Why Do Cars Backfire?

Robert Himler's Underground Racing Gallardo shoots flames

Backfiring basically happens when your car’s internal combustion engine has a moment of external combustion, or at least one happening elsewhere than the combustion chamber.  A little backfiring on deceleration can exist by design in modern cars, adding to the sporting sound.  It also channels what we hear when race cars do it and see when flames come out their exhausts.  But when it gets excessive, it’s time to look at these reasons why cars backfire.

Air/fuel mix.  When this gets sideways, backfiring happens.  The balance is struck with manual adjustments in carbureted cars, but fuel-injected cars manage this automatically by design.  If yours is the latter and like the majority of cars on the road today, there’s probably no adjustment to be done.

Hopefully only minor repairs are needed.  Start with common culprits like missing or leaking vacuum hoses and cleaning the mass airflow (MAF) sensor before moving to more serious hardware like the engine control unit (ECU).

Spark.  And by this, it can be the result of timing too far off.  In older cars, it’s a fairly straightforward process of advancing or retarding timing.  Modern cars have it done automatically by the ECU, so this again could be the reason behind backfiring.  But before you go to that trouble and expense, check and replace spark plugs as needed, along with the plug wires.

Emissions/exhaust.  If you’re muscling 500 horsepower on a spirited drive, you’ll probably get some backfires when downshifting and coasting.  Meandering through your neighborhood in a minivan, not so much.  Unwanted occurrences can also originate with the emissions and/or exhaust components.  The most audible and sometimes visual backfiring happens here, especially when the catalytic converter is missing or malfunctioning.  There isn’t a lot to check, clean or replace on your own here without proper tools and know-how.  When in doubt, have a qualified technician inspect and diagnose.       

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