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Home >> Isuzu >> 2003 >> Axiom Base, RWD >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Basic Diagnostic Procedures >> Fuel System >> Fuel System Diagnosis (Ascender 5.3L) >> Testing Procedure
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Testing Procedure

  1. Perform DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS in SELF-DIAGNOSTICS - ASCENDER 5.3L article. When system check is complete, go to next step.
    2. NOTE: Before you proceed with this diagnostic, inspect the fuel system for damage and for external leaks.
  2. Turn ignition on, with engine off. Command the fuel pump relay ON with a scan tool. Does the fuel pump trun ON when commanded with a scan tool? If yes, go to next step. If no, see INTERMITTENTS in TROUBLE SHOOTING - NO CODES article.
    3. CAUTION: Wrap a shop towel around the fuel pressure connection in order to reduce the risk of fire and personal injury. The towel will absorb any fuel leakage that occurs during the connection of the fuel pressure gauge. Place the towel in an approved container when the connection of the fuel pressure gauge is complete.
    NOTE: Before you proceed with this diagnostic, inspect the fuel system for damage and for external leaks.
    NOTE: You may need to command the fuel pump ON several times, in order to obtain the highest possible fuel pressure.
  3. Turn ignition off. Turn OFF all accessories. Install the Fuel Pressure Gauge (J 34730-1A). See FUEL PRESSURE GAUGE INSTALLATION & REMOVAL (ASCENDER)  . Turn ignition on, with engine off. DO NOT start engine. Command the fuel pump ON with a scan tool. Observe the fuel pressure gauge with the fuel pump commanded ON. Is the fuel pressure within the specified value? If yes, go to next step. If no, go to step  12 .
    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, then remain constant.
  4. Observe the fuel pressure gauge for one minute. Does the fuel pressure drop more than 5 psi (34 kPa)? If yes, go to step  9 . If no, go to next step.
  5. Relieve the fuel pressure to 10 psi (69 kPa). Observe the fuel pressure gauge for one minute. Does the fuel pressure drop more than 2 psi (14 kPa)? If yes, go to step  24 . If no, go to next step.
  6. Turn OFF the ignition for 15 seconds. Turn ON the igniton, with the engine OFF. Monitor the fuel pressure gauge. Start the engine. Does the fuel pressure drop 3-10 psi (21-69 kPa)? If yes, go to next step. If no, go to step  8 .
  7. Operate the vehicle within the conditions of the customer's concerns. Monitor the fuel-related parameters with a scan tool. Do any of the scan tool parameters indicated a lean condition? If yes, go to step  17 . If no, see SYMPTOMS in TROUBLE SHOOTING - NO CODES.
  8. Disconnect the vacuum hose from the fuel pressure regulator. Monitor the fuel pressure gauge. Idle the engine. Apply 12-14 inches of vacuum to the fuel pressure regulator with a hand-held vacuum pump. Does the fuel pressure drop 3-10 psi (21-69 kPa). If yes, go to step  21 . If no, go to step  23 .
  9. Turn ignition off. Disconnect the vacuum hose from the fuel prssure regulator. Turn ignition on, with engine off. Command the fuel pump ON with a scan tool. Inspect for a fuel leak from the vacuum port of the fuel pressure regulator. Is the fuel pressure regulator leaking fuel? If yes, go to step  23 . If no, go to next step.
  10. Turn ignition off. Relieve the fuel pressure. See FUEL SYSTEM PRESSURE RELEASE (ASCENDER 5.3L)  . Disconnect the fuel feed hose and the fuel return hose from the fuel rail. 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 ON with a scan tool. Bleed the air from the fuel pressure gauge. Command the fuel pump ON and OFF with a scan tool. Close the fuel feed pipe shut-off valve. Monitor the fuel pressure gauge for one minute. Does the fuel pressure remain constant? If yes, go to step  19 . If no, go to next step.
