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Fuel System Pressure Release (CNG)

WARNING: This page is about a different car, the 2002 Pontiac Sunfire and 2002 Chevrolet Cavalier. However, it is still accessible from the selected car via links, so may be relevant.
NOTE: If High Pressure Lock-Off (HPL) is replaced, or tank is to be cleaned, replaced or shipped, also perform FUEL TANK VENTING (CNG) .
  1. Connect scan tool to Data Link Connector (DLC). Clear all existing DTC's. Start engine and verify engine is operating on CNG. Using scan tool, select High Pressure Lock-Off (HPL) output control. While the engine is operating on CNG, command the HPL solenoid closed and observe the Fuel Pressure Sensor parameter on the scan tool. If Fuel Pressure Sensor parameter fails to decrease, go to next step. When the engine stalls or a switch-over to gasoline is observed, perform the following:
    • Turn the ignition off.
    • Remove the CNG fuse (20-amp) in order to prevent a CNG prime pulse from occurring.
    • 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.

  2. Remove spare tire cover. Remove jack and tool kit. Remove spare tire. Remove 4 tank cover bolts. Remove tank cover.
  3. Close manual lock-down screw located in center of HPL manual shutoff fitting by rotating the Allen screw clockwise with manual vent tool until fully seated. See Fig 1 and Fig 2 . Turn ignition off. Disconnect negative battery cable.
    4. CAUTION: Loosen, but DO NOT remove, fitting in this step.
  4. Loosen fuel line fitting at Low Pressure Lock-Off (LPL) solenoid inlet port several turns. If fuel is not venting with LPL inlet fitting loosened, relieve fuel line tension by pulling line away from LPL solenoid.
    5. CAUTION: Loosen, but DO NOT remove, fitting in this step.
  5. After fuel has stopped venting at the LPL fitting perform the following:
    • Loosen fuel line fitting at High Pressure Line (HPL) several turns.
    • Relieve fuel line tension by pulling line away from HPL.

    Pressure is released when fuel line can be pulled away from HPL and fuel fails to vent.

Fig 1: Identifying Manual Vent Tool
G00071007Courtesy of GENERAL MOTORS CORP.
Fig 2: Identifying CNG Fuel System Components
G00056057Courtesy of GENERAL MOTORS CORP.
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