GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 1997 🢒 Trans Sport Ext. 🢒 Engine 🢒 Engine Controls - 3.4L (LA1) 🢒 Backfire

Checks

Actions

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

Preliminary Checks
    •  Refer to Important Preliminary Checks .
    •  Refer to Visual/Physical Checks .
    •  Search for bulletins.

Sensor/System
    • Check for the following intermittent ignition system conditions:
       -  Intermittent CKP 24X 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 Test .
    •  Ensure each injector harness is connected to the correct injector/cylinder. Refer to Engine Controls Schematics .

Ignition System
    • Check for proper ignition voltage output using the following steps:
       -  Clip the spark tester J 26792 to engine ground.
       -  Connect one end of a spark plug wire to the 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 spark tester, a crisp blue 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 - 7000ohms (5K - 7Kohms)

       -  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
7000ohms (7Kohms)

    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 Engine Mechanical for diagnosis procedures.

Additional Checks
    • Check Intake and exhaust manifold(s) for casting flash. Refer to Engine Mechanical .
    •  Check TCC operation. Refer to Transaxle 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 System.