Backfiring basically happens when your carâs 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, itâs time to look at these reasons why cars backfire. Air/fuel mix. When this gets sideways, backfiring happens.