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Home >> Chevrolet >> 2002 >> Blazer 4D Utility, 4WD, Part Time >> Repair and Diagnosis >> External Pages >> Different car >> Section 251 (Engine Controls - Removal & Installation) >> Fuel Systems (CNG) >> Fuel Tank Venting

Fuel Tank Venting

WARNING: This page is about a different car, the 2003 Pontiac Sunfire and 2003 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.
WARNING: Venting of CNG to the atmosphere may be prohibited in some areas. Before venting the CNG fuel tank, contact the local gas company to ensure that all local, state and federal regulations are observed.
WARNING: When venting a CNG fuel tank, take the following precautions to prevent fire and personal injury:
  • Use proper grounding procedure to an earth ground to prevent a build-up of static electricity, which can lead to an electrical discharge.
  • Use CNG Tank Venting Kit (J 42435) with a vent stack.
  • Vent CNG fuel tank in a well ventilated work area.
  • Keep sparks, flames, and smoking material away from vent area.
  • Wear appropriate hearing protection during venting procedure. A sudden pressure loss from an uncontrolled gas leak can cause hearing damage.
CAUTION: Venting cannot be stopped once the High Pressure Lock-Off (HPL) has been opened using manual vent tool. The manual vent tool can damage main seat of HPL if used improperly. Use this tool only when HPL is not in operation.
CAUTION: Replace all "O" rings that are disconnected or loosened.
NOTE: Venting the CNG fuel tank will damage the HPL. Venting is only necessary for High Pressure Lock-Off (HPL) replacement, internal tank cleaning, tank replacement or tank shipping.
NOTE: Numbers in text correspond to numbered components in referenced figures.
  1. Remove spare tire cover. Remove jack and tool kit. Remove spare tire. Remove 4 tank cover bolts. Remove tank cover. See Fig 1.
  2. Release fuel system pressure. See FUEL SYSTEM PRESSURE RELEASE (CNG) .
  3. Turn ignition switch to OFF position. Disconnect negative battery cable.
  4. Remove the fuel line vent boot cover nuts. See Fig 2. Remove fuel line vent boot cover.
  5. Disconnect all harness connectors to remove the vent boot cover from the tank area.
  6. Disconnect the CNG fuel fill line from the HPL. See Fig 3.
  7. Remove the Fuel Tank Pressure (FTP) sensor (2). See Fig 4.
  8. Disconnect the CNG fuel outlet line from the HPL fitting (1).
  9. Install a cap (9) supplied with CNG tank venting kit onto the fuel outlet line fitting (8). Tighten the caps to specification. See TORQUE SPECIFICATIONS . See Fig 5.
  10. Install an appropriate fitting from the CNG Tank Venting Kit, on FTP sensor port. Tighten fitting to specification. See TORQUE SPECIFICATIONS . See Fig 5.
  11. Install a cap supplied from the CNG Tank Venting Kit, to FTP sensor port fitting. Tighten the cap to specification. See TORQUE SPECIFICATIONS . See Fig 5.
  12. Remove the fill line HPL fitting (3). See Fig 4.
  13. Install the fitting (4) supplied in the CNG Tank Venting Kit that threads into both the HPL fill line port and the CNG vent hose female fitting. Tighten the fitting to specification. See TORQUE SPECIFICATIONS . See Fig 5.
  14. Connect the female end of the vent hose to the fill line fitting. See Fig 6.
    15. NOTE: Refer to the CNG Tank Venting Kit container for instructions on how to properly construct a vent stack.
  15. Connect the male end of the vent hose (6) to a vent stack.
  16. Connect the ground strap (4) to the FTP port fitting (3) and to the vent stack earth ground (5). See Fig 6.
    17. CAUTION: 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.
  17. 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.
  18. 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.
  19. Remove the manual shut-off fitting from the HPL. See Fig 7.
  20. Obtain the manual vent tool supplied from the CNG Tank Venting Kit.
  21. Rotate the screw on the manual vent tool counterclockwise to fully retract the plunger.
  22. Install the manual vent tool into the HPL port with the plunger fully retracted. Tighten the manual vent tool to specification. See TORQUE SPECIFICATIONS .
  23. Slowly turn the manual vent tool screw clockwise until the plunger is fully extended. See Fig 8.
  24. Open the CNG Tank Venting Kit in-line valve. The tank should always contain some pressure and begin to vent.
    25. CAUTION: DO NOT leave the tank unattended during venting.
  25. Allow the CNG fuel tank to vent until the pressure gauge reads zero psi (zero kPa). If the tank stops venting and pressure is indicated on the CNG Tank Venting Kit gauge, check the vent line for a frozen fitting. The fitting will eventually thaw and fuel will again continue to vent.
  26. When the tank is completely vented remove the vent line (6) from the fuel fitting. See Fig 6.
  27. Remove the fitting and cap (3) from the FTP port. See Fig 6.
  28. Remove the fuel fitting cap from the fuel outlet port.
  29. Remove the fitting from the fuel fill line port.
  30. Remove the manual vent tool (2). See Fig 6.
  31. If the tank is to be re-used replace the high pressure lock-off.
Fig 1: Removing CNG Fuel Tank Cover
G00196195Courtesy of GENERAL MOTORS CORP.
Fig 2: Removing CNG Fuel Tank Vent Cover
G00196201Courtesy of GENERAL MOTORS CORP.
Fig 3: Disconnecting Fuel Line From HPL
G00196196Courtesy of GENERAL MOTORS CORP.
Fig 4: Removing FTP Sensor
G00196197Courtesy of GENERAL MOTORS CORP.
Fig 5: Installing Caps Onto Fuel Outlet Line Fitting
G00196198Courtesy of GENERAL MOTORS CORP.
Fig 6: Connecting Vent Hose to Fill Line
G00196199Courtesy of GENERAL MOTORS CORP.
Fig 7: Removing Manual Shut-Off Fitting From HPL
G00196200Courtesy of GENERAL MOTORS CORP.
Fig 8: Turning Manual Vent Tool
G00196202Courtesy of GENERAL MOTORS CORP.
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