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For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2007 🢒 Sierra Classic - 4WD 🢒 Diagnostic Navigation 🢒 Vehicle Diagnostic Information 🢒 Backfire

Inspections

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

Preliminary Inspections
    • Refer to Important Preliminary Inspections Before Starting in Symptoms - Engine Controls .
    • Search for bulletins.
    • Verify that the powertrain control module (PCM) grounds are clean, tight, and in the proper locations. Refer to Power and Grounding Component Views and Engine Controls Schematics .

Fuel System
    • Inspect for incorrect fuel pressure. Refer to Fuel System Diagnosis .
    • Inspect for a contaminated fuel condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Verify that each injector harness is connected to the correct injector or cylinder. Relocate injector harnesses as necessary.

Sensor/System
    • Inspect the air intake system and crankcase for air leaks.
    • Inspect the crankcase ventilation valve for proper operation. Refer to Crankcase Ventilation System Inspection/Diagnosis .
    • Inspect for an inaccurate speedometer.
    • Use a scan tool in order to monitor the knock sensor (KS) system for excessive spark retard activity. Refer to Knock Sensor (KS) System Description .

Ignition System
    • Inspect for proper ignition voltage output with the J 26792 Spark Tester .
    • Remove spark plugs and inspect for the following:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
       - Refer to Spark Plug Inspection .
    • An improper spark plug gap will cause a driveability problem. Refer to Spark Plug Inspection . Gap the spark plugs using a wire gage gap tool . Refer to Spark Plug Replacement .
    • Determine the cause of the fouling before replacing the spark plugs. Refer to Spark Plug Inspection for diagnosis.
    • Visually and physically inspect secondary ignition for the following:
       - Ignition wires arcing to ground
       - Ignition coils arcing to ground
    • Soak the secondary ignition system with water from a spray bottle. Soaking the secondary ignition system may help locate damaged or deteriorated components. Look and 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 misfiring cylinder can be located, use the DTC P0300 table for diagnosis. Refer to DTC P0300 .
    • Inspect for loose ignition coil grounds. Refer to Electronic Ignition (EI) System Diagnosis .

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

Engine Mechanical
    • Inspect engine mechanical for the following:
       - Compression--Refer to Engine Compression Test .
       - Sticking or leaking valves
       - Worn camshaft lobes
       - Valve timing
       - Bent push rods
       - Worn rocker arms
       - Broken valve springs
       - Excessive oil in combustion chamber or leaking valve seals--Refer to Oil Consumption Diagnosis .
    • For incorrect basic engine parts--Inspect the following:
       - The camshaft--Refer to Camshaft and Bearings Cleaning and Inspection .
       - The cylinder heads--Refer to Cylinder Head Cleaning and Inspection .
       - The pistons, etc.--Refer to Piston, Connecting Rod, and Bearing Cleaning and Inspection .
    • Refer to Symptoms - Engine Mechanical for diagnosis procedures.

Additional Inspections
    • Visually and physically inspect the vacuum hoses for splits, kinks, and proper connections and routing as shown on the Vehicle Emission Control Information label.
    • Inspect the intake manifold and the exhaust manifold passages for casting flash.
    • Inspect the transmission torque converter clutch (TCC) operation. The scan tool should indicate an RPM drop when the TCC is commanded ON. Refer to Torque Converter Diagnosis for the 4L60-E transmission or Torque Converter Diagnosis for the 4L80-E transmission.
    • Inspect the exhaust system for possible restrictions. Inspect the following:
       - Inspect the exhaust system for damaged or collapsed pipes.
       - Inspect the mufflers for heat distress or possible internal failure.
       - Inspect for possible plugged catalytic converters. Refer to Restricted 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, inspect for high voltage components near the ignition control circuits.
    • Inspect the park/neutral position (PNP) switch operation. Refer to Park/Neutral Position Switch Adjustment for the 4L60-E transmission.
    • Inspect for faulty motor mounts. Refer to Engine Mount Inspection .
    • Inspect the intake manifold and the exhaust manifold passages for casting flash