GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1998 🢒 GMC C Pickup - 2WD (CNG) 🢒 Engine 🢒 Engine Controls - 5.0L and 5.7L 🢒 Fuel Pressure Relief

Fuel Pressure Relief 1998

Preliminary Checks

The vehicle is designed to start and operate on CNG. However, the vehicle will start automatically on gasoline if any one of the following conditions exists. To return to CNG operation, perform the recommended action:

    • The CNG fuel tank is empty.
        Recommended Action: Inspect the fuel level in the CNG fuel tank. Add CNG if necessary.
    • The engine coolant temperature is less than 10°F (-12°C).
        Recommended Action: Drive the vehicle if possible. After the first decel above 30 mph (48 km/h), the vehicle will automatically switch to CNG operation.
    • Every 100th start.
        Recommended Action: Drive the vehicle if possible. After the first decel above 30 mph (48 km/h), the vehicle will automatically switch to CNG operation.
    • A CNG fuel system fuse is bad.
        Recommended Action: Check the CNG fuel system fuses located in the underhood fuse/relay center.

Under certain conditions, it may not be possible to return to CNG operation. This can occur under the following conditions:

    • The engine crank time is more than eight seconds.
    • The AF OBD (On-Board Diagnostics) system detects a CNG fuel system problem. The Malfunction Indicator Lamp (Service Engine Soon Light) may come on.

Certain DTCs will cause an automatic switch to gasoline. The vehicle will not attempt to run again on CNG until the next key cycle. The vehicle will start on gasoline if the fault exists. However, the CNG fuel system can still be evacuated because the alternative fuels engine control unit (AF ECU) attempts to prime the CNG fuel system during crank.

Close the CNG manual shut-off valve (MSV) and perform the following procedure in order to evacuate the CNG fuel system when an automatic switch DTC is set:

  1. Turn the ignition ON.
  2. Start the engine.
  3. Turn the ignition OFF.
  4. Wait ten seconds.
  5. Repeat this procedure five times.

The prime pulse evacuates the CNG fuel system by cycling the engine on and off.

Using the CNG Manual Shut-Off Valve (MSV)

Perform this procedure only if the vehicle starts and operates on CNG. Refer to Preliminary Checks above if the vehicle operates no gasoline (fuel indicator light illuminated).

Relieve the CNG fuel pressure as follows in order to service components forward of the MSV:


    Object Number: 296395 Size: SH
  1. Close the CNG manual shut-off valve by turning the handle ¼-turn clockwise.
  2. Start and run the engine until it stalls. The CNG fuel system has now been evacuated from forward of the MSV.
  3. Turn the ignition OFF.
  4. A small amount of CNG will remain in the fuel lines. Slowly open the fitting in order to release any remaining pressure when disconnecting a fuel line.

Using the High Pressure Lock-Off (HPL)

Perform this procedure only if the vehicle starts and operates on CNG. Refer to Preliminary Checks above if the vehicle operates on gasoline (fuel indicator light illuminated).

Relieve the CNG fuel pressure as follows in order to service components from the fill valve to the MSV:


    Object Number: 296507 Size: SH
  1. Disconnect the high pressure lock-off (HPL) electrical connector (1).
  2. Start and run the engine until the fuel indicator light on the dash illuminates. The CNG fuel system has now been evacuated, and the vehicle is operating on gasoline.
  3. Turn the ignition OFF.
  4. A small amount of CNG will remain in the fuel lines. Slowly open the fitting in order to release any remaining pressure when disconnecting a fuel line.