GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2004 🢒 GTO 🢒 Engine 🢒 Engine Electrical 🢒 Backfire

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DEFINITION : The 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 - Engine Controls .
    • Search for bulletins.
    • Inspect the powertrain control module (PCM) grounds. Ensure that they are clean, tight and in the proper locations. Refer to Engine Controls Schematics .

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

Sensor/System
    • Inspect the air intake system and crankcase for air leaks.
    • Test the crankcase ventilation valve for proper operation. Place a finger over the inlet hole in the valve end several times. The valve should snap back. If not, replace the valve.
    • Verify 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 Diagnostic System Check - Instrument Cluster in Instrument Panel, Gages 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
    • Test for proper ignition voltage output with spark tester J 26792 . Refer to Electronic Ignition (EI) System Diagnosis .
    • Inspect for an intermittent ignition system malfunction in the following circuits:
       - Ignition control circuit
       - Power and ground to the ignition coils
    • 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 causes a driveability condition. Gap the spark plugs using a wire gauge gap tool. Refer to Spark Plug Replacement .
    • Determine the cause of the fouling before replacing the spark plugs. If the spark plugs are gas fouled, refer to DTC P0172 or P0175 . If the spark plugs are coolant fouled or oil fouled, refer to Oil Consumption Diagnosis in Engine Mechanical-5.7L or Loss of Coolant in Engine Cooling.
    • Inspect the secondary ignition for the following conditions:
       - Ignition wires arcing to ground
       - Ignition coils arcing to ground
    • Wet the secondary ignition system with water from a spray bottle. Wetting the secondary ignition system may help to locate damaged or deteriorated components. When you apply the water, look and listen for arcing or misfiring.
    • 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 Cooling System Draining and Filling in Engine Cooling.
    • Test the engine thermostat for proper operation and heat range. Refer to Thermostat Diagnosis in Engine Cooling.

Engine Mechanical
    • Inspect engine mechanical for the following:
       - Test the engine compression. Refer to Engine Compression Test in Engine Mechanical-5.7L.
       - Sticking or leaking valves
       - Worn camshaft lobes
       - Valve timing
       - Bent push rods
       - Worn rocker arms
       - Broken valve springs
       - Excessive oil in combustion chamber due to leaking valve seals. Refer to Oil Consumption Diagnosis in Engine Mechanical-5.7L.
    • Inspect for incorrect basic engine parts. Inspect the following components:
       - The camshaft
       - The cylinder heads
       - The pistons

Additional Checks
    • Inspect the vacuum hoses for splits, for kinks, for 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. Refer to Exhaust Manifold Cleaning and Inspection and Intake Manifold Cleaning and Inspection in Engine Mechanical-5.7L.
    • Test the Transmission Torque Converter Clutch (TCC) operation. The scan tool should indicate an RPM drop when the TCC is commanded ON. Refer to Diagnostic Starting Point - Automatic Transmission in Automatic Transmission-4L60-E.
    • Inspect the exhaust system for possible restrictions. Inspect the following components:
       - Inspect the exhaust system for damaged or collapsed pipes. Refer to Restricted Exhaust in Engine Exhaust.
       - Inspect the exhaust manifold for a collapsed inner wall. Refer to Exhaust Manifold Cleaning and Inspection in Engine Exhaust.
       - Inspect the mufflers for heat distress or possible internal failure. Refer to Symptoms - Engine Exhaust in Engine Exhaust.
       - Inspect for a possible plugged 3-way catalytic converters by comparing the exhaust system back pressure on each side of the engine. Test the back pressure by removing the AIR check valves near the exhaust manifolds or the Heated Oxygen sensors. Refer to Restricted Exhaust 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 the RPM, with little change in actual engine RPM change may indicate that EMI is present. Inspect for high voltage components near the ignition control circuits if a condition exists.
    • Test the PNP switch circuit. Refer to Park/Neutral Position Switch Adjustment in Automatic Transmission-4L60-E.
    • Inspect for faulty engine mounts. Refer to Engine Mount Inspection in Engine Mechanical-5.7L.