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 🢒 Backfire

Checks

Action

DEFINITION: The fuel ignites in the intake manifold or in the exhaust system. This produces a loud popping noise.

Preliminary Checks

Refer to Symptoms .

Sensor Checks

Inspect for an open NGO enable circuit.

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 on the gasoline system. Test the system accordingly.

    • Inspect for the proper ignition coil output voltage using 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 the ignition system. Refer to Enhanced Ignition System Diagnosis in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual.
    • Inspect and adjust the CMP.
    • Inspect for crossfire between the spark plugs (the distributor cap, the spark plug wires and the proper routing of the plugs wires).

Engine Mechanical Checks

Important:: The AF system works on a fumigation principle of fuel introduction. It is more sensitive to intake manifold leakage than the gasoline fuel supply system.

    • Inspect the engine for the following conditions:
       - Improper valve timing
       - Engine compression
       - Manifold vacuum leaks
       - Intake manifold gasket leaks.
       - Sticking or leaking valves.
       - Exhaust system leakage
    • Verify the EGR system operation for being open at all times. Refer to EGR System Diagnosis in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual.
    • Inspect the intake and exhaust system for casting flash or other restrictions.

Fuel System Checks
    • Perform a fuel system diagnosis. Refer to Fuel System Diagnosis .
    • Verify the correct bi-fuel VCM software is installed.

Additional Checks
    • Inspect for the proper VCM software calibrations.
    • Inspect the EGR gasket for a faulty or loose fit.
    • Verify the EGR valve operation for being open all the time. Refer to EGR System Diagnosis in Engine Controls - 5.0, 5.7L in the C/K Truck Service Manual.