Backfires

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

PRELIMINARY STEPS
^ Refer to Important Preliminary Checks and Visual and Physical Checks before starting. Testing and Inspection

^ Search for technical service bulletins.

^ Check PCM grounds for being clean, tight and in their proper locations. Refer to Diagrams / Powertrain Control Module for PCM ground numbers and locations.

SENSOR/SYSTEM

^ Check for low fuel pressure. Refer to Fuel System Diagnosis.

^ Check for restricted fuel filter. Refer to Fuel System Diagnosis. System Diagnosis - Fuel System

^ Check for contaminated fuel. Refer to Alcohol/Contaminants in-Fuel Test.

^ 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. 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. Refer to Diagrams / Electrical / Vehicle Speed Sensor.

^ Using a Scan Tool, monitor Throttle Position (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 Acceleration Slip Regulation (ASR)/Traction Control System (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 Knock Sensor (KS) system for excessive spark retard activity. Refer to Knock Sensor. Knock Sensor

IGNITION SYSTEM

^ Check for proper ignition voltage output with spark tester J 26792. Refer to Distributor Ignition System for procedure.

^ 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's 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. System Check - Distributor Ignition (DI)

^ 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 / Specifications for correct gap. Specifications

^ The spark plug gapping should be done using a wire gauge gap tool (J 41319). Refer to Spark Plug Replacement. Service and Repair

^ If spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs. To determine the rich condition, refer to DTCs:

P0172 Fuel Trim System Rich Bank 1 P0172
OR
P0175 Fuel Trim System Rich Bank 2. P0175

For the oil fouling condition, refer to Engine. Engine

^ 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. System Check - Distributor Ignition (DI)

^ 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 / System Diagnosis. Testing and Inspection

^ Check engine thermostat for having a faulty part or for wrong heat range. Refer to Cooling System / Thermostat / Application and ID.

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 / System Diagnosis for diagnosis procedures. Engine

ADDITIONAL CHECKS

^ Visually (physically) check: Vacuum hoses for splits, kinks, and proper connections and routing as shown on the underhood Vehicle Emission Control Information label.

^ Check Intake and exhaust manifold(s) for casting flash. Refer to:

Engine / Intake Manifold. Intake Manifold
OR
Exhaust / Exhaust Manifold. Exhaust Manifold

^ Check Torque Converter Clutch (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 under this heading. System Check - Restricted Exhaust

^ 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 Park/Neutral Position (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.

^ Check for faulty motor mounts. Refer to Engine / Drive Belts, Mounts, Brackets and Accessories / Motor Mounts for inspection of mounts. Engine Mount