There are a variety of factors that can cause your car to backfire, but the most common ones are having a poor air-to-fuel ratio, a misfiring spark plug, or good old-fashioned bad timing. This explosive sound is scary by itself, but it’s even more alarming when followed by a loss of vehicle power. If you’re wondering, “what causes backfire in an engine,” read on to learn how to deal with it.
If too much fuel is added to the engine, it may not all burn up before the exhaust valves open — letting unburned gasoline into the red-hot exhaust headers, where it can combust and lead to a backfire. Too much fuel could be getting into your engine due to damaged and leaking fuel injectors or bad engine sensors.
What is Backfire In a Car?
A backfire or afterburn is a combustion or explosion produced by a running internal combustion engine that occurs in the exhaust system, rather than inside the combustion chamber.
It is also sometimes referred to as an afterfire, especially in cases where the word backfire is used to mean a fuel burn that occurs while an intake valve is open, causing the fire to move backward through the system and out through the intake instead of the exhaust.
When the flame moves backward it may also be called a “pop-back”. A backfire can be caused either by ignition that happens with an exhaust valve open or unburnt fuel making its way into the hot exhaust system.
A visible flame may momentarily shoot out of the exhaust pipe. A backfire is often a sign that the engine is improperly tuned.
Why do Car Engines Backfire?
Common causes of backfire are running rich (too much fuel going into cylinders) or faulty ignition, possibly a fouled (dirty) spark plug, coil, or plug wire.
#1. Bad Engine Timing.
An engine backfire is what occurs when the combustion event takes place outside the engine’s combustion cylinders. Inside each cylinder, fuel and air are mixed in a precise ratio at the exact right time. A spark ignites the entire mixture, and the resulting explosions are what power your car.
If the timing of the spark is a bit too late in the engine cycle, unburnt fuel and air may be allowed to flow through the exhaust. As that ultra-hot fuel reaches the tailpipe, it mingles with the highly-flammable oxygen present in the outside air. This combusts the unspent fuel in the exhaust, rather than in the engine — resulting in a loud bang and sometimes even visible flames coming from the exhaust tip.
Modern engine timings are controlled by computer, and can even vary on the fly. That means most engines will burn up 100% of the fuel in each combustion chamber each and every time. This kind of backfire is more common on older, carbureted engines with distributor caps.
#2. Lean Air/Fuel Mixture.
Not having enough fuel added to the engine can also cause this problem! See, if there isn’t enough fuel mixed with the air in the engine, the resulting mixture will actually combust more slowly — meaning, when the exhaust valves open, there will still be unburned gasoline ready to get dumped into the exhaust.
This problem could be due to clogged fuel injectors, a failing fuel pump or even a pressure leak elsewhere in the fuel system.
#3. Rich Air/Fuel Mixture.
Not only does your engine need to fire its spark and close its valves at the right time, it also needs to burn the right ratio of air to fuel. If too much fuel is added to the engine, it may not all burn up before the exhaust valves open — letting unburned gasoline into the red-hot exhaust headers, where it can combust and lead to a backfire.
Too much fuel could be getting into your engine due to damaged and leaking fuel injectors or bad engine sensors. If the engine sensors are sending incorrect information to the computer, it may try to compensate by adding more fuel than necessary. To fix this, you may need fuel system cleaning service, some O2 sensors replaced or the mass airflow sensor replaced on your vehicle.
#4. Bent Or Damaged Valve.
As you can see, timing is critical to the safe, smooth operation of your engine. In interference engines, a type of engine where the valves open into the space that will soon be occupied by the rising piston, those valves must close before the piston arrives.
If the engine timing system fails, this could allow the pistons to strike the open valves, bending or cracking them. This will prevent them from forming a good seal when closed, and could allow the engine to backfire. While uncommon, this kind of damage is very severe and you’ll need a significant engine service to get your engine back in working order.
#5. Incorrect Spark Firing Order.
Nowadays, electronically controlled engines with coil-on-plug ignition systems basically eliminate this problem — but on older models with spark distributors and spark plug wires, it’s possible that your spark plugs could be hooked up to the distributor in the wrong order.
If the spark fires in the wrong cylinder at the wrong time, this is sure to lead to a backfire, among other problems! This problem can also occur if the spark plug wires are crossed, say, after having your spark plugs replaced — but, again, modern cars have engineered this problem away for the most part. Hooray for progress!
Why Do Cars Backfire When Shifting?
Not all backfires occur when you start the engine. Sometimes one can happen when you shift gears. However, that loud pop that you hear when shifting is, in fact, an afterfire.
Most of these occur on manual vehicles where a clutch is used to shift gears. As you press in your clutch and go from one gear to the next, fuel can continue to enter the cylinders, especially if you switch gears at higher RPMs.
All this unburnt fuel builds up in the exhaust and ignites when you release the clutch. Though this might sound alarming when it happens, and after a fire when shifting isn’t necessarily damaging your vehicle.
Is backfire bad for a car?
Backfires and afterfires are worth paying attention to since they can cause engine damage, power loss, and decreased fuel efficiency. There are a variety of factors that can cause your car to backfire, but the most common ones are having a poor air-to-fuel ratio, a misfiring spark plug, or good old-fashioned bad timing.