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Fuel Pressure Relief Procedure

WARNING: This page is about a different car, the 2003 GMC Sierra and 2003 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
NOTE: CNG Tank Venting Kit (J-42435-A) is required for this procedure. See Figure .
  1. Using a scan tool, if communication can be established with the Power Control Module (PCM), go to step  9 . If communication cannot be established, see DIAGNOSTIC SYSTEM CHECK - ALTERNATIVE FUELS - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM in.
  2. Store the DTC Information for any FICM DTCs that may be set.
  3. Clear all FICM DTC Information.
  4. Turn the ignition OFF.
    5. NOTE: If the engine fails to operate on CNG or start at all, proceed to step  9 .
  5. Start and idle the engine. Ensure that the engine is operating on CNG by observing that the Fuel Indicator Lamp (FIL) is OFF and that AFO Enable scan tool display indicates the engine is operating on alternative fuel.
  6. With a scan tool, select the High Pressure Lock-Off (HPL) solenoid special function.
  7. While the engine is operating on CNG command the HPL solenoid OFF and observe the FUEL TANK PRESSURE sensor parameter. The fuel tank pressure should slowly decrease. If the fuel pressure fails to decrease proceed to next step. When the engine stalls or a switch-over to gasoline is observed, perform the following:
    • Turn OFF the ignition.
      • NOTE: Turning the ignition ON with the AF Lock-off relay disconnected may set a DTC. Clear DTC Information after repairs are complete.
    • Remove the CNG 20-amp fuse in order to disable the CNG fuel system.
    • A small amount of CNG will remain in the fuel lines and components. Slowly open the fittings in order to release any remaining pressure when disconnecting a fuel line.
    • Fuel Pressure Relief for the CNG system is now complete. Go to step  17 .
    Turn OFF the ignition. Remove the CNG 20-amp fuse in order to disable the CNG fuel system. A small amount of CNG will remain in the fuel lines and components. Slowly open the fittings in order to release any remaining pressure when disconnecting a fuel line. Fuel Pressure Relief for the CNG system is now complete. Go to step  17 .
  8. Remove the fuel tank cover, if necessary. See FUEL TANK COVER  .
  9. Close the manual lock-down screw located on the high pressure lock-off (HPL) solenoid. See Fig 1 . The manual shut-off fitting is closed by rotating the Allen screw clockwise until fully seated.
  10. Turn ignition OFF.
  11. Disconnect the negative battery cable(s).
    12. WARNING: When relieving the Compressed Natural Gas (CNG) fuel system pressure, take the following precautions in order to prevent fire and personal injury: vent the CNG fuel system in a well ventilated work area, keep sparks, flames, and smoking material away from the area and wear the appropriate hearing protection during the venting procedure. A sudden pressure loss from an uncontrolled gas leak can cause hearing damage.
  12. Slowly loosen the fuel line fitting at the Low Pressure Lock-Off (LPL) inlet port several turns. Fuel Pressure Relief for the CNG system is complete when the fuel line can be pulled away from the "O" ring face seal and fuel does not vent.
  13. Slowly loosen the fuel line fittings at the tee and at the check valve several turns.
  14. Fuel Pressure Relief for the CNG system is complete when the fuel lines can be pulled away from the "O" ring face seal and fuel does not vent.
    15. WARNING: Natural gas is stored under high pressure. If a leak occurs, the high pressure could cause a personal injury. In order to prevent Compressed Natural Gas (CNG) fuel leaks, always replace the CNG fuel system "O" rings during service.
  15. Replace the "O" ring face seals at all fittings that were disconnected or loosened.
  16. After all necessary repairs are complete, tighten any loosened fittings to the correct torque. See TORQUE SPECIFICATIONS  . Open the manual lock-down screws located on the HPL solenoid manual shut-off fittings by rotating the Allen screws counterclockwise until fully seated on any vehicle that does not require venting of the CNG fuel tanks. Leave the manual lock-down screw closed on any vehicle that will require CNG fuel tank venting.
    17. NOTE: If the service necessary for the vehicle requires that the ignition be turned ON with CNG components or lines disconnected, keep the Alternate Fuels Lock-off relay disconnected in order to prevent CNG fuel venting during an ignition ON prime pulse. Turning the ignition ON with the AF Lock-off relay disconnected may set a DTC. Clear DTC Information after repairs are complete.
  17. If the service necessary for the vehicle requires the CNG fuel tank be relieved of all internal pressure, vent tank. See FUEL TANK VENTING PROCEDURE  .
Fig 1: Closing manual Lock-Down Screw
G00232192Courtesy of GENERAL MOTORS CORP.
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