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Home >> GMC >> 1997 >> C3500 HD 7.4 J, Automatic >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Basic Diagnostic Procedures - 7.4L >> Basic Fuel System Checks >> Fuel System Pressure Test >> Diagnostic Procedure
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Diagnostic Procedure

  1. Perform ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK  . After performing OBD system check, go to next step.
  2. Relieve fuel pressure. See FUEL SYSTEM PRESSURE RELIEF  . Connect Fuel Pressure Gauge (J-34730-1A) to fuel pressure test port. Turn ignition on with engine off. Bleed air out of fuel pressure gauge into container. The ignition may have to be cycled off for 10 seconds, then back on more than once in order to obtain maximum fuel pressure. Monitor fuel pressure with fuel pump running. Fuel pressure should be 60-66 psi (4.2-4.6 kg/cm2 ) on Central Sequential Fuel Injection (CSFI) fuel system and 56-62 psi (4.0-4.4 kg/cm2 ) on Sequential Fuel Injection (SFI) fuel system, and should hold steady. If fuel pressure is as specified, go to next step. If fuel pressure is not as specified, go to step 8).
  3. Turn ignition off for 10 seconds with A/C off. Turn ignition on. Fuel pump should run for 2 seconds. Observe fuel pressure after pump stops running. Fuel pressure may vary a few pounds, then should hold steady. After fuel pump stops, fuel pressure should stabilize and remain constant at 55-60 psi (3.9-4.2 kg/cm2 ) on CSI system and 48-52 psi (3.4-3.7 kg/cm2 ) on SFI system. If pressure is within specification and holds, go to step 5). If pressure is not within specification, or is within specification but does not hold, go to next step.
  4. Turn ignition off for 10 seconds with A/C off. Relieve fuel pressure. See FUEL SYSTEM PRESSURE RELIEF  . Disconnect fuel filter. Drain any remaining fuel from fuel pipes into container. Install an appropriate fuel line shutoff valve in place of fuel filter. Open shutoff valve. Turn ignition on. Check for leaks at shutoff valve. Start engine and idle for 30 seconds. Turn ignition off for 10 seconds. Turn ignition on with engine off. Using scan tool, command fuel pump on. Wait for fuel pressure to build. Close shutoff valve. Fuel pressure should remain constant at 55-60 psi (3.9-4.2 kg/cm2 ) on CSI system or 48-52 psi (3.4-3.7 kg/cm2 ) on SFI system. If pressure is as specified and holds, go to step 12). If pressure is not as specified or is as specified but does not hold, go to step 6).
  5. Start engine. Warm engine to normal operating temperature. From idle, open throttle quickly, while observing fuel pressure. The fuel pressure should approach the top of the specified range. If fuel pressure is not within specification, or if fuel pressure is within specification, but fuel pressure is suspected of decreasing during acceleration, cruise or hard cornering, go to step 27). If fuel pressure is within specification and fuel pressure is not suspected of decreasing during acceleration, cruise or hard cornering, see FUEL INJECTION TROUBLE SHOOTING under ENGINE PERFORMANCE in TROUBLE SHOOTING - BASIC PROCEDURES article in GENERAL INFORMATION.
  6. Inspect the fuel feed line between the CSI/SFI unit and the shut-off adapter. If fuel feed line is faulty, go to step 25). If fuel feed line is okay, go to next step.
  7. Open shutoff valve. Relieve fuel pressure. See FUEL SYSTEM PRESSURE RELIEF  . Disconnect fuel return pipe union located near fuel filter. Drain remaining fuel from fuel pipes into container. Install appropriate fuel line shutoff valve in place of fuel return pipe union. Open shutoff valve. Using scan tool, command fuel pump on. Check for leaks at fuel shutoff valve fittings. Start engine and idle for 30 seconds. Turn ignition off for 10 seconds. Turn ignition on with engine off. Using scan tool, command fuel pump on. Wait for fuel pressure to build. Close shutoff valve installed on fuel return pipe. If fuel pressure remains constant, go to step 22). If fuel pressure does not remain constant, go to step 29).
  8. Turn ignition on with engine off. Using scan tool, command fuel pump on. Fuel pump should run for 2 seconds, and then turn off. Observe fuel pressure after fuel pump stops running. If fuel pressure is present, go to step 13). If fuel pressure is not present, go to next step.
  9. Using scan tool, command fuel pump on. Listen for operation of fuel pump. If fuel pump operates, go to next step. If fuel pump does not operate, see NO-START DIAGNOSIS  .
  10. Check for the following conditions:
    • No fuel in the fuel tank (do not rely on fuel gauge reading).
    • Plugged in-line fuel filter.
    • Plugged or damaged fuel pump strainer.
    • Restricted fuel line.
    • On dual tank models, check the auxiliary tank for fuel.
    If a problem is found, go to next step. If no problem is found, go to step 12).
  11. Repair or replace as necessary. After repairs, go to step 30).
  12. Check for a leaking fuel feed pipe in between the shut-off adapter and the modular fuel sender. Repair as necessary. If no leak is present, replace the fuel sending unit. See FUEL SYSTEM in appropriate REMOVAL & INSTALLATION article. After repairs, go to step 30).
  13. Turn ignition on with engine off. Using scan tool, command fuel pump on. Fuel pump should run for 2 seconds. Observe fuel pressure. If fuel pressure is greater than 66 psi (4.6 kg/cm2 ) on CSI system or greater than 62 psi (4.4 kg/cm2 ) on SFI system, go to next step. If fuel pressure is not as specified, go to step 15).
  14. Relieve fuel pressure. See FUEL SYSTEM PRESSURE RELIEF  . Disconnect engine compartment fuel return line. Attach a flexible hose to fuel return line. Place other end of hose in container. Using scan tool, command fuel pump on and observe fuel pressure within 2 seconds. Fuel pressure should be 60-66 psi (4.2-4.6 kg/cm2 ) on CSI system and 56-62 psi (4.0-4.4 kg/cm2 ) on SFI system. If fuel pressure is as specified, go to step 16). If fuel pressure is not as specified, go to step 17).
  15. Turn ignition on with engine off. Using scan tool, command fuel pump on. Fuel pump should run for 2 seconds. Observe fuel pressure. Fuel pressure should be 60-66 psi (4.2-4.6 kg/cm2 ) on CSI system and 56-62 psi (4.0-4.4 kg/cm2 ) on SFI system. If fuel pressure is less than specified, go to step 19). If fuel pressure is not less than specified, go to step 30).
  16. Locate and repair restriction in fuel return line. After repairs, go to step 30). If fuel return line is not restricted, go to next step.
  17. Check for restriction in engine fuel return outlet pipe at point where fuel line was disconnected. If fuel return pipe is restricted, go to next step. If fuel return pipe is not restricted, go to step 22).
  18. Repair/replace engine fuel pipes as necessary. After repairs, go to step 30).
  19. Inspect in-line fuel filter and fuel line for restriction. If restriction is found, go to next step. If restriction is not found, go to step 21).
  20. Repair/replace in-line fuel filter or fuel line as necessary. After repairs, go to step 30).
  21. Relieve fuel pressure. See FUEL SYSTEM PRESSURE RELIEF  . Disconnect fuel return pipe union located near fuel filter. Drain remaining fuel from fuel pipes into container. Install appropriate fuel line shutoff valve in place of fuel return pipe union. Open shutoff valve. Using scan tool, command fuel pump on. Bleed air out of fuel pressure gauge into container. DO NOT allow fuel pressure to exceed 75 psi (4.5 kg/cm2 ). Excess pressure may damage fuel pressure regulator. Slowly close shutoff valve installed on fuel return pipe. Fuel pressure should increase to greater than 66 psi (4.6 kg/cm2 ) on CSI system or 62 psi (4.4 kg/cm2 ) on SFI system. If fuel pressure increases as specified, go to next step. If fuel pressure does not increase as specified, go to step 23).
  22. Replace fuel pressure regulator. After repairs, go to step 30).
  23. Check for the following conditions:
    • Low fuel pump voltage due to high resistance connections.
    • High fuel pump voltage due to internal pump binding.
    • Poor fuel pump ground.
    • Partially restricted fuel pump strainer.
    • Incorrect or faulty fuel pump.

    If any of the listed conditions are present, go to next step. If none of the listed conditions are present, go to step 12).

  24. Repair or replace as necessary. After repairs, go to step 30).
  25. Replace fuel feed line. After repairs, go to step 30).
  26. Start engine and allow it to reach normal operating temperature. Open throttle quickly. Observe fuel pressure. Fuel pressure should increase to near 66 psi (4.6 kg/cm2 ) on CSI system or 62 psi (4.4 kg/cm2 ) on SFI system. If fuel pressure increases quickly, see appropriate TROUBLE SHOOTING - NO CODES article. If fuel pressure does not increase quickly, go to next step.
  27. Check for the following conditions:
    • In-line fuel filter for restriction.
    • Fuel feed pipe for restriction.
    • Mispositioned or damaged fuel pump strainer.
    • Fuel pump flex pipe for leaks.
    • Vehicle is equipped with correct fuel pump.
    • Fuel pump electrical wiring for high resistance.

    If any of the listed conditions are present, go to next step. If none of the listed conditions are present, go to step 12).

  28. Repair or replace as necessary. After repairs, go to step 30).
  29. Locate and replace any leaking injector(s). See FUEL INJECTOR BALANCE TEST under FUEL SYSTEM in appropriate SYSTEM & COMPONENT TESTING article. After repairs, go to next step.
  30. Using scan tool, select CAPTURE INFO, REVIEW INFO function. If additional DTCs are present, diagnose DTCs. See appropriate SELF-DIAGNOSTICS article. If DTCs are not present, system is okay.
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