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Fuel System Pressure Release (CNG)

WARNING: This page is about a different car, the 2003 Pontiac Sunfire, 2003 Pontiac Grand Am, 2003 Oldsmobile Alero, and 2003 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.
WARNING: ALWAYS release fuel pressure before disconnecting any fuel injection-related component. DO NOT allow fuel to contact engine or electrical components. When relieving the Compressed Natural Gas (CNG) fuel system pressure, take the following precautions 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.
  • Wear the appropriate hearing protection during the venting procedure. A sudden pressure loss from an uncontrolled gas leak can cause hearing damage.
NOTE: If High Pressure Lock-Off (HPL) is replaced, or tank is to be cleaned, replaced or shipped, also perform FUEL TANK VENTING (CNG) under FUEL SYSTEMS (CNG) in REMOVAL & INSTALLATION - CAVALIER & SUNFIRE article.
  1. Using a scan tool communicate with the PCM. If communication cannot be established and the DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS (ALTERNATIVE FUELS) under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 2.2L CAVALIER - BI-FUEL - CNG/GASOLINE article, has already been performed proceed to step  9.
  2. Store the DTC Information using the scan tool.
  3. Clear the DTC Information using the scan tool.
  4. Turn ignition switch to OFF position.
  5. Start and idle the engine. If the engine fails to operate on CNG or start at all, go to step  9.
  6. Ensure that the engine is operating on CNG by observing if the Fuel Indicator Light (FIL) is OFF, or the AFO Enable scan tool display indicates the engine is operating on alternative fuel.
  7. Using a scan tool, select the HIGH PRESSURE LOCK-OFF (HPL) solenoid special function.
  8. While the engine is operating on CNG command the HPL solenoid OFF and observe the fuel tank pressure sensor data 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 ignition switch to OFF position.
      • NOTE: Turning the ignition ON with the AF Lock-off fuse removed may set a DTC. Clear DTC Information after repairs are complete.
    • Remove the ALT. FUELS LOCK-OFF 10-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. Proceed to step  16.
  9. Close the manual lock-down screw located on the end of the High Pressure Lock-Off (HPL) solenoid. The manual shut-off fitting is closed by rotating the Allen screw clockwise until fully seated. See Fig 1.
  10. Turn ignition switch to OFF position.
  11. Disconnect the negative battery cable.
  12. Slowly loosen the fuel line fitting at the Low Pressure Lock-Off (LPL) solenoid elbow several turns. See Fig 2.
  13. 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.
    14. CAUTION: Natural gas is stored under high pressure. If a leak occurs, the high pressure could cause a personal injury. To prevent Compressed Natural Gas (CNG) fuel leaks, always replace the CNG fuel system "O" rings during service.
  14. Replace the "O" ring face seals at all fittings that were disconnected or loosened.
  15. After all necessary repairs are complete perform the following:
    • Tighten any loosened fittings to specification. See TORQUE SPECIFICATIONS in REMOVAL & INSTALLATION - CAVALIER & SUNFIRE article.
    • Open the manual lock-down screw located on the HPL manual shut-off fitting by rotating the Allen screw counter-clockwise until fully seated on any vehicle that does not require venting of the CNG fuel tanks.
    16. NOTE: If the service necessary for the vehicle requires that the ignition be turned ON with CNG components or lines disconnected, keep the ALT. FUELS LOCK-OFF fuse (10-amp) removed to prevent CNG fuel venting during an ignition ON prime pulse. Turning the ignition switch to RUN position with the AF Lock-off fuse removed may set a DTC. Clear DTC Information after repairs are complete.
  16. If the service necessary for the vehicle requires the CNG fuel tanks to be relieved of all internal pressure, go to FUEL TANK VENTING (CNG) under FUEL SYSTEMS (CNG) in REMOVAL & INSTALLATION - 2.2L CAVALIER & SUNFIRE article.
Fig 1: Turning Manual Vent Tool Screw
G00196202Courtesy of GENERAL MOTORS CORP.
Fig 2: Locating LPL Solenoid Elbow & Fuel Line Fitting
G00196194Courtesy of GENERAL MOTORS CORP.
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