Engine backfire is the result of incomplete combustion inside the combustion chamber. The excess air and fuel mixture can travel to the intake manifold or through the exhaust manifold and detonate prematurely, causing a backfire.
There are two types of engine backfires: an "after fire" (a detonation inside the exhaust manifold) and an actual "back fire" (the ignition of air and fuel inside the intake manifold and throttle body).
Improper Engine Timing
Improper timing can result in both intake and exhaust backfires. A timing that is too advanced will detonate the spark plugs even before the intake valve gets a chance to fully close and seal the system. The partially burnt gasses will travel back into the intake valve and through the intake manifold where an explosion or "back fire" occurs during the next combustion cycle. An engine that runs lean will produce an incomplete combustion -- the partially burnt air and fuel mix will travel to the exhaust manifold and detonate upon the next combustion cycle.
Improper or Damaged Ignition Wiring
The ignition system relies on well-maintained components to properly operate. Damaged ignition cables and worn-out distributor and rotor systems will all contribute to improper timing -- thereby contributing to engine backfires.
Low Fuel Pressure
Dirty injectors or carburettors, worn out fuel filters, clogged fuel lines or a faulty fuel pump will all result in a loss of pressure in the fuel system. The engine needs to maintain a specific fuel pressure to properly inject or provide the right amount of fuel in the combustion process. Any or all of the above-mentioned factors will contribute engine backfires as a result of low fuel pressure.
Damaged Catalytic Converters
A faulty, damaged or missing catalytic converter will also contribute to engine misfires. This is because the catalytic converter is an important part of the exhaust system and is monitored by the engine ECU. A damaged converter will result in miscalculated engine readings and cause the system to advance or retard the timing unnecessarily -- thereby causing an "after fire" or backfire at the exhaust.
How to Prevent and Eliminate Engine Backfires
Preventive maintenance (as prescribed in your owner's manual) will help prevent engine backfires. Change the fuel filter in your car every year, or as indicated in the manual, to always maintain the right fuel pressure. Regular oil changes and tune-ups will also keep your engine in tip-top shape. Cars with an automatic ignition system will need the spark plugs replaced every 12 months or as prescribed in the owner's manual.