Fuel System Diagnosis Fuel Supply System

Notice: Do not use fuels containing methanol in order to prevent damage and corrosion to the fuel system.

Alcohol-in-fuel can be detrimental to the fuel system components. Alcohol-in-fuel can 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 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 in order to detect the presence of alcohol in the fuel. In this procedure, water is used to extract the alcohol from the fuel. If contaminated fuel is suspected, the fuel must be completely drained from the tank and replaced with known good fuel.

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 and the fuel system should be cleaned. Refer to Fuel System Cleaning .

1. Using a 100 ml cylinder with 1 ml graduation marks, fill with fuel to the 90 ml mark.
2. Add 10 ml of water to bring the total fluid volume to 100 ml.
3. Install a stopper.
4. Shake vigorously for 10 to 15 seconds.
5. Carefully loosen the stopper to release the pressure.
6. Close the stopper.
7. Shake vigorously again for 10 to 15 seconds.
8. Put the graduated 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 measure greater than 10 ml.

For example, if the volume of the lower layer is increased to 15 ml, it will indicate at least 5 % alcohol in fuel. The actual amount of alcohol may be somewhat greater because this procedure does not extract all of the alcohol from the fuel.

Refer to Fuel System Pressure Test .

An inoperative fuel pump would cause a no start condition. A fuel pump which does not provide enough pressure can result in poor performance. Refer to Fuel System Pressure Test for procedures.

The diagnosis of the fuel filter is covered in Fuel System Pressure Test .

A plugged fuel filter may cause a restricted fuel delivery or a no start condition.

The diagnosis of gasoline odor may be a condition of a leaking fuel feed, a return pipe or hose. Fuel pipes that are pinched, plugged, or mis-routed may cause restricted fuel delivery.

The diagnosis of gasoline odor may be a condition of a leaking fuel tank, filler neck, or filler cap.

A defective filler cap or a plugged or pinched vapor pipe can cause a collapsed fuel tank.

Loose mounting straps, or any foreign material in the tank may cause a rattle at the fuel tank.

Check for correct cable routing or binding. Correct as necessary.

Fuel System Diagnosis Central SFI

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 Electrical 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 relay circuit, refer to Fuel Pump Electrical Circuit Diagnosis .

Some failures of this system will result in an Engine Cranks But Will Not Run symptom. If this condition exists, refer to Engine Cranks but Does Not Run .

This Table determines if the problem is caused by the ignition system, VCM, or fuel pump circuit. If it is determined to be a fuel problem, refer to Fuel System Pressure Test .

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 Relay Circuit Diagnosis. Refer to Fuel Pump Electrical 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 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 Sensor 1 .

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