4 Common Causes of Car Misfires
If you’ve ever been walking down a busy street and heard a gunshot-like sound that didn’t come from a firearm, you’ve probably heard a car backfiring.
It could be just a “pop” sound you hear while watching a tuned car on the track, or a series of bangs followed by crackles, or maybe a long, loud, high-speed bang.
Some people might think a misfire is cool, but a misfire is actually a sign that there’s something wrong with your car. Here we explain what causes a misfire and what damage it can cause if left unchecked for too long.
What is a misfire? A misfire occurs when unburned fuel in the exhaust system heats up to the point of ignition and causes an explosion somewhere other than the engine.
This explosion is what causes the strange “bang” noise you hear when accelerating, decelerating, or starting your car for the first time on a cold morning.
If this gush of fuel gets too hot, it can explode like a Molotov cocktail, sending flames spewing out of the back of your car. (Please don’t set off a burnout near a field during open flame bans, kids.)
What does it sound like?
The sound of a misfire sounds a bit like a gunshot, and maybe some fireworks along with it. A good misfire is like good wine…it needs to be loud, but not so loud that it bursts your buddies’ eardrums.
When tuning your car for a misfire, it can take slightly more fuel than normal combustion requires, which can cause excess fuel to be pumped out of the exhaust system.
A slight buzzing sound when you release the throttle will drop the RPMs, but unburned fuel continues to flow into the exhaust system. That “buzzing” sensation you hear from the driver’s seat fills your mind.
Still not sure what a misfire sounds like? This might help:
One surprising thing about misfires is that you can even see flames coming out of the back of the exhaust system at night.
Causes of misfires
1) Bent valves
Typically, each cylinder has at least one intake valve and one exhaust valve. These intake valves open and close to let in fuel and air, and the exhaust valves open at the end of each combustion cycle to let out emissions/excess gases and fuel. So when the seals wear out or the valves become bent, the seals are no longer able to prevent unwanted fuel from entering the cylinders.
If there is too much fuel in the cylinder during compression, there will be more unburned fuel in the exhaust gases. Unlike a “rich” mixture, this amount can be irregular if the valves are bent. Fortunately, the cause of the misfire is probably not due to a bent valve; repairing it at your local mechanic is very expensive.
2) Poor ignition timing
The spark plug will only fire when both valves are closed, hopefully achieving the most efficient combustion. However, if the ignition timing is off, fuel will burn when any of the valves are open or closed, causing a disproportionate amount of fuel to exit the exhaust system.
If too much unburned fuel flows through a glowing header, it can cause the header to ignite and cause an explosion in the exhaust system.
3) Rich Air-Fuel Mixture
Maintaining this will extend the life of your engine and cost you less in the long run. The car becomes sluggish and may not be able to participate in road races. Remember, an efficient engine is a smooth running engine, and a quiet engine is a fast one.
4) Lean Air/Fuel Mixture
If your fuel injectors are clogged, your fuel filter is dirty, or your fuel pump is failing, it can create a “lean” air-fuel mixture in the combustion cycle. Lean means there isn’t enough air to burn all of the fuel in time for proper combustion.
It’s all about maintaining the correct balance of air, spark, and fuel. If any of these elements are missing in any way, your engine’s Check Engine Light will come on and excess unburned fuel will pass through the exhaust system.