By measuring the voltage, the VCM calculates the incoming air temperature.

The IAT sensor signal is used to adjust spark timing according to incoming air density.

The scan tool displays temperature of the air entering the engine, which should read close to ambient air temperature when engine is cold, and rise as underhood temperature increases. If the engine has not been run for several hours (overnight) the IAT sensor temperature and engine coolant temperature should read close to each other. A failure in the IAT sensor circuit should set DTC P0112 or DTC P0113.

DTC P0107 or DTC P0108 indicates a failure in the MAP sensor circuit, which may effect fuel metering.

The exhaust heated oxygen sensor (HO2S 1) is mounted in the exhaust manifold where it can monitor the oxygen content of the exhaust gas stream.

The oxygen content in the exhaust reacts with the sensor to produce voltage output. This voltage should constantly fluctuate from approximately 100 mV (high oxygen content - lean mixture) to 900 mV (low oxygen content - rich mixture). The heated oxygen sensor voltage can be monitored with a scan tool.

By monitoring the voltage output of the heated oxygen sensor, the VCM calculates what fuel mixture command to give to the injector (lean mixture-low HO2S 1 voltage=rich command, rich mixture-high HO2S 1 voltage=lean command).

The heated oxygen sensor circuit, if open, should set a DTC P0134 and the scan tool will display a constant voltage between 350-550 mV. A constant voltage below 250 mV in the sensor circuit should set DTC P0131, while a constant voltage above 750 mV in the circuit should set DTC P0132. DTC P0131 and DTC P0132 could also be set as a result of fuel system problems.

In order to control emissions of hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx), a 3-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also converts the NOx to nitrogen. The VCM has the capability to monitor this process using HO2S 2. The HO2S 2, located in the exhaust stream past the 3-way catalytic converter, produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalyst's ability to convert exhaust emissions effectively. A problem with the HO2S 2 electrical circuits should set DTC P0137, P0138 or P0140, depending on the specific condition. If the catalyst is functioning correctly, the HO2S 2 signal will be far less active than that produced by the HO2S 1. If a problem exists which causes the VCM to detect excessive HO2S 2 activity outside of an acceptable range for an extended period of time, the VCM will set DTC P0420, indicating that the 3-way catalytic converter's oxygen storage capacity is below a threshold considered acceptable.

The throttle position (TP) sensor is a potentiometer connected to the throttle shaft on the throttle body. By monitoring the voltage on the signal line, the VCM calculates throttle position. As the throttle valve angle is changed (accelerator pedal moved), the TP sensor signal also changes.

At a closed throttle position, the output of the TP sensor is low. As the throttle valve opens, the output increases so that at wide open throttle (WOT), the output voltage should be above 4 volt.

The VCM calculates fuel delivery based on throttle valve angle (driver demand). A broken or loose TP sensor may cause intermittent bursts of fuel from an injector and unstable idle because the VCM thinks the throttle is moving. A problem in the TP sensor 5 volt reference or signal circuits should set either a DTC P0122 or DTC P0123. A problem with the TP sensor ground circuit may set DTCs P0123 and P0117. Once a DTC is set, the VCM will use an artificial default value based on mass air flow for TP sensor and some vehicle performance will return. A high idle may result when either DTC P0122 or DTC P0123 is set.

This check should be performed when TP sensor attaching parts have been replaced. A scan tool can be used to read the TP signal output voltage.

1. Connect the DMM from TP sensor connector terminal B (BLK wire) to terminal C (DK Blue wire). Jumpers for terminal access can be made using terminals 1214836 and 12014837.
2. With ignition ON, engine OFF, the TP signal voltage should be less than 1.25 volts if more than 1.25 volts verify free throttle movement. If still more than 1.25V, replace TP sensor.
3. Remove the voltmeter and jumpers, reconnect the TP sensor connector to the sensor.

When there is a problem with the idle refer to Idle Air Control (IAC) System Diagnosis .

The scan tool scan tool displays crankshaft position sensor data as engine speed (RPM). An error in the crankshaft position sensor circuit should set a DTC P0336, P0337, P0338, or a P0339. The crankshaft position sensor provides a signal through the ignition control module which the VCM uses as reference to calculate RPM and crankshaft position.

The scan tool scan tool will display the camshaft position sensor data as a 0 and 1. As the sensor pulses, the scan data will switch from 0 to 1. All camshaft position sensor data should be checked at idle. An error in the camshaft position sensor circuit should set a DTC P0340. The camshaft position sensor sends a signal to the VCM which uses it as a sync pulse to trigger the injectors in proper sequence.

The VCM uses this signal to determine the position of the #1 piston during its power stroke. This signal is used by the VCM to calculate fuel injection mode of operation. A loss of this signal will set DTC P0340.

If the cam signal is lost while the engine is running, the fuel injection system will shift to a calculated fuel injection mode based on the last fuel injection pulse, and the engine will continue to run. The engine can be restarted and will run in the calculated mode as long as the fault is present .

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.

(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. The extension spring also checks the pressure between the ball and seat and the injector armature and fuel tube shutoff. The flow control solenoid assembly (2) is electrically activated to allow fuel to flow from the injector to the flexible fuel line to the poppet nozzle.

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 with Tech 2 . Refer to 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, with 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, reduced 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 .

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 observed 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 commands additional counts 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 affects the idle speed 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 or rough 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 Throttle Position Sensor Replacement .