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Home >> Pontiac >> 2002 >> Aztek AWD >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Basic Diagnostic Procedures >> Basic Fuel System Checks (CNG) >> Fuel System Pressure Release
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Fuel System Pressure Release

  1. Using scan tool, attempt to communicate with Alternative Fuel Electronic Control Unit (AF ECU). If communication is possible, go to next step. If communication is not possible and system check has been performed, go to step  7.
  2. Read and record any AF ECU DTCs. Go to next step.
  3. Clear all AF ECU DTCs. Turn ignition off. Go to next step.
  4. Start engine. Ensure vehicle is operating on CNG by noting if fuel indicator light (located on fuel gauge select switch) is off and Natural Gas Operation (NGO) in scan tool data is enabled. If vehicle is operating on CNG, go to next step. If vehicle does not start or is not operating on CNG, go to step  7.
  5. Using scan tool, select High Pressure Lock-Off (HPL) output control. Go to next step.
  6. Monitor Fuel Pressure Sensor (FPS) data. While vehicle is operating on CNG, use scan tool to close HPL. If FPS data does not change, go to next step. When engine stalls or changes to gasoline operation, turn ignition off. Remove CNG fuse (20-amp) located in underhood fuse block. Slowly open fuel system fittings to vent remaining pressure.
  7. Remove fuel tank cover. Close manual lock down screw on HPL by rotating Allen screw clockwise until fully seated. Go to next step.
  8. Turn ignition off. Disconnect negative battery cable. Ensure vehicle is in a well ventilated area, away from sparks and flame. Go to next step.
  9. Slowly open fuel line fitting at Low Pressure Lock-Off (LPL) inlet port. Do not remove fitting. Go to next step.
  10. If fuel is not venting, relieve fuel line tension by pulling line away from LPL. Go to next step.
    11. NOTE: If ignition will be turned on while system is vented, ensure CNG fuse is removed.
  11. After fuel has stopped venting, slowly loosen fuel line fitting at High Pressure Regulator (HPR) inlet. Do not remove fitting. Fuel pressure relief is complete when fuel line at HPR inlet stops venting. After repair, replace "O" rings at all fittings that were loosened. Tighten all loosened fittings to specification. Open manual lock down screw counterclockwise to fully seated position.
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