Backfires

SYMPTOM DESCRIPTION: Fuel ignites in intake manifold, or in exhaust system, making a loud popping noise.

SYSTEM OR COMPONENT TO CHECK

COMPUTER INPUTS/OUTPUTS
CHECK:
AIR MASS METER: Damaged or defective
THROTTLE POSITION SENSOR: Improperly adjusted, damaged or defective
CRANKSHAFT POSITION/SPEED SENSOR: Defective, improperly installed, or damaged.
OXYGEN SENSOR: Excessive lean air/fuel mixture
FUEL OR IGNITION CONTROL UNIT: Supply voltage, poor ground, defective control unit(s)

IGNITION
CHECK:
IGNITION COIL: Check for proper output voltage with diagnostic scope.
SPARK PLUGS: Remove spark plugs, check for wet plugs, cracks, wear, improper gap, burned electrodes, or heavy deposits. Replace as necessary.
SECONDARY CIRCUIT: For cross fire between spark plugs (distributor cap, spark plug wires and proper routing of plug wires.)
IGNITION CONTROL UNIT: Control unit defective
POWER STAGE: Defective

ENGINE
CHECK:
COMPRESSION: Perform a compression check, look for sticking or leaking valves.
CARBON DEPOSITS: Heavy carbon deposits on intake valves (most apparent during warm-up periods).
VACUUM LEAKS: Intake manifold gaskets and hoses. Crankcase air leaks.
RESTRICTIONS: Intake and exhaust system for casting flash or other restrictions.

FUEL
CHECK:
FUEL INJECTORS: Poor spray pattern, clogged or stuck injector.
FUEL DELIVERY: Check fuel pump pressure and volume output.
FUEL QUALITY: Check for fuel contamination, water, diesel, in fuel?