Mechanics

Backfire Repair

Step by step guide on how to repair an engine backfire problem. This guide pertains to most non hybrid vehicles.

Difficulty Scale: 4 of 10

Tools and Supplies Needed

  • Fuel pressure gauge
  • Engine code scanner
  • Protective eyewear and clothing
Begin with the car on level ground, in park with the emergency brake set, engine cool.

Step 1 - If  the check engine or service engine soon light is illuminated visit - Scan for trouble codes

Step 2 - The ignition system is designed to operate at a specific voltage and amperage output. If this voltage/amperage is forced to spike due to faulty or inferior tune up parts it could cause the system to crossfire. A crossfire condition can allow the electrical discharge for a particular spark plug circuit to transfer to a alternative cylinder which can produce a backfire in either the intake or exhaust system.

Step 3 - A fuel system is designed to produce a consistent fuel pressure. If this fuel pressure drops, it can cause a lean mixture which can cause the engine to backfire. Visit - Fuel system test

Step 4 - Engine ignition timing is set at a specific degree, in relation to the crankshaft of the engine. If this timing becomes misadjusted it can cause low power, poor gas mileage, engine detonation (pinging) and backfiring. Visit - Set engine ignition timing

Step 5 - On distributor style ignition systems, if moisture is present inside of the distributor cap, it can cause internal ignition crossfire. This crossfire condition can allow the electrical discharge for a particular spark plug circuit to transfer to an alternative cylinder which produced a backfire. To check for this condition remove the distributor cap and inspect for moisture or arcing. If moisture is present dry components thoroughly and reassemble.

Step 6 - The engine is designed to run on a specific ratio of air to fuel (14.5 to 1). If this ratio increases by withholding fuel, it can cause the engine to backfire through the intake system. The number one reason for this condition is a plugged fuel filter. Visit - Fuel filter replacement

Step 7 - A failed air intake boot can cause a backfire by altering the feedback voltage to the PCM. Visit - Air intake boot replacement

Step 8 - Some engine exhaust systems have an air injection system designed to help dilute the exhaust gasses. This air is only added in operation during power demands. If the air injection check valve or sometime referred to as a gulp valve fails it can allow extra air into the exhaust system causing backfiring. To check your systems valve, remove it to check air flow. Air should only travel one direction. If air travels in both directions, the valve has failed and needs replacement.

Step 9 - Small leaks in the exhaust system can take in cool air during deceleration of the engine causing a popping or backfiring in the exhaust system. Inspect the exhaust system for leaks, such as broken welds or rust holes.

Helpful Information

There are two kinds of backfires, one is produced from the engine air intake and the other is produced from the tail pipe of the exhaust system. The cause for a backfire condition can vary depending on the malfunction of a particular system. Some backfires can be extreme enough to cause damage to related components. Anytime a backfire is observed immediately diagnose and repair as needed. If the engine is running rough it can produce a backfire. A backfire occurs when there is an imbalance in the air to fuel ratio required for your vehicle to operate properly.

A common backfire situation occurs when there is a small leak in the air injection system that feeds the exhaust system. This can cause unburned fuel to explode suddenly. One of the most common causes is a stuck or faulty air intake or gulp valve near the exhaust manifold. Backfiring can also occur with a sudden drop in fuel pressure. This may be due to a faulty fuel pump or a plugged fuel filter. Correcting problems in the fuel system usually resolves these issues.

 If the fuel mixture is too lean (not enough fuel) you may have a backfire in the intake manifold, or too rich (too much fuel) you may get a backfire out of the exhaust system. It is important to observe the location of the backfire. The information below is separated into two sections; the first section contains possible causes for backfires in the engine intake. The second section contains possible causes for backfires in the exhaust system. (Note: if the engine is misfiring and the engine backfires while the engine is under load please visit, engine misfires.

Best Practices

  • Change the fuel filter at regular intervals.
  • Tune up and service the fuel injection system in accordance with regular maintenance.

AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-09-13)