When you turn ON the ignition switch, the Vehicle Control Module (VCM) turns ON the in-tank fuel pump. The pump remains ON as long as the engine is cranking or running and the VCM is receiving reference pulses. If there are no reference pulses, the VCM shuts the fuel pump OFF within 2 seconds after the ignition was switched to the ON position or if the engine stops. The VCM will also turn ON the fuel pump for 2 seconds when the ignition is turned to the OFF position.

An electric fuel pump pumps the fuel through an in-line filter to the Central SFI unit. The pump is attached to the fuel level meter assembly inside of the fuel tank. The pump is designed to provide fuel pressure above what is needed by the fuel injectors. The pressure regulator keeps the fuel available to the injectors at a regulated pressure. Unused fuel is returned to the fuel tank by the fuel return pipe.

Tools Required

The numbers below refer to the step numbers on the diagnostic table.

2. Wrap a shop towel around the fuel pressure connection to absorb any small amount of fuel leakage that may occur when installing the fuel pressure gauge. Turn ON the ignition leaving the engine OFF, the fuel pressure should be 415-455 kPa (60-66 psi). This pressure is controlled by a spring pressure within the regulator assembly.

4. The fuel pressure that continues to fall is caused by one of the following items:

•	The in-tank modular fuel sender is not holding pressure.

•	The fuel pressure regulator valve is leaking.

•	A Central SFI injector is leaking.

•	Fuel line is leaking.

15. If the fuel pressure is less than 415 kPa (60 psi), it falls into the following 3 areas:

•	The pressure is regulated but less than 415 kPa (60 psi).The amount of fuel reaching the injector is sufficient, but the pressure is too low. The system will run lean, hard starting cold, no start, overall poor performance, and may set a DTC. Refer to Engine Cranks but Does Not Run

•

A restricted flow causing pressure drop -- Normally, a vehicle with a fuel pressure of less than 300 kPa (44 psi) at idle is inoperable. However, if the pressure drop occurs only while driving, the engine will normally surge then stop running as the pressure begins to drop rapidly. This is most likely caused by a restricted fuel line or plugged filter.

•	A leaking or contaminated pressure regulator valve or seat interface may not allow the regulated pressure to be achieved.

21. Notice: DO NOT allow the fuel pressure to exceed 517 kPa (75 psi). Excessive pressure may damage the fuel system.

    Restricting the fuel return line allows the fuel pressure to build above regulated pressure. With the battery voltage applied to the fuel pump test terminal, the pressure should rise above 455 kPa (66 psi), as the valve in the return line is partially closed.

26. When the engine is idling, the manifold pressure is low (high vacuum) and is applied to the fuel pressure regulator diaphragm. This will offset the spring and result in a lower fuel pressure. This idle pressure will vary somewhat depending on the barometric pressure; however, the pressure at idle should be less indicating the pressure regulator control.

The fuel pump relay is mounted in the underhood electrical center located in the engine compartment. For diagnosis of the fuel pump relay circuit, refer to Fuel Pump Relay Circuit Diagnosis.The fuel pump and engine oil pressure indicator switch is mounted in the engine block near the distributor. For diagnosis of the fuel pump circuit, refer to Fuel Pump Relay Circuit Diagnosis. Some failures of this system will result in an Engine Cranks But Will Not Run symptom. This table determines if the problem is caused by the ignition system, VCM, or fuel pump circuit.

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 fuel trim 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 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, OFF for 5 seconds.

The IAC valve affects the idle characteristics 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.

For diagnosis of the Fuel Pump Circuit refer to Fuel Pump Electrical Circuit Diagnosis .

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

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

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 5V 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 5V 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 three-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 reduces NOx, converting it to nitrogen. The VCM has the capability to monitor this process using HO2S 2. HO2S 2, located in the exhaust stream past the three-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 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 three-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 4V.

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 5V 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 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.