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HOMESERVICE MANUALSCHEVROLET1998SUBURBAN K2500, 7.4 J, PART TIMEREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 170 (ENGINE PERFORMANCE SYSTEM REMOVAL, OVERHAUL & INSTALLATION - BI-FUEL)FUEL SYSTEMSFUEL SYSTEM PRESSURE RELEASEFUEL PRESSURE RELIEF PROCEDURE
1998 Chevrolet Suburban K2500, 7.4 J, Part Time

Fuel Pressure Relief Procedure

1998 Chevrolet Suburban K2500, 7.4 J, Part TimeSECTION Fuel Pressure Relief Procedure
WARNING: This page is about a different car, the 2002 GMC Savana and 2002 Chevrolet Chevy Express. However, it is still accessible from the selected car via links, so may be relevant.
NOTE: If the service necessary for the vehicle requires that the ignition be turned ON with CNG components or lines disconnected, keep the CNG (20-amp) fuse removed in order to prevent CNG fuel venting during an ignition ON prime pulse. In order to prevent CNG fuel leaks, always replace the CNG fuel system "O" rings during service. Replace the "O" ring face seals at all fittings that were disconnected or loosened.
  1. Using a scan tool, communicate with the Alternative Fuels Engine Control Module (AF ECM). If communication cannot be established and alternative fuels diagnostic system check has already been performed, go to step  7 .
  2. Store the DTC Information for any AF ECM DTCs that may be set.
  3. Clear all AF ECM DTC Information and then turn the ignition OFF.
  4. 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 both the scan tool Engine Data-Alt. Fuels displays of AFO Signal and AFO Signal Command indicate the AF ECM is in command of engine fueling. If the engine fails to operate on CNG or start at all, go to step  7 .
  5. Using the scan tool, select the high pressure lock-off relay special function.
  6. While the engine is operating on CNG command the lock-off relay OFF and observe the scan tool fuel tank pressure sensor data parameter. If the fuel pressure fails to decrease, go to next step. When the engine stalls or a switch-over to gasoline is observed, turn ignition OFF. Remove the CNG (20-amp) fuse in order to prevent a CNG prime pulse from occurring during service. 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  12 .
  7. Close the manual lock-down screws located on both of the high pressure lock-off solenoid manual shut-off fittings by rotating the allen screws clockwise until fully seated. Holes are provided in the side of the fuel tank frame in order to allow access to the manual shut-off fittings.
  8. Turn ignition OFF and disconnect the negative battery cable.
  9. Slowly loosen the fuel line fitting at the High Pressure Regulator (HPR) inlet port several turns. DO NOT remove the fitting.
  10. If fuel is not venting with the HPR inlet fitting loosened, relieve fuel line tension by pulling the line away from the HPR.
  11. After fuel has stopped venting at the HPR fitting, slowly loosen the fuel line fitting at the Low Pressure Lock-Off (LPL) solenoid inlet port several turns. DO NOT remove the fitting. Relieve fuel line tension by pulling the line away from the LPL solenoid. Fuel pressure relief for the CNG system is complete when the fuel line at the LPL solenoid inlet port can be pulled away from the "O" ring face seal and fuel fails to vent.
  12. 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 screws closed on any vehicle that will require CNG fuel tank venting. 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  .
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Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.