6 Symptoms of an Engine Misfire (& 7 Common Causes)

Misfires happen when the combustion fails in your engine. But what are the signs of misfires, and what causes it? Let's find out

Symptoms Of An Engine Misfire

Do you suspect that your car is misfiring? Misfires are one of the most common problems when it comes to modern gas engines.

There are also many different things that could cause a misfire, and it’s not easy to troubleshoot if you do not know where to begin.

In this guide, we will go through the most common bad engine misfire symptoms and the different things that could cause it. Let’s begin by taking a quick look at the signs you should look for.

The most common symptoms of engine misfires are rough acceleration or rough idle. Other signs you may notice are a check engine light and poor engine performance. You may also notice that the check engine light is blinking during the misfires.

These are the most common signs and not all of them. Here is a more detailed list of the most common symptoms of engine misfires:

Car Engine Misfires Symