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 System Unwanted Switch to Gasoline

Checks

Action

DEFINITION: The vehicle switches to gasoline operation during normal operation when there is still enough CNG pressure in the tank (over 1/8 tank). The Malfunction Indicator Lamp (MIL) does not illuminate.

Preliminary Checks

Refer to Symptoms .

Sensor Checks
    • Inspect the Crankshaft Position Sensor (CKP).
    • Inspect the Mass Air Flow (MAF) sensor.

Fuel System Checks
    • Inspect the fuel pressure. Refer to Fuel System Diagnosis .
    • Inspect the Gas Distribution Adapter (GDA) for leakage.
    • Inspect for an intermittent NGO enable circuit.
    • Inspect for a skewed or disconnected Fuel Sending Unit (FSU) (1997) or Fuel Pressure Sensor (FPS) (1998).
    • Inspect for a faulty Mixture Control Valve (MCV).

Ignition System Checks

Important:: CNG is a gaseous fuel and requires a higher secondary ignition system voltage for the equivalent gasoline operating conditions. Do not discount the possibility of a CNG-only ignition system failure if a problem is reported on the CNG system and not the gasoline system. Test the system accordingly.

    • Inspect for the proper ignition voltage output with the J 26792 Spark Tester or the equivalent.
    • Inspect the spark plugs. Remove the plugs and inspect them for the following conditions:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
    • Inspect for bare or shorted ignition wires.
    • Inspect for moisture in the distributor cap (if applicable).
    • Inspect for loose ignition coil connections.
    • Inspect for loose control module or ignition system grounds.
    • Inspect the Ignition Control (IC) circuit for an open or short to ground. Refer to Enhanced Ignition System Diagnosis in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual.
    • Remove the Crankshaft Position (CKP) sensor if the engine starts but then immediately stalls. Inspect the CKP sensor for improper resistance or faulty connections. Refer to Enhanced Ignition System Diagnosis in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual.