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Causes of Exhaust Backfire (Top 7 Reasons)

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Last Updated on: 7th September 2023, 12:41 am

Backfiring may be cool for car enthusiasts, but there may be more sinister reasons why it’s happening.

A backfire is caused by combustion of unburned fuel in the exhaust system. When a vehicle backfires, you may see a flame protrude from the exhaust tip, but most of the time you will only hear a loud popping or banging noise.

Unburned fuel entering the exhaust system may result from a number of mechanical issues, or it could simply be the way the car is designed, the following are some of the most typical causes of a backfire.

In this article, we will discuss the causes of an exhaust backfire.

7 Causes of Exhaust Backfire

There are multiple causes of backfiring, some are more sinister than others. However, sometimes the vehicle is designed to backfire and crackle upon letting off the throttle.

  • Rich air/fuel ratio
  • Lean air/fuel ratio
  • Bent valves
  • Timing issues
  • Damaged distributor
  • Spark plug tracking
  • Incorrect firing order

1. Rich Air/Fuel Ratio

A rich fuel to air ratio occurs when your engine receives more gasoline than it need. When there is residual fuel in your car’s exhaust and cylinders, it explodes and makes a loud popping sound.

2. Lean Air/Fuel Ratio

Not only may a high air/fuel ratio generate a backfire, but so can a mixture with too little fuel. A lean air to fuel ratio has insufficient fuel and much air.

Low fuel pressure due to a failed fuel pump, a blocked fuel filter, or clogged fuel injectors might cause this.

When a lean ratio combusts, it burns more slowly, so when the exhaust valves open, there may still be some air and fuel left behind, resulting in a backfire.

3. Bent Valves

There are at least one intake valve and one exhaust valve in each of your engine’s cylinders. They’re designed to let air and fuel enter the cylinders before closing when combustion begins.

The exhaust valves open after the air and fuel have been combusted, allowing the exhaust fumes to exit the tailpipe.

The valves will not establish a suitable seal if they get deformed or damaged. This will enable air and fuel to return to the intake or exhaust, where they can burn.

Fortunately, this is a relatively rare cause of backfiring. Replacing damaged valves or valve seals necessitates disassembling the whole engine and can be expensive.

4. Timing Issues

We’re talking about delayed time here, which is what produces the backfire.

Delayed timing refers to the engine’s ignition cycle which begins late in the combustion cycle and ignites the fuel when the exhaust valve opens.

Timing issues can be caused by a host of problems itself, the most common cause is a stretched or worn timing chain or belt.

5. Damaged Distributor

A distributor cap and a wire set distribute the electrical pulse to the spark plugs in cars that don’t have ignition coils on individual spark plugs.

If the distributor cap is damaged, moisture may get inside, causing the electrical spark to jump to the incorrect cylinder, resulting in a backfire.

This may not be obvious at first and may also be a temperamental issue.

6. Spark Plug Tracking

Spark plug carbon tracking may occur in a number of settings.

For example, sparks created by wires on a distributor cap might begin to bounce from one wire to another extremely fast, forming a carbon track that acts as a shortcut for the spark.

When spark plug wires or ignition coils are put directly onto the spark plug, the electrical spark splits routes, leaving gasoline in the cylinder.

When the second spark is produced, it contacts the fuel that has been left behind, causing a quick burn while the exhaust valve is open, resulting in a backfire.

Carbon tracking is normally caused by a route to ground being created over the spark plug insulator by oil, dirt, or moisture, a badly eroded spark plug electrode may also raise firing voltages to the point where the spark will seek the path of least resistance.

7. Incorrect Firing Order

Nowadays, electronically controlled engines with coil-on-plug ignition systems almost remove this issue.

On earlier vehicles with spark distributors and spark plug wires, your spark plugs may be connected to the distributor in the incorrect sequence.

If the spark hits the incorrect cylinder at the wrong moment, it can result in a backfire, among other issues.

This problem may also occur if the spark plug wires are crossed, such as after having your spark plugs changed, most modern vehicles have mostly eliminated this issue.

  • Andy Lewin

    Andy Lewin is a senior mechanic, ASE qualified master technician, and an experienced automotive engineer. He's passionate about serving the automotive community with the highest-quality and trustworthy information on all things automotive. He loves to write about car repairs, maintenance, car modifications and tuning, faults, and much more.

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