GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC K Sierra - 4WD 🢒 Engine 🢒 Engine Controls - 4.8L, 5.3L, and 6.0L 🢒 Backfire

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DEFINITION: Fuel ignites in the intake manifold or in the exhaust system, making a loud popping noise.

Preliminary Checks
    • Refer to Important Preliminary Checks Before Starting in Symptoms .
    • Search for bulletins.
    • Check the PCM grounds for being clean, tight and in the proper locations. Refer to Engine Controls Schematics .

Fuel System
    • Check for incorrect fuel pressure. Refer to Fuel System Diagnosis .
    • Check for a restricted fuel filter. Refer to Fuel System Diagnosis .
    • Check for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Check that each injector harness is connected to the correct injector/cylinder according to the firing order (firing order: 1-8-7-2-6-5-4-3). Relocate injector harnesses as necessary.

Sensor/System
    • Check the Secondary Air Injection (AIR) system.
    • Check the air intake system and crankcase for air leaks.
    • Check the crankcase ventilation valve for proper operation. Place a finger over inlet hole in valve end several times. The valve should snap back. If not, replace the valve.
    • Check for proper calibration of speedometer. Connect Signal Generator J 33431-B to VSS electrical connector. Ignition ON, tester ON and monitor the speedometer. The speedometer should indicate 86 km/h (54 mph). Refer to Speedometer Inaccurate (J 38522 Unavailable) in Instrument Panel, Gauges and Console.
    • Use a scan tool in order to monitor the KS system for excessive spark retard activity. Refer to Knock Sensor (KS) System Description .

Ignition System
    • Check for proper ignition voltage output with spark tester J 26792 .
    • Check for an intermittent ignition system malfunction in the following circuits:
       - Intermittent ignition control circuit.
       - Use the scan tool's Snapshot feature in order to help locate an intermittent ignition failure.
    • Remove spark plugs and check for the following:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits

Refer to Spark Plug Visual Diagnosis in Engine Electrical.

    • An improper spark plug gap will cause a driveability problem. Refer to Spark Plug Visual Diagnosis in Engine Electrical. Gap the spark plugs using a wire gauge gap tool (J 41319). Refer to Spark Plug Replacement in Engine Electrical.
    • Determine the cause of the fouling before replacing the spark plugs if the spark plugs are gas, coolant, or oil fouled. Refer to DTC P0172 Fuel Trim System Rich Bank 1 or DTC P0175 Fuel Trim System Rich Bank 2 for diagnosis of the rich condition. Refer to Spark Plug Visual Diagnosis in Engine Electrical for diagnosis of coolant or oil fouled spark plugs.
    • Visually/Physically inspect secondary ignition for the following:
       - Ignition wires arcing to ground
       - Ignition coils arcing to ground
    • Wet down the secondary ignition system with water from a spray bottle. Wetting down the secondary ignition system may help locate damaged or deteriorated components. Look/listen for arcing or misfiring as you apply the water.
    • Monitor the Misfire Current Counters while driving the vehicle in the conditions that the misfire occurred. If a cylinder can be located with a misfire use the DTC P0300 table for diagnosis.
    • Check for loose ignition coil grounds. Refer to Electronic Ignition (EI) System Diagnosis .

Engine Cooling System
    • Check the engine coolant level for being low. Refer to Loss of Coolant in Engine Cooling.
    • Check the engine thermostat for proper operation and for the correct heat range. Refer to Thermostat Diagnosis in Engine Cooling.

Engine Mechanical
    • Check engine mechanical for the following:
       - Check compression. Refer to Engine Compression Test in Engine Mechanical 4.8L, 5.3L, 6.0L.
       - Sticking or leaking valves. Refer to Valve Train Diagnosis in Engine Mechanical 4.8L, 5.3L, 6.0L.
       - Worn camshaft lobe(s). Refer to Valve Train Diagnosis in Engine Mechanical 4.8L, 5.3L, 6.0L.
       - Valve timing. Refer to Valve Train Diagnosis in Engine Mechanical 4.8L, 5.3L, 6.0L.
       - Bent push rods. Refer to Valve Train Diagnosis in Engine Mechanical 4.8L, 5.3L, 6.0L.
       - Worn rocker arms. Refer to Valve Train Diagnosis in Engine Mechanical 4.8L, 5.3L, 6.0L.
       - Broken valve springs. Refer to Valve Train Diagnosis in Engine Mechanical 4.8L, 5.3L, 6.0L.
       - Excessive oil in combustion chamber-Leaking valve seals. Refer to Oil Consumption Diagnosis in Engine Mechanical 4.8L, 5.3L, 6.0L.
       - Low cylinder compression. Refer to Engine Compression Test in Engine Mechanical 4.8L, 5.3L, 6.0L.
    • For incorrect basic engine parts. Inspect the following:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.
    • Refer to Engine/Engine Mechanical for diagnosis procedures.

Additional Checks
    • Visually (physically) check the vacuum hoses for splits, kinks, and proper connections and routing as shown on Vehicle Emission Control Information label.
    • Check the intake manifold and the exhaust manifold passages for casting flash. Refer to Engine/Engine Mechanical.
    • Check the Transmission Torque Converter Clutch (TCC) operation. The scan tool should indicate an RPM drop when the TCC is commanded ON. Refer to Functional Test in Automatic Transmission (4L60E) or Functional Test in Automatic Transmission (4L80E).
    • Check the exhaust system for possible restrictions. Check the following:
       - Inspect the exhaust system for damaged or collapsed pipes.
       - Inspect the exhaust manifold for a collapsed inner wall.
       - Inspect the mufflers for heat distress or possible internal failure.
       - Inspect for possible plugged Three-Way catalytic converters by comparing exhaust system back pressure on each side of engine. Check back pressure by removing AIR check valves near exhaust manifolds. Refer to Restricted Exhaust System Check in Engine Exhaust.
    • Electromagnetic Interference (EMI) on the reference circuit can cause an engine miss condition. A scan tool can usually detect EMI by monitoring the engine RPM. A sudden increase in RPM with little change in actual engine RPM change may indicate that EMI is present. If a problem exists, check for high voltage components near the ignition control circuits.
    • Check the Park/Neutral Position (PNP) switch operation. Refer to DTC P0705 Trans Range Switch Circuit and DTC P0706 Trans Range Switch Performance .
    • Check for faulty motor mounts. Refer to Engine Mount Inspection in Engine Mechanical 4.8L, 5.3L, 6.0L.
    • Check the intake manifold and the exhaust manifold passages for casting flash. Refer to Engine/Engine Mechanical.