  11. Turn ignition off. Open the shut-off valve for the fuel feed pipe. Turn ignition on, with engine off. Command the fuel pump ON and OFF with a scan tool. Close the shut-off valve for the fuel return pipe. Monitor the fuel pressure gauge for one minute. Does the fuel pressure remain constant? If yes, go to step  23 . If no, go to step  20 .
  12. Is the fuel pressure more than 62 psi (427 kPa) for VIN V, T and U (gasoline) or 54 psi (375 kPa) for VIN: Z (ethanol)? If yes, go to step  14 . If no, go to next step.
  13. Is the fuel pressure more than 0 psi (0 kPa)? If yes, go to step  16 . If no, go to step  17 .
  14. Turn ignition off. Relieve the fuel pressure. See FUEL SYSTEM PRESSURE RELEASE (ASCENDER 5.3L)  . 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 the engine off. Monitor the fuel pressure gauge with the fuel pump operating. Is the fuel pressure within 55-62 psi (385-425 kPa) for VIN V, T and U (gasoline) or 48-54 psi (335-375 kPa) for VIN: Z (ethanol). If yes, go to step  22 . If no, go to next step.
  15. Inspect the fuel rail outlet passage for a restriction. Did you find and correct the condition? If yes, go to step  25 . If no, go to step  23 .
    16. NOTE: Do not allow the fuel pressure to exceed 75 psi (517 kPa). Excessive pressure may damage the fuel system.
  16. Turn ignition off. Relieve the fuel pressure. See FUEL SYSTEM PRESSURE RELEASE (ASCENDER 5.3L)  . Disconnect the fuel return hose from the fuel rail return pipe. Install the Fuel Line Shut-Off Adapters (J 37287) between the fuel return pipe and the fuel rail. Open the valve on the fuel pipe shut-off adapter. Turn ignition on, with engine off. Command the fuel pump ON with a scan tool. Bleed the air from the fuel system. Monitor the fuel pressure gauge. Slowly close the valve in the fuel pipe shut-off adapter. Command the fuel pump ON with a scan tool. Does the fuel pressure rise above 62 psi (425 kPa) for VIN V, T and U (gasoline) 54 psi (375 kPa) VIN: Z (ethanol). If yes, go to step  23 . If no, go to next step.
  17. Inspect the fuel filter and fuel feed pipe for restrictions. Did you find and correct the conditon? If yes, go to step  25 . If no, go to next step.
  18. Test for an intermittent and for a poor connection at the harness connectors and the ground circuits of the fuel pump. Did you find and correct the condition? If yes, go to step  25 . If no, go to next step.
  19. Remove the fuel sender assembly. See FUEL LEVEL SENDER in REMOVAL & INSTALLATION article. Inspect the following components:
    • The fuel pump flex hose for damage.
    • The in-tank fuel pump harness connectors for bad connections.
    • The fuel strainer for restrictions.
    • The fuel tank for contaminants.
    Did you find and correct the condition? If yes, go to step  25 . If no, go to step  24 .
  20. Turn ignition off. Raise the fuel rail, with the fuel lines connected. Turn ignition on, with engine off. Command the fuel pump relay ON with a scan tool. Locate and replace the leaking fuel injector. FUEL RAIL & FUEL INJECTORS in REMOVAL & INSTALLATION article. When replacement is complete, go to step  25 .
  21. Repair the vacuum source to the fuel pressure regulator. When repair is complete, go to step  25 .
  22. Locate and repair the restricted fuel return line. When repair is complete, go to step  25 .
    23. NOTE: Before you replace the fuel pressure regulator, inspect for a missing or damaged fuel pressure regulator "O" ring seal.
  23. Replace the fuel pressure regulator. See FUEL SYSTEM in REMOVAL & INSTALLATION article. When repair is complete, go to step  25 .
  24. Replace the fuel pump. See FUEL PUMP in REMOVAL & INSTALLATION article. When repair is complete, go to step  25 .
  25. Operate the system in order to verify the repair. Did you correct the condition? If yes, the system is okay. If no, go to step  3 .
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