Engine backfire is caused by an imbalance in the air-to-fuel ratio of the vehicle. Backfires occur in one of two places. A backfire in the intake manifold is caused by a ratio that is too lean (not enough fuel). A backfire out of the exhaust system is caused by a ratio that is too rich (too much fuel).
Backfire issues are corrected by troubleshooting and fixing the fuel-to-air ratio. Vacuum leaks, airflow sensors and oxygen sensors are all possible trouble spots allowing more air into the system. A clogged fuel filter or faulty fuel pump can also be the cause of an imbalance in the air-to-fuel ratio.
Another possibility is incorrect engine timing. Bad engine timing is often the issue behind intake backfires, but they can also cause exhaust backfires. A car with bad timing runs and idles badly or erratically. Adjusting engine timing is not difficult and requires a service manual and timing light.
To troubleshoot a backfiring problem, identify the possible causes and replace any necessary parts. Clogged fuel filters cause low fuel pressure and set up a backfire situation. Change the fuel filter regularly, and service the fuel-injection system. Replace sensors if they are not functioning correctly, and check for vacuum leaks.