GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 1998 🢒 Firebird 🢒 Engine 🢒 Engine Controls - 5.7L 🢒 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 Symptoms , Important Preliminary Checks Before Starting.
    • Search for bulletins.
    • Inspect the PCM grounds for being clean, tight and in the proper locations. Refer to Ground Distribution Schematics in Wiring Systems.

Fuel System
    • Test for a low fuel pressure condition. 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 .
    • Verify 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
    • Inspect the Secondary Air Injection (AIR) system. Refer to Secondary Air Injection System Description .
    • Inspect the Air intake system and crankcase for air leaks.
    • Test 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.
    • Test 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 Available) 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
    • Test for proper ignition voltage output with spark tester J 26792 .
    • Test 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 inspect for the following:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
    • An improper spark plug gap will cause a driveability concern. Do the spark plug gapping 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 or oil fouled. Refer to DTC P0172 Fuel Trim System Rich Bank 1 and DTC P0175 Fuel Trim System Rich Bank 2 for diagnosis of the rich condition or Oil Consumption Diagnosis in Engine Electrical for diagnosis of the oil fouling condition.
    • Visually/Physically inspect secondary ignition for the following:
       - Ignition wires for cross firing
       - 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.
    • 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 in Engine Cooling.
    •  Test the engine thermostat for proper operation or for the wrong heat range. Refer to Thermostat Diagnosis in Engine Cooling.

Engine Mechanical
    • Inspect engine mechanical for the following:
       - Test compression. Refer to Engine Compression Test in Engine Mechanical 5.7L.
       - Sticking or leaking valves. Refer to Valve Train Diagnosis in Engine Mechanical 5.7L.
       - Worn camshaft lobe(s). Refer to Valve Train Diagnosis in Engine Mechanical 5.7L.
       - Valve timing. Refer to Valve Train Diagnosis in Engine Mechanical 5.7L.
       - Bent push rods. Refer to Valve Train Diagnosis in Engine Mechanical 5.7L.
       - Worn rocker arms. Refer to Valve Train Diagnosis in Engine Mechanical 5.7L.
       - Broken valve springs. Refer to Valve Train Diagnosis in Engine Mechanical 5.7L.
       - Excessive oil in combustion chamber-Leaking valve seals. Refer to Oil Consumption Diagnosis in Engine Mechanical 5.7L.
       - Low cylinder compression
    • For incorrect basic engine parts. Inspect the following:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.

Additional Checks
    • Visually (physically) inspect the vacuum hoses for splits, kinks, and proper connections and routing. Refer to Emission Hose Routing Diagram .
    • Inspect the intake manifold and the exhaust manifold passages for casting flash.
    • Test the TCC operation. The scan tool should indicate an RPM drop when the TCC is commanded ON. Refer to Torque Converter Clutch (TCC) Diagnosis in Automatic Transmission 4L60E.
    • Inspect the exhaust system for possible restrictions. Inspect 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. Refer to Restricted Exhaust System Check .
    • 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 condition exists, inspect the routing of the secondary ignition wires or high voltage components near ignition control circuits.
    • Check the PNP switch circuit. Refer to Park/Neutral Position Switch Diagnosis .
    • Check for faulty motor mounts.
    • Check the intake manifold and the exhaust manifold passages for casting flash. Refer to Intake Manifold Clean and Inspect and Exhaust Manifold Cleaning and Inspection in Engine Mechanical 5.7L.