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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 1168 (Engine Controls - 6.0L - Alternative Fuels (Introduction)) >> Repair Instructions >> Fuel Tank Venting Procedure >> Fuel Tank Venting

Fuel Tank Venting

WARNING: This page is about a different car, the 2004 GMC Sierra and 2004 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
    IMPORTANT: Venting of natural gas to the atmosphere may be prohibited in some areas. Before venting the compressed natural gas (CNG) fuel tank, contact the local gas company to ensure that all local, state and federal regulations are observed.
    Venting cannot be stopped once the high pressure lock-off (HPL) has been opened using the manual vent tool.
    Venting the CNG fuel tank will damage the HPL. Venting is only necessary for the following repairs:
    • High pressure lock-off replacement
    • Internal tank cleaning
    • Tank replacement
    • Tank shipping
  1. Remove the CNG fuel tank cover. Refer to Fuel Tank Cover Replacement . Remove the truck body or side panel as required on ZW9 equipped vehicles in order to gain access to the right side of the CNG fuel tank.
  2. Relieve the fuel system pressure. Refer to Fuel Pressure Relief Procedure .
  3. Disconnect the negative battery cable. Refer to Battery Negative Cable Disconnect/Connect Procedure (Single Battery) Battery Negative Cable Disconnect/Connect Procedure (Auxiliary Battery) in Engine Electrical in the C/K Pickup service manual.
    4. Fig 1: Identifying HPL Line Fittings And Fuel Lines
    GM1489671Courtesy of GENERAL MOTORS CORP.
  4. Disconnect the fuel outlet line (4) from the HPL tee fitting (3).
  5. Disconnect the tee fitting (3) from the HPL inlet fitting (2).
  6. Remove the HPL inlet fitting (2).
    7. Fig 2: Identifying HPL Line Fittings
    GM763013Courtesy of GENERAL MOTORS CORP.
  7. Remove the fuel tank pressure (FTP) sensor (2).
  8. Install fittings (2, 4, 8) supplied from the J 42435  into all of the HPL fuel ports as necessary. See Special Tools and Equipment .

    It is not necessary to install a fitting and cap in the pressure relief device (PRD) outlet port (6).

    Tighten:  Tighten the fittings to 48 N.m (35 lb ft).

  9. Install caps supplied from the J 42435  onto the FTP port fitting (2) and the fuel outlet fitting (8) as necessary. See Special Tools and Equipment .

    Tighten:  Tighten the caps to 37 N.m (27 lb ft).

    10. Fig 3: High Pressure Lock-Off (HPL) Solenoid Components
    GM790835Courtesy of GENERAL MOTORS CORP.
  10. Connect the female end of the vent hose to the fuel line fitting.
  11. Connect the male end of the vent hose (6) to a vent stack. Refer to the J 42435  container for instructions on how to properly construct a vent stack. See Special Tools and Equipment .
    12. CAUTION: When venting the Compressed Natural Gas (CNG) fuel tank take the following precautions in order to prevent fire and personal injury:
    • Use the proper grounding procedure to an earth ground in order to prevent a build-up of static electricity which can lead to an electrical discharge.
    • Use the J 42435 CNG Tank Vent Kit with a vent stack.
    • Vent the CNG fuel tank in a well ventilated work area.
    • Keep sparks, flames, and smoking material away from the area.
    • Wear the appropriate hearing protection during the venting procedure. A sudden pressure loss from an uncontrolled gas leak can cause hearing damage.
  12. Connect the ground strap (4) to the FTP port fitting (3) and to the vent stack earth ground (5).
    13. IMPORTANT: If you were required to close the manual lock-down screw during the Fuel Pressure Relief Procedure  the tank may begin to vent when the lock-down screw is opened.
  13. Ensure that the manual lock-down screw of the manual shut-off fitting (2) is open by rotating the Allen screw counterclockwise until fully seated.
  14. If the tank begins venting when the manual lock-down screw is opened do not proceed to the next step until the tank stops venting.
    15. Fig 4: HPL Manual Shut-Off Fitting
    GM296440Courtesy of GENERAL MOTORS CORP.
  15. Remove the manual shut-off fitting from the HPL.
  16. Obtain the manual vent tool supplied from the J 42435  . See Special Tools and Equipment .
  17. Rotate the screw on the manual vent tool counterclockwise in order to fully retract the plunger.
  18. Install the manual vent tool into the HPL port with the plunger fully retracted.

    Tighten:  Tighten the manual vent tool to 12 N.m (108 lb in).

    19. Fig 5: Identifying HPL Solenoid Manual Lock-Down Screw
    GM296441Courtesy of GENERAL MOTORS CORP.
  19. Slowly turn the manual vent tool screw clockwise until the plunger is fully extended.
  20. Open the J 42435  in-line valve. See Special Tools and Equipment .

    The tank should always contain some pressure and begin to vent.

    21. IMPORTANT: Do not leave the tank unattended during venting.
  21. Allow the CNG fuel tank to vent until the pressure gauge reads 0 kPa (0 psi). If the tank stops venting and pressure is indicated on the J 42435  gauge, inspect the vent line for a frozen fitting. See Special Tools and Equipment . The fitting will eventually thaw and fuel will again continue to vent. Inspect for freezing near the HPL. A frozen HPL may stop fuel flow temporarily and show 0 kPa (0 psi) on the pressure gauge.
    22. Fig 6: High Pressure Lock-Off (HPL) Solenoid Components
    GM790835Courtesy of GENERAL MOTORS CORP.
  22. When the tank is completely vented remove the vent line (6) from the fuel fitting.
  23. Remove all the fittings and caps as necessary.
  24. Remove the manual vent tool (2).
  25. If the tank is to be re-used replace the high pressure lock-off. Refer to High Pressure Lock-Off (HPL) Solenoid Replacement .
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