GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 1999 🢒 Grand Prix 🢒 Engine 🢒 Engine Controls - 3.1L 🢒 Backfire

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

Preliminary Checks
    •  Refer to Intermittent Conditions .
    •  Search for bulletins.

Sensor/System
    • Check for the following intermittent ignition system conditions:
       -  Intermittent 24X CKP Signal.
       -  Intermittent ignition feed circuit or sensor ground circuit to the crankshaft position sensor or the camshaft position sensor.
       -  Intermittent 3X reference signal.
       -  Intermittent camshaft position signal
    •  Check the Mass Air Flow sensor conditions. Ensure that the MAF Sensor honeycomb is not damaged.

Fuel System
    • Check for low fuel pressure. Refer to Fuel System Pressure Test .
    •  Check for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    •  Ensure each injector harness is connected to the correct injector/cylinder.

Ignition System
    • Check for proper ignition voltage output using the following steps:
       -  Clip the J 26792 spark tester to engine ground.
       -  Connect one end of a spark plug wire to the J 26792 spark tester; leave the other end of the spark plug wire to the coil being tested.
       -  Connect one end of another spark plug wire to the other coil tower; connect the other end of the spark plug wire to ground.
       -  Crank the engine while observing the J 26792 spark tester; a spark should be observed.
       -  Repeat the above steps for each coil.
    •  If adequate spark is not present at the coil(s), check for the following conditions:
       -  Coil(s) -- Cracks, carbon tracking/arcing, or a resistance value outside the specified range.

Coil Resistance
5000 - 7000 ohms (5K - 7K ohms).

       -  Spark Plug wires -- Signs of arcing/cross firing, cracks, carbon tracking, plug boot damage, pinched, improper routing, or a resistance value outside the specified range.

Spark Plug Wire Resistance
600ohms per foot (1 968ohms per meter).

    Important: :  Spraying the secondary ignition wires with a light mist of water may help locate an intermittent problem. Ignition components will arc to ground when a secondary component is faulty.

       -  Defective module.
       -  Ignition System Wiring -- Loose ignition module feed or ground connection, or damaged system wiring.
    •  Remove spark plugs and check for the following:
       -  Fouled plugs
       -  Cracks
       -  Wear
       -  Improper gap
       -  Burned or damaged electrodes
       -  Improper heat range or reach
    •  If spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs.

Engine Mechanical

Check engine mechanical:

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

Additional Checks
    • Check Intake and exhaust manifold(s) for casting flash.
    •  Check TCC operation. Refer to Torque Converter Diagnosis .
    •  Check exhaust system for possible restriction. Check the following:
       -  Inspect exhaust system for damaged or collapsed pipes.
       -  Inspect mufflers for heat distress or possible internal failure.
       -  For possible plugged Three-Way catalytic converter. Refer to Restricted Exhaust System Check and Exhaust Leakage - Not OBD II .