Backfires
DEFINITION: Fuel ignites in the intake manifold, or in the exhaust system, making loud popping noise.PRELIMINARY CHECKS
^ Refer to Symptoms, Important Preliminary Checks Before Starting. Testing and Inspection
^ Search for bulletins.
^ Check PCM grounds for being clean, tight and in their proper locations. Refer to Diagrams. Diagrams
SENSOR/SYSTEM
^ Check for low fuel pressure. System Diagnosis - Fuel System
^ Check for restricted fuel filter. System Diagnosis - Fuel System
^ Check for contaminated fuel. Refer to Alcohol/Contaminants in-Fuel Testing Procedure. Fuel
^ Check each injector harness is connected to the correct injector/cylinder according to the firing order (firing order: 1-8-4-3-6-5-7-2). Relocate injector harnesses as necessary.
SENSOR
^ Check Exhaust Gas Recirculation (EGR) for being loose or for a faulty gasket. Check EGR operation for being open all the time. Refer to EGR Valve
^ Check Secondary Air Injection (AIR) system.
^ Check Air intake system and crankcase for air leaks.
^ Check Crankcase ventilation valve for proper operation. Place finger over inlet hole in valve end several times. Valve should snap back. If not, replace valve.
^ Check for proper calibration of speedometer. Connect Signal Generator (J 33431-B) to Vehicle Speed Sensor (VSS) electrical connector. Ignition ON, tester ON and monitor the speedometer. The speedometer should indicate 86 km/h (54 mph).
^ Using a Scan Tool, monitor TP sensor voltage. The TP sensor voltage should be less then 0.9 volt. If the TP sensor voltage is greater than 0.9 volt, inspect the ASR/TCS adjuster assembly (If so equipped). Also inspect throttle body and accelerator cables for binding. Refer to Adjuster Assembly Throttle And Cruise Control Cables.
^ Using a scan tool, monitor KS system for excessive spark retard activity. Refer to Knock Sensor.
IGNITION SYSTEM
^ Check for proper ignition voltage output with spark tester J 26792.
^ Check for an intermittent ignition system malfunction in the following circuits:
- Intermittent Low resolution circuit.
- Intermittent High resolution circuit.
- Intermittent Ignition Control circuit.
- Using the scan tool Snapshot feature may help locate an intermittent ignition failure. Review low and high resolution signals. Abnormal low and high resolution signals can cause a backfire on acceleration. Refer to Distributor Ignition System Check
^ Remove spark plugs and check for the following:
- Wet plugs
- Cracks
- Wear
- Improper gap
- Burned electrodes
- Heavy deposits
^ An improper spark plug gap will cause a driveability problem. Refer to spark plug gap specification is Section OB. The spark plug gapping should be done using a wire gauge gap tool (J 41319). Refer to Spark Plug Replacement. If spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs. Refer to DTC P0172 Fuel Trim System Rich Bank 1 and DTC P0175 Fuel Trim System Rich Bank 2 to determine the Rich condition or Engine Mechanical for the oil fouling condition.
^ Visually/physically inspect Secondary ignition for the following:
- Ignition wires for cross firing.
- Ignition wires arcing to ground.
- Ignition wires for proper routing.
- Distributor cap for cracks or carbon tracking.
^ Wetting down the secondary ignition system with water from a spray bottle may help locate damaged or deteriorated components. Look/listen for arcing or misfiring as water is applied.
^ Check for loose ignition coil ground. Refer to Distributor Ignition System Check
^ Check distributor vent system for being restricted or open. Refer to Distributor Vent System Check. System Check - Distributor Vent
ENGINE COOLING SYSTEM
^ Check engine coolant level for being low. Refer to Cooling System.
^ Check engine thermostat for having a faulty part or for wrong heat range. Refer to Cooling System.
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 for diagnosis procedures.
ADDITIONAL CHECKS
^ Visually (physically) check: Vacuum hoses for splits, kinks, and proper connections and routing as shown on Vehicle Emission Control Information label.
^ Check Intake and exhaust manifold(s) for casting flash. Refer to Engine Mechanical.
^ Check TCC operation. The scan tool should indicate an RPM drop when the TCC is commanded ON. Refer to Transmission Control Systems.
^ 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 converters by comparing exhaust system back pressure on each side of engine. Check back pressure by removing AIR check valves near exhaust manifolds. Refer to Restricted Exhaust System Check and Exhaust System.
^ An engine miss condition can be caused by Electromagnetic Interference (EMI) on the reference circuit. EMI can usually be detected by monitoring engine RPM with a scan tool. A sudden increase in RPM with little change in actual engine RPM change, indicates EMI is present. If a problem exists, check routing of secondary ignition wires, high voltage components (Near Ignition Control circuits). Check distributor ground circuit for being open or loose.
^ Is Traction Control Adjuster Assembly out of adjustment? Refer to Adjuster Assembly-Throttle And Cruise Control Cables.
^ Check PNP switch circuit. Refer to PNP Switch Check, or use the Tech 1 and be sure the scan tool indicates vehicle is in drive with gear selector in drive or overdrive. PNP Switch Check
^ Check for faulty motor mounts. Refer to Engine for inspection of mounts.
^ Check for intake and exhaust manifold passages for casting flash. Refer to Engine.