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Home >> GMC >> 2004 >> Jimmy 2D Utility, Part Time, Standard >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System - 4.3L (Troubleshooting & Diagnosis) >> Troubleshooting >> Fuel System Diagnosis >> System Description
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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 two 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 two seconds if ignition reference pulses cease to be detected and the ignition remains ON.

The fuel tank stores the fuel supply. The electric fuel pump supplies fuel through an in-line fuel filter to the fuel injection system. The pump provides fuel at a higher rate of flow than is needed by the fuel injection system. The fuel pressure regulator maintains the correct fuel pressure to the fuel injection system. A separate pipe returns unused fuel to the fuel tank.

Fuel System Diagnosis

Step Action Values Yes No
1 Did you perform the Diagnostic System Check-Engine Controls? - Go to Step 2  Go to Diagnostic System Check - Engine Controls
2
IMPORTANT: Inspect the fuel system for damage or external leaks before proceeding with this diagnostic.
  1. Turn ON the ignition, with the engine OFF.
  2. Command the fuel pump relay ON with a scan tool.
Does the fuel pump operate?		 - Go to Step 3  Go to Fuel Pump Electrical Circuit Diagnosis 
3
IMPORTANT: Verify that adequate fuel is in the fuel tank before proceeding with this diagnostic.
  1. Turn OFF the ignition.
  2. Turn OFF all accessories.
  3. Install a fuel pressure gage. Refer to Fuel Pressure Gage Installation and Removal .
  4. Turn ON the ignition, with the engine OFF.
    5. 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.
  5. Command the fuel pump relay ON with a scan tool.
  6. Observe the fuel pressure gage with the fuel pump commanded ON.
Is the fuel pressure within the specified range?		 379-427 kPa (55-62 psi) Go to Step 4  Go to Step 10 
4
IMPORTANT: The fuel pressure may vary slightly when the fuel pump stops operating. After the fuel pump stops operating, the fuel pressure should stabilize and remain constant.
Monitor the fuel pressure gage for 1 minute.Does the fuel pressure decrease by more than the specified value?		 34 kPa (5 psi) Go to Step 8  Go to Step 5 
5
  1. Relieve the fuel pressure to the first specified value.
  2. Monitor the fuel pressure gage for 5 minutes.
Does the fuel pressure decrease by more than the second specified value?		 69 kPa (10 psi)
14 kPa (2 psi)		 Go to Step 22  Go to Step 6 
6
  1. Turn ON the ignition, with the engine OFF.
  2. Monitor the fuel pressure gage.
  3. Start the engine.
Does the fuel pressure decrease by the specified value when the engine is started?		 21-69 kPa (3-10 psi) Go to Step 7  Go to Step 21 
7
  1. Operate the vehicle within the conditions to reproduce the original symptoms.
  2. Monitor the O2 and the fuel trim parameters with a scan tool.
Do the scan tool parameters indicate a lean condition?		 - Go to Step 15  Go to Symptoms - Engine Controls 
8
  1. Turn OFF the ignition.
  2. Relieve the fuel pressure. Refer to Fuel Pressure Relief Procedure .
  3. Raise the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
  4. Disconnect the fuel feed pipe from the fuel filter outlet. Refer to Quick Connect Fitting(s) Service (Plastic Collar) .
  5. Disconnect the rear fuel return pipe from the chassis fuel pipe. Refer to Quick Connect Fitting(s) Service (Plastic Collar) .
  6. Drain any remaining fuel into an approved gasoline container.
  7. Install the J 37287  Fuel Line Shut-off Adapters between the rear fuel pipes and the chassis fuel pipes.
  8. Open the valves on the fuel pipe shut-off adapters.
  9. Turn ON the ignition, with the engine OFF.
  10. Command the fuel pump relay ON with a scan tool.
  11. Bleed the air from the fuel pressure gage.
  12. Command the fuel pump relay ON and then OFF with a scan tool.
  13. Close the fuel feed pipe shut-off valve.
  14. Monitor the fuel pressure gage for 1 minute.
Does the fuel pressure remain constant?		 - Go to Step 17  Go to Step 9 
9
  1. Turn OFF the ignition.
  2. Open the fuel feed pipe shut-off valve.
  3. Turn ON the ignition, with the engine OFF.
  4. Command the fuel pump relay ON and then OFF with a scan tool.
  5. Close the fuel return pipe shut-off valve.
  6. Monitor the fuel pressure gage for 1 minute.
Does the fuel pressure remain constant?		 - Go to Step 21  Go to Step 18 
10 Is the fuel pressure more than the specified value? 427 kPa (62 psi) Go to Step 12  Go to Step 11 
11 Is the fuel pressure more than the specified value? 0 kPa (0 psi) Go to Step 14  Go to Step 15 
12
  1. Turn OFF the ignition.
  2. Relieve the fuel pressure. Refer to Fuel Pressure Relief Procedure .
  3. Raise the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
  4. Disconnect the rear fuel return pipe from the chassis pipe. Refer to Quick Connect Fitting(s) Service (Plastic Collar) .
  5. Attach a length of flexible hose to the chassis fuel pipe.
  6. Place the open end of the flexible fuel hose into an approved gasoline container.
  7. Turn ON the ignition, with the engine OFF.
  8. Monitor the fuel pressure gage while the fuel pump is operating.
Is the fuel pressure within the specified range?		 379-427 kPa (55-62 psi) Go to Step 20  Go to Step 13 
13 Inspect the engine fuel return pipe and the chassis fuel return pipe for a restriction.
Did you find and correct the condition?		 - Go to Step 23  Go to Step 21 
14
  1. Turn OFF the ignition.
  2. Relieve the fuel pressure. Refer to Fuel Pressure Relief Procedure .
  3. Raise the vehicle. Refer to Lifting and Jacking the Vehicle in General Information.
  4. Disconnect the rear fuel return pipe from the chassis pipe. Refer to Quick Connect Fitting(s) Service (Plastic Collar) .
  5. Install the J 37287  Fuel Line Shut-off Adapter between the rear fuel return pipe and the chassis fuel pipe.
  6. Open the valve in the fuel pipe shut-off adapter.
  7. Turn ON the ignition, with the engine OFF.
  8. Bleed the air from the fuel pressure gage.
  9. Monitor the fuel pressure gage.
    10. NOTE: DO NOT allow the fuel pressure to exceed 517 kPa (75 psi). Excessive pressure may damage the fuel system.
  10. Command the fuel pump relay ON with a scan tool.
  11. Slowly close the valve in the fuel return pipe shut-off adapter, while the fuel pump is operating.
Does the fuel pressure increase to more than the specified value?		 427 kPa (62 psi) Go to Step 21  Go to Step 15 
15 Inspect the following components for a restriction:
  • The fuel filter
  • The fuel feed pipe

Did you find and correct the condition?		 - Go to Step 23  Go to Step 16 
16 Inspect the harness connectors and the ground circuits of the fuel pump for poor connections. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.
Did you find and correct the condition?		 - Go to Step 23  Go to Step 17 
17
  1. Remove the fuel sender assembly. Refer to Fuel Sender Assembly Replacement (Utility/Crew Cab) or Fuel Sender Assembly Replacement (Pick-Up) .
  2. Inspect the following items:
    • The fuel pump flex hose for damage
    • The in-tank fuel pump harness connectors for poor connections
    • The fuel strainer for a restriction
    • The fuel tank for contaminants
Did you find and correct the condition?		 - Go to Step 23  Go to Step 22 
18
  1. Open the fuel return pipe shut-off valve.
  2. Remove the upper intake manifold. Refer to Intake Manifold Replacement - Upper in Engine Mechanical-4.3L.
  3. Install the engine compartment fuel feed and fuel return pipes to the fuel meter body.
  4. Turn ON the ignition, with the engine OFF.
  5. Inspect for a fuel leak from the fuel pressure regulator.
Is the fuel pressure regulator leaking fuel?		 - Go to Step 21  Go to Step 19 
19 Remove the fuel injectors from the intake manifold in order to isolate the leaking fuel injector. Refer to Fuel Injector Replacement (4.3L (RPO LU3) Engine) .
Did you complete the replacement?		 - Go to Step 23  -
20 Repair the restricted fuel return pipe between the chassis fuel pipe and the fuel tank.
Did you complete the repair?		 - Go to Step 23  -
21
IMPORTANT: Inspect for a missing or damaged O-ring seal before replacing the fuel pressure regulator.
Replace the fuel pressure regulator. Refer to Fuel Pressure Regulator Replacement (4.3L (RPO LU3) Engine) .Did you complete the replacement?		 - Go to Step 23  -
22 Replace the fuel sender. Refer to Fuel Sender Assembly Replacement (Utility/Crew Cab) or Fuel Sender Assembly Replacement (Pick-Up) .
Did you complete the replacement?		 - Go to Step 23  -
23 Operate the system in order to verify the repair.
Did you correct the condition?		 - System OK Go to Step 3 
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