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Home >> Isuzu >> 2003 >> Axiom Base, 4WD >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Basic Diagnostic Procedures >> Fuel System >> Fuel System Diagnosis (Ascender 4.2L) >> Testing Procedure
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Testing Procedure

  1. Perform DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS in SELF-DIAGNOSTICS - 4.2L ASCENDER article. When the check is complete, go to next step.
    2. NOTE: Inspect the fuel system for damage or external leaks before proceeding with this diagnostic.
  2. Turn ignition on, with engine off. Command the fuel pump relay ON with a scan tool. If the fuel pump operates, go to next step. If the fuel pump does not operate, see FUEL PUMP ELECTRICAL CIRCUIT DIAGNOSIS  .
    3. NOTE: Verify that adequate fuel is in the fuel tank before proceeding with this diagnostic.
    NOTE: The fuel pump relay may need to be commanded ON a few times in order to obtain the highest possible fuel pressure.
  3. Turn ignition off. Turn OFF all accessories. Install a fuel pressure gauge, see FUEL PRESSURE GAUGE INSTALLATION & REMOVAL (ASCENDER)  . Turn ignition on, with engine off. DO NOT start the engine. Command the fuel pump relay ON with a scan tool. Observe the fuel pressure gauge with the fuel pump commanded ON. If the fuel pressure is within 48-54 psi (334-375 kPa) go to next step. If the fuel pressure is not within the specified range, go to 10 step .
    4. NOTE: 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.
  4. Monitor the fuel pressure gauge for one minute. If the fuel pressure decreases by more than 5 psi (34 kPa) go to step  7 . If the fuel pressure does not decrease by more than 5 psi (34 kPa) go to next step.
  5. Relieve the fuel pressure to 10 psi (69 kPa). Monitor the fuel pressure gauge for 5 minutes. If the fuel pressure decreases by more than 2 psi (14 kPa) go to step  21 . If the fuel pressure does not decrease by more than 2 psi (14 kPa) go to step  6 .
  6. Operate the vehicle within the conditions to reproduce the original symptoms. Monitor the O2 and the fuel trim parameters with a scan tool. Do any of the scan tool parameters indicate a lean condition? If yes, go to step  15 . If no, go to SYMPTOMS in TROUBLE SHOOTING - NO CODES article.
  7. Turn ignition off. Remove the vent hose from the fuel pressure regulator. Turn ignition on, with engine off. Command the fuel pump relay ON with a scan tool. Inspect for a fuel leak from the fuel pressure regulator atmospheric vent. If the fuel pressure regulator is leaking fuel, go to step  20 . If the fuel pressure regulator is not leaking fuel, go to next step.
  8. Turn ignition off. Relieve the fuel pressure. See FUEL SYSTEM PRESSURE RELEASE (ASCENDER 4.2L)  . Disconnect the fuel feed hose and the fuel return hose from the fuel rail pipes. Install the Fuel Line Shut-Off Adapters (J 37287) between the fuel hoses and the fuel rail pipes. Open the valves on the fuel pipe shut-off adapters. Turn ignition on, with engine off. Command the fuel pump relay ON with a scan tool. Bleed the air from the fuel pressure gauge. Command the fuel pump relay ON and then OFF with a scan tool. Close the fuel feed pipe shut-off valve. Monitor the fuel pressure gauge for one minute. If the fuel pressure remains constant, go to step  17 . If the fuel pressure does not remain constant, go to next step.
  9. Turn ignition off. Open the fuel feel pipe shut-off valve. Turn ignition on, with engine off. Command the fuel pump relay ON and then OFF with a scan tool. Close the fuel return pipe shut-off valve. Monitor the fuel pressure gauge for one minute. If the fuel pressure remains constant, go to step  20 . If the fuel pressure does not remain constant, go to step  18 .
  10. Is the fuel pressure more than 54 psi (375 kPa)? If yes, go to step  12 . If no, go to next step.
  11. Is the fuel pressure more than 0 psi (0 kPa)? If yes, go to step  14 . If no, go to step  15 .
  12. Turn ignition off. Relieve the fuel pressure. See FUEL SYSTEM PRESSURE RELEASE (ASCENDER 4.2L)  . Disconnect the fuel return hose from the fuel rail return pipe. Attach a length of flexible fuel hose into an approved gasoline container. Turn ignition on, with engine off. Monitor the fuel pressure gauge while the fuel pump is operating. If the fuel pressure is within 48-54 psi (334-375 kPa), go to step  19 . If the fuel pressure is not within 48-54 psi (334-375 kPa), go to next step.
  13. Inspect the fuel rail outlet passage and the fuel rail return pipe for a restriction. If there is no restriction, go to step  22 . If there is a restriction, go to step  20 .
    14. NOTE: DO NOT allow the fuel pressure to exceed 75 psi (517 kPa). Excessive pressure may damage the fuel system.
  14. Turn ignition off. Relieve the fuel pressure. See FUEL SYSTEM PRESSURE RELEASE (ASCENDER 4.2L)  . Disconnect the fuel return hose from the fuel rail return pipe. Install the Fuel Line Shut-Off Adapter between the fuel return pipe and the fuel rail. Open the valve in the fuel pipe shut-off adapter. Turn ignition on, with engine off. Bleed the air from the fuel pressure gauge. Monitor the fuel pressure gauge. Command the fuel pump relay ON with a scan tool. Slowly close the valve in the fuel return pipe shut-off adapter, while the fuel pump is operating. If the fuel pressure increases to more than 54 psi (375 kPa), go to step  20 . If the pressure does not exceed 54 psi (375 kPa), go to next step.
  15. Inspect the fuel filter and the fuel feel pipe for restrictions. If the problem was found and corrected, go to step  22 . If the problem was not found and corrected, go to next step.
  16. Inspect the harness connectors and the ground circuits of the fuel pump for poor connections. If the problem was found and corrected, go to step  22 . If the problem was not solved and corrected, go to next step.
  17. Remove the fuel sender assembly. 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.
    If the condition was corrected, go to step  22 . If the condition was not corrected, go to step  21 .
  18. Turn ignition off. Raise the fuel rail with the fuel pipes connected. Turn ignition on, with engine off. Command the fuel pump relay ON with a scan tool. Locate and replace the leaking fuel injector. After replacement is complete, go to step  22 .
  19. Repair the restricted fuel return pipe. When the repair is complete, go to step  22 .
    20. NOTE: Inspect for a missing or damaged "O" ring seal before replacing the fuel pressure regulator.
  20. Replace the fuel pressure regulator. When procedure is complete, go to  22 .
  21. Replace the fuel sender. When procedure is complete, go to  22 .
  22. Operate the system in order to verify the repair. If the condition was corrected, the system is okay. If the system was not corrected, go to step  3 .
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