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Home >> GMC >> 2003 >> Safari Van Cargo, AWD >> Repair and Diagnosis >> External Pages >> Different car >> Section 1000 (Engine Controls - 6.6L (Lb7) (Troubleshooting & Diagnosis) >> Troubleshooting >> Fuel System Diagnosis (Cab Chassis) >> Fuel System Air Leak Check

Fuel System Air Leak Check

WARNING: This page is about a different car, the 2004 GMC Sierra, 2004 GMC Cab & Chassis Sierra, 2004 Chevrolet Silverado, and 2004 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
  1. Install the J 44638  Vacuum Gage to the fuel system service port on the right front side of the engine.
  2. Remove the air duct from the air cleaner assembly to the turbo inlet.
  3. Remove the generator for visibility of the fuel supply lines. Refer to Generator Replacement (4.3L Engine) or Generator Replacement (4.8L, 5.3L, and 6.0L Engines) or Generator Replacement (6.6L (LB7) Engine) or Generator Replacement (6.6L (LLY) Engine) or Generator Replacement (8.1L Engine) in Engine Electrical.
  4. Prime the fuel manager until 10 psi is indicated on the J 44638  .
  5. Check for external fuel leaks between the fuel filter/heater assembly and the fuel injection pump.
    • If fuel is leaking externally, replace or repair the leaking component.
    • If no external fuel leaks are found, go to step 6.
  6. Reprime the fuel system until 10 psi is indicated on the J 44638  .
    • If the fuel pressure drops from 10 psi to 2 psi in less than 1 minute, go to step 23.
    • If the fuel pressure drops from 10 psi to 2 psi in more than 1 minute, go to step 7.
  7. Disconnect the fuel supply line (2) at the engine. Refer to Quick Connect Fitting(s) Service (Metal Collar) .
  8. Cap off the fuel supply pipe (2) on the engine with a 1/2 inch rubber plug (3) and clamp.
  9. Disconnect the fuel return line (1) at the engine. Refer to Quick Connect Fitting(s) Service (Metal Collar) .
    10. Fig 1: Fuel Supply & Return Line At Engine
    GM761595Courtesy of GENERAL MOTORS CORP.
  10. Install a J 23738-A  Mityvac on the fuel return pipe on the engine.
  11. Check the fuel system components on top of the engine for vacuum leaks.
    IMPORTANT: There is an orifice in the fuel injection pump between the fuel supply and fuel return systems. This will delay the vacuum being pulled from the fuel supply lines.
    1. Apply at least 25 inches Hg of vacuum to the fuel return pipe with the J 23738-A  and wait more than 1 minute for the system to stabilize.
    2. Repeat previous substep 6 times.
    3. Observe the J 23738-A  .
      • If the vacuum does not drop below 8 inches within 10 minutes, go to step 12.
      • If the vacuum drops below 8 inches within 10 minutes, go to step 18.
  12. Reconnect the fuel supply line and the return line at the engine.
  13. Install a transparent hose between the fuel injection control module (FICM) outlet and the fuel filter inlet, with a vertical loop to observe the incoming fuel for air bubbles.
  14. Install the generator. Refer to Generator Replacement (4.3L Engine) or Generator Replacement (4.8L, 5.3L, and 6.0L Engines) or Generator Replacement (6.6L (LB7) Engine) or Generator Replacement (6.6L (LLY) Engine) or Generator Replacement (8.1L Engine) in Engine Electrical.
  15. Prime the fuel system 30 times to remove the air from the fuel hoses.
    16. IMPORTANT: If the engine does not start in the next step, go to step 23.
    IMPORTANT: The engine must be run for at least 10 minutes to allow the fuel system to stabilize and remove all air from the fuel system.
  16. Start and run the engine.
  17. Observe the fuel in the transparent hose for air bubbles.
    • If there are no air bubbles of any kind entering the transparent hose with the incoming fuel, the fuel suction system is OK. Go to step 28.
    • If any air bubbles are entering the transparent hose with the incoming fuel, stop the engine and go to step 19.
  18. Check the fuel injection pump and the on-engine fuel suction system for air leakage.
    1. Disconnect the fuel outlet hose (2) from the outlet pipe (1) of the fuel filter assembly.
    2. Open the drain on the fuel filter assembly and drain the fuel into a suitable container.
    3. Close the drain.
    4. Install a J 23738-A  on the outlet port (1) of the fuel filter assembly.
      5. Fig 2: Fuel Outlet Hose & Pipe
      GM761594Courtesy of GENERAL MOTORS CORP.
    5. Apply 15 inches Hg of vacuum to the fuel pipe and observe the J 23738-A  .
      • If the vacuum does not drop, connect the fuel supply line at the engine and go to the next substep.
      • If the vacuum drops, repair the air leak between the fuel supply line fitting and the fuel filter assembly.
    6. Remove the J 23738-A  from the outlet port (1) of the fuel filter assembly and install the J 23738-A  to the fuel outlet hose (2).
      7. Fig 3: Fuel Outlet Hose & Pipe
      GM761594Courtesy of GENERAL MOTORS CORP.
    7. Remove the Air Intake Pipe. Refer to Air Intake Pipe Replacement .
    8. Remove the supply hose (3) from the fuel injection pump (4) and plug the hose.
      9. Fig 4: Fuel Injection Pump, Supply & Return Hoses
      GM896820Courtesy of GENERAL MOTORS CORP.
    9. Apply 15 inches Hg of vacuum and observe the reading.
      • If the vacuum drops, repair the leak between the fuel filter assembly outlet and the fuel injection pump inlet.
      • If the vacuum does not drop, go to the next substep.
    10. Disconnect the fuel injection pump return hose (1).
      11. Fig 5: Fuel Injection Pump, Supply & Return Hoses
      GM896820Courtesy of GENERAL MOTORS CORP.
    11. Cap off the fuel return pipe on the fuel injection pump with a 5/16 inch rubber plug, and clamp the plug.
    12. Install a J 23738-A  to the fuel inlet pipe of the fuel injection pump.
    13. Apply 20 inches of vacuum and observe the J 23738-A  .
      • If the vacuum drops, replace the fuel injection pump. Refer to Fuel Injection Pump Replacement . Then go to step 28.
      • If the vacuum does not drop, replace the fuel injection pump inlet supply hose (3). Then go to step 28.
  19. Check the fuel sender for air leakage.
    1. Remove the front fuel tank. Refer to Fuel Tank Replacement (Pickup) or Fuel Tank Replacement (Cab/Chassis - Front) or Fuel Tank Replacement (Cab/Chassis - Rear) .
    2. Remove the fuel sender from the fuel tank.
    3. Remove the strainer from the fuel sender and plug the bottom end of the pickup tube.
    4. Apply 15 inches Hg of vacuum to the upper end of the pickup tube and observe the J 23738-A  .
      • If the vacuum holds, inspect the O-ring sealing surfaces of the fuel sending unit and the chassis fuel line for damage and correct as necessary. If the sealing surfaces are not damaged, replace the fuel line between the chassis line and the fuel sending unit. Then go to Step 20.
      • If the vacuum does not hold replace the fuel sender, reinstall the fuel tank, and go to step 20.
  20. Prime the fuel system 30 times or until fuel pressure is 10 psi to remove the air from the fuel system. Air will be forced out of the system within 2 minutes.
  21. Start and run the engine.
  22. Observe the fuel for air bubbles.
    • If there are no air bubbles of any kind entering the transparent hose with the incoming fuel, the fuel suction system is OK. Go to step 28.
    • If any air bubbles are entering the transparent hose with the incoming fuel, stop the engine and go to step 8.
  23. Prime the fuel system 30 times or until fuel pressure is 10 psi to remove the air from the fuel system. Air will be forced out of the system within 2 minutes.
  24. Remove the hose from the fuel filter assembly outlet fitting and plug the hose.
  25. Verify that the J 44638  is installed.
  26. Remove the ignition 1 relay.
  27. Crank the engine 2-3 times in 15 second intervals and observe the J 44638  .
    • If the J 44638  indicates 5 inches Hg of vacuum or more, go to step 28.
    • If the J 44638  indicates less than 5 inches Hg of vacuum, replace the fuel injection pump. Refer to Fuel Injection Pump Replacement . Then go to step 28.
  28. Remove the transparent hose if necessary and attach any disconnected components or fuel lines.
  29. Prime the fuel system 30 times to remove the air in the fuel hoses
  30. Clean any fuel spillage.
  31. Start the engine. If the engine starts and stalls, repeat step 29.
  32. Run the engine to check for fuel leakage.
  33. Verify that the customer concern is corrected. If the customer concern is not corrected, go to Symptoms - Engine Controls .
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