Fuel System Diagnosis

Fuel System Diagnosis

Diagnostic Fault Information

Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Initial Inspection and Diagnostic Overview

Circuit/System Description

The control module enables the fuel pump relay when the ignition switch is turned ON. The control module will disable the fuel pump relay within 2 seconds unless the control module detects ignition reference pulses. The control module continues to enable the fuel pump relay as long as ignition reference pulses are detected. The control module disables the fuel pump relay within 2 seconds if ignition reference pulses cease to be detected and the ignition remains ON.

The fuel tank stores the fuel supply. An electric turbine style fuel pump attaches to the fuel pump module inside the fuel tank. The fuel pump supplies high pressure fuel through the fuel filter and the fuel feed pipe to the fuel injection system. The fuel pump provides fuel at a higher rate of flow than is needed by the fuel injection system. The fuel pump also supplies fuel to a venturi pump located on the bottom of the fuel pump module. The function of the venturi pump is to fill the fuel pump module reservoir. The fuel pressure regulator, a part of the fuel pump module, maintains the correct fuel pressure to the fuel injection system. The fuel pump module contains a reverse flow check valve. The check valve and the fuel pressure regulator maintain fuel pressure in the fuel feed pipe and the fuel rail in order to prevent long cranking times.

Circuit/System Verification

Important:
* Inspect the fuel system for damage or external leaks before proceeding.
* Verify that adequate fuel is in the fuel tank before proceeding.
* The fuel pump relay may need to be commanded ON a few times in order to obtain the highest possible fuel pressure.
* Before proceeding with this test review the User Manual CH 48027-5 for Safety Information and Instructions.

1. Ignition ON, engine OFF, command the fuel pump relay ON with a scan tool. You should hear the fuel pump turn ON and OFF.

^ If the fuel pump does not operate, refer to Fuel Pump Electrical Circuit Diagnosis. Component Tests and General Diagnostics

2. Ignition OFF, all accessories OFF, install a CH-48027-100 Digital Pressure Gage. Refer to Fuel Pressure Gage Installation and Removal. Fuel Pressure Gage Installation and Removal
3. Ignition ON, engine OFF, command the fuel pump relay ON with a scan tool. Verify the fuel pressure is between 345-414 kPa (50-60 psi) and does not decrease more than 34 kPa (5 psi) in 5 minutes.

Circuit/System Testing

1. Important:
* The fuel pump relay may need to be commanded ON a few times in order to obtain the highest possible fuel pressure.
* DO NOT start the engine.

Ignition ON, engine OFF, command the fuel pump relay ON with a scan tool and observe the fuel pressure gage while the fuel pump is operating. Verify the fuel pressure is between 345-414 kPa (50-60 psi).

^ If the fuel pressure is greater than the specified range, replace the fuel pump module.
^ If the fuel pressure is less than the specified range, test, inspect, and repair the items listed below. If all items test normal, replace the fuel pump module.

* Restricted fuel feed pipe
* Restricted or plugged fuel filter
* Restricted or plugged strainer
* Inspect the harness connectors and the ground circuits of the fuel pump for poor connections.

2. Verify that the fuel pressure does not decrease more than 34 kPa (5 psi) in 5 minutes.

^ If the fuel pressure decreases more than the specified value, perform the following procedure:

1. Ignition OFF, relieve the fuel pressure. Refer to Fuel Pressure Relief.
2. Disconnect the chassis fuel feed hose from the engine compartment fuel pipe.
3. Connect the chassis fuel feed hose to the inlet side of SA9127E-7 Fuel Pressure/Flow Adapter using the J 43937 Fuel Pressure Adapter Lines.
4. Connect the engine compartment fuel pipe to the ON/OFF valve side of the SA9127E-7 using the J 43937.
5. Cap the pressure test port on the SA9127E-7 with the J 43936.
6. Open the valve on the SA9127E-7.
7. Ignition ON, command the fuel pump relay ON with a scan tool and bleed the air from the CH-48027-100.
8. Command the fuel pump relay ON for a minimum of 10 seconds with a scan tool.
9. Close the valve on the SA9127E-7.
10. Monitor the fuel pressure for 1 minute.

^ If the fuel pressure decreases more than 34 kPa (5 psi) within the specified time, locate and replace the leaking fuel injector(s).
^ If the fuel pressure does not decrease more than 34 kPa (5 psi) within the specified time, replace the fuel pump module.

3. Relieve the fuel pressure to 69 kPa (10 psi). Verify that the fuel pressure does not decrease more than 14 kPa (2 psi) in 5 minutes.

^ If the fuel pressure decreases more than the specified value, replace the fuel pump module.

4. Operate the vehicle within the conditions of the customer's concern while monitoring the fuel pressure with the CH-48027-100. The fuel pressure should not drop off during acceleration, cruise, or hard cornering.

^ If the fuel pressure drops off, test, inspect, and repair the items listed below. If all items test normal, replace the fuel pump module.

* Restricted fuel feed pipe
* Restricted or plugged fuel filter
* Restricted or plugged strainer
* Inspect the harness connectors and the ground circuits of the fuel pump for poor connections.

5. If the fuel system tests normal, refer to Symptoms - Computers and Control Systems. - Symptoms - Engine Controls

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure. Verification Tests

* Fuel Filter Replacement
* Fuel Hose/Pipes Replacement - Chassis
* Fuel Injector Replacement
* Fuel Pump Module Replacement