GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2000 🢒 GMC C Pickup - 2WD (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Fuel Tank Venting

Caution: Always use the J 42435-B Compressed Natural Gas (CNG) Fuel Tank Venting Kit in conjunction with the General Motors approved Fuel Tank Venting service procedure to vent CNG fuel tanks. Failure to use the J 42435-B and follow the approved Fuel Tank Venting procedure could result in death, serious injury or vehicle and component damage.

The J 42435-B is required for the following reasons:

   • Natural gas fuel vapor can also contain water vapor.
   • The pressure drop associated with venting high pressure gaseous fuel can cause fuel vapor, tank and line temperatures to reach -73°C (-100°F) or colder.
   • The below freezing temperature can cause the water vapor to freeze
   • The J 42435-B vent hose contains an orifice which is intended to freeze before the CNG high pressure lock-off (HPL) tank valve freezes.
   •  The J 42435-B pressure gauge can indicate a restricted orifice if fuel flow ceases.
   • Internal fuel tank ice build-up may result when venting the CNG fuel tank without the J 42435-B.
      - Internal fuel tank ice build-up can completely restrict the venting of fuel.
      - Venting may cease indefinitely depending upon ambient temperature conditions.
      - The ceasing of fuel tank venting may falsely indicate that the fuel tank pressure has been completely vented.
      - CNG fuel tanks with exceptionally high water content are suceptible to internal fuel tank ice build-up even with the use of the J 42435-B.
The CNG fuel tank is completely vented when all of the following conditions have been met:
  1. Venting natural gas is no longer audible from the vent stack.
  2. Close the J 42435-B ¼-turn valve and stabilize the fuel tank and line assembly at a temperature greater than 7°C (45°F) for at least 4 hours.
    3.         If the ambient temperature is less than 7°C (45°F), warm the fuel tank and vent line assembly manually, either with warm water or by placing the tank and line assembly into a warm environment.
  3. The J 42435-B pressure gauge indicates 0 kPa (0 psi).

Important: 

   • If possible, operate the vehicle on CNG until most of the fuel tank pressure has been depleted.
   • Venting of natural gas to the atmosphere may be prohibited in some areas. Before venting the compressed natural gas (CNG) fuel tank, contact the authority having jurisdiction to ensure that all local, state and federal regulations are observed.
   • Transportation of a pressurized CNG fuel tank is prohibited by law. The CNG fuel tank must be vented prior to consignment.
   • Storing a pressurized CNG fuel tank outside of the vehicle may be prohibited. Contact the authority having jurisdiction prior to removing and storing a pressurized CNG fuel tank.
   • A pressurized CNG fuel tank must be properly restrained and protected from unauthorized access when outside of the vehicle. Restrain the fuel tank to a shipping pallet or with the CNG fuel tank brackets from the vehicle and move to a secure area.
   • Protect the tank from ultra-violet (UV) radiation and physical damage while removed from the vehicle. Inspect for damage any CNG fuel tank that has been removed from the vehicle prior to installation.
   • Venting cannot be stopped once the high pressure lock-off (HPL) tank valve has been opened with the manual vent tool. The J 42435-B contains a ¼ turn valve that allows the vent operation to be paused temporarily.
   • Venting the CNG fuel tank with the manual vent tool will damage the HPL. Venting the internal tank pressure is only necessary for the following:
      - When required by the authority having jurisdiction
      - HPL replacement
      - Internal tank cleaning
      - Tank replacement
      - Tank shipping
      - Tank storage outside of the vehicle (varies by local ordinance)

Tools Required

J 42435-B CNG Tank Venting Kit

  1. Remove the CNG fuel tank cover if applicable. Refer to CNG Fuel Tank Cover Replacement in Body Rear End.
  2. Relieve the fuel system pressure. Refer to Fuel Pressure Relief if not previously performed.

    3. Caution: Unless directed otherwise, the ignition and start switch must be in the OFF or LOCK position, and all electrical loads must be OFF before servicing any electrical component. Disconnect the negative battery cable to prevent an electrical spark should a tool or equipment come in contact with an exposed electrical terminal. Failure to follow these precautions may result in personal injury and/or damage to the vehicle or its components.

  3. Disconnect the negative battery cable.

    4. Object Number: 296443 Size: SH
  4. Disconnect the fuel outlet line from the tank (if not previously disconnected).
  5. In order to prevent freeze damage to the fill line and the FPS, the tank fittings must be capped by performing the following steps:
    6. 5.1. Remove the fuel pressure sensor (FPS).
    5.2. Install a fitting and cap (2) supplied from the J 42435-B CNG Tank Venting Kit into the FPS port (1).
    5.3. Disconnect the fill line hose (if not previously disconnected).

            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-B 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.

    5.4. Install a cap (3) supplied from the kit onto the fill line port fitting (4).

    Object Number: 296445 Size: SH
  6. Connect the vent hose (1) to the fuel outlet fitting and to the vent stack.
  7. Connect the ground strap (2) attached to the vent hose to the HPL and to the vent stack (earth) ground.

    8. 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.

  8. Ensure the manual lock-down screw of the manual shut-off fitting is open by rotating the allen screw counter-clockwise until fully seated.
  9. 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.

    10. Object Number: 296440 Size: SH
  10. Remove the manual shut-off fitting from the HPL.
  11. Before installing the manual vent tool, count the number of turns required to rotate the tool from fully retracted to fully extended. When venting the CNG tank ensure that you rotate the tool the same number of turns. This will ensure that the HPL pilot valve is un-seated and the tank is vented. Some pressure should always vent from the CNG tank.

    12. Verify that the manual vent tool supplied from the J 42435-B is in the non-venting position with the allen screw fully seated counter-clockwise.

  12. Install the manual vent tool into the HPL port.

    13. Object Number: 296441 Size: SH
  13. Slowly turn the manual vent tool allen screw clockwise until natural gas begins to vent. Do not leave the tank unsupervised.
  14. The CNG fuel tank is completely vented when all of the following conditions have been met:
    15. 14.1. Venting natural gas is no longer audible from the vent stack.
    14.2. Close the J 42435-B ¼-turn valve and stabilize the fuel tank and line assembly at a temperature greater than 7°C (45°F) for at least 4 hours.

    If the ambient temperature is less than 7°C (45°F), warm the fuel tank and vent line assembly manually, either with warm water or by placing the tank and J 42435-B line assembly into a warm environment.

    14.3. The J 42435-B pressure gauge indicates 0 kPa (0 psi).
  15. When the tank is completly vented, remove the vent line from the fuel outlet fitting.
  16. Remove the fitting and cap from the FPS port and the cap from the fill line port.
  17. Remove the manual vent tool.
  18. Replace the O-rings at every line and fitting location that was loosened or disconnected.
  19. Tighten any loosened fittings to the correct torque. Refer to Fastener Tightening Specifications .