2005 Buick Century BaseFACTORY OEM SERVICE PROCEDURE

Engine Controls (Troubleshooting & Diagnosis): Troubleshooting: Backfire

Backfire

Inspection/Test Actions
DEFINITION: Fuel ignites in the intake manifold, or in the exhaust system, making loud popping noise.
Preliminary
Sensor/System
  • Inspect for the following intermittent ignition system conditions:
    • Monitor the 24X crank sensor and the CMP sensor signal present parameter on the scan tool. If both are not responding, test the sensor feed circuit. Both sensors use a separate feed circuit but are internally connected to power. Test all CKP sensor A and CMP sensor circuits for intermittents. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
    • Monitor the 3X parameter on the scan tool. If the 3X is not responding, inspect the CKP sensor B and circuits for intermittents. Inspect the ignition control (IC) circuit, IC timing control circuit, low resolution engine speed signal circuit, and the low reference circuit for intermittents. If these circuits become open, or shorted may not set a DTC immediately, but are capable of causing driveability complaints. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
    • Test the CKP sensor B for an internal short. Refer to Electronic Ignition (EI) System Diagnosis .
  • Inspect the mass air flow (MAF) sensor conditions. Ensure that the MAF sensor honeycomb is not damaged.
Fuel System
Ignition System
  • Inspect for proper ignition voltage output using the following steps:
    1. Attach the J 26792  Spark Tester to engine ground. See Special Tools .
    2. Connect the spark plug end of the spark plug wire to the J 26792  . See Special Tools . Connect the other end of the spark plug wire to the coil being tested.
    3. Connect the spark plug end of the companion spark plug wire to ground. The companion spark plug wire is the wire attached to the corresponding coil tower.
    4. Crank the engine while observing the J 26792  . See Special Tools . A spark should be observed.
    5. Repeat the above steps for each coil.
  • If spark is not present at the coils, inspect for the following conditions:
    • Coils-Cracks, carbon tracking/arcing, or a resistance value outside the specified range.

      Coil Resistance:  5000-8000 Ω (5K-8K Ω)

    • 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:  9868 Ω per meter (3000 Ω per foot)

      IMPORTANT: Spraying the secondary ignition wires with a light mist of water may help locate an intermittent condition. Ignition voltage will arc to ground when a secondary component is faulty.
    • Defective ignition module
    • Ignition system wiring-Loose ignition module feed or ground connection, or damaged system wiring.
  • Remove spark plugs and inspect for the following conditions:
    • 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. Refer to Spark Plug Inspection .
Engine Mechanical Inspect engine mechanical for the following conditions:
  • Test the compression-Refer to Engine Compression Test in Engine Mechanical.
  • Sticking or leaking valves
  • Worn camshaft lobes
  • Valve timing
  • Bent push rods
  • Worn rocker arms
  • Broken valve springs
  • Excessive oil in combustion chamber-Leaking valve seals
  • Low cylinder compression
  • Inspect for incorrect engine parts, such as the following basic components:
    • Camshaft
    • Cylinder heads
    • Pistons, etc.
  • Refer to Symptoms - Engine Mechanical in Engine Mechanical.
Additional
  • Inspect intake and exhaust manifolds for casting flash.
  • Test the torque converter clutch (TCC) operation.
  • Inspect the exhaust system for possible restriction:
    • Inspect the exhaust system for damaged or collapsed pipes.
    • Inspect the mufflers for heat distress or possible internal failure.
    • Test for possible catalytic converter restrictions by testing the exhaust system back pressure.
Factory Service Tools

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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.