System Diagnosis - Fuel Metering System

The fuel pump relay is located in the rear relay center (1) next to the rear fuse block (2) under the rear seat cushion. Refer to Driveability and Emissions for the locator view. For diagnosis of the fuel pump relay circuit, refer to DTC P0231 Fuel Pump Feedback Circuit Low Voltage.

Always start with the Powertrain OBD System Check This will reduce diagnosis time and prevent unnecessary parts replacement. Diagnosis of the fuel metering system is covered in this starting with the, Fuel System Diagnosis. This table cover the diagnosis of the injectors, pressure regulator, and the fuel pump.

If a malfunction occurs in the fuel metering system, it usually results in either a rich or lean exhaust condition. This condition is sensed by the HO2S, and causes the PCM to change the fuel calculation (injector pulse width). The change made to the fuel calculation is indicated by a change in the short term fuel trim 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 are around 128, but vary slightly from engine to engine.

Note 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 conditions with the system in control and out of control.

^ A momentary lean condition (system is in control) will appear on the scan tool as:

- Short term fuel trim value above 128 (adding fuel).

- Long term fuel trim value around 128.

^ A prolonged lean condition (system is in control) will appear on the scan tool as:

- Short term fuel trim value around 128.

- Long term fuel trim value above 128 (added fuel).

^ A prolonged lean condition (system is out of control) will appear on the scan tool as:

- Short term fuel trim value well above 128 (adding fuel).

- Long term fuel trim value well above 128 (added fuel).

If both fuel trim values are fixed well above 128, refer items which can cause a lean system.

^ A momentary rich condition (system is in control) will appear on the scan tool as:

- Short term fuel trim value less than 128 (reducing fuel).

- Long term fuel trim value around 128.

^ A prolonged rich condition (system is in control) will appear on the scan tool as:

- Short term fuel trim value around 128.

- Long term fuel trim value less than 128 (reduced fuel).

^ A prolonged rich condition (system is out of control) will appear on the scan tool as:

- Short term fuel trim value much less 128 (reducing fuel).

- Long term fuel trim value much less 128 (reduced fuel).

Note: If both fuel trim values are fixed well below 128, refer to items which can cause the system to run rich. If a driveability symptom exists, refer to the particular symptom in Symptoms for additional items to check.

Alcohol concentrations greater than 10% in fuel can be detrimental to fuel system components and may cause driveability problems such as hesitation, lack of power, stall, no start, etc.

The problems may be due to fuel system corrosion and subsequent fuel filter plugging, deterioration of rubber components, and/or air-fuel mixture leaning.

Various types and concentrations of alcohol are used in commercial fuel. Some alcohol is more detrimental to fuel system components than others. If an excessive amount of alcohol in the fuel is suspected as the cause of a driveability condition, the following procedure may be used to detect the presence of alcohol in the fuel.


Testing Procedure
The fuel sample should be drawn from the bottom of the tank so that any water present in the tank will be detected. The sample should be bright and clear. If the sample appears cloudy, or contaminated with water, as indicated by a water layer at the bottom of the sample, this procedure should not be used. The fuel system should be cleaned, refer to Fuel System Cleaning 4.0L, Fuel System Cleaning 3.8L.

1. Take a 100 ml specified cylinder with 1 ml graduation marks, fill the cylinder with fuel to the 90 ml mark.

2. Add 10 ml of water in order to bring the total fluid volume to 100 ml and install a stopper.

3. Shake the cylinder vigorously for 10 to 15 seconds.

4. Carefully loosen the stopper in order to release the pressure.

5. Re-install the stopper and shake the cylinder vigorously again for 10 to 15 seconds.

6. Put the cylinder on a level surface for approximately 5 minutes in order to allow adequate liquid separation.

If alcohol is present in the fuel, the volume of the lower layer, which would now contain both alcohol and water, will be greater than 10 ml. For example, if the volume of the lower layer is increased to 15 ml, this indicates at least 5 percent alcohol in the fuel. The actual amount of alcohol may be somewhat greater because this procedure does not extract all of the alcohol from the fuel.


Circuit Description
Refer to Injectors 1-4-6-7 wiring view and Injectors 2-3-5-8 wiring view for an electrical diagram of this circuit.

The Fuel System Check provides a quick analysis of the fuel system. This chart then identifies more detailed diagnosis required based on the symptoms present.

The Fuel Leak/Odor Check provides a means of identifying a fuel system component that is causing a leak. This also provides information on how to diagnosis a fuel odor condition.


Diagnostic Aids
^ Nylon fuel lines are not repairable and must be replaced.

^ Evaporative Emission Control (EVAP) system problems may cause a fuel odor condition.

^ Rich engine operation may also cause a fuel odor condition.


Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.

5. The vehicle should be operated the same conditions as when the leak/odor occurs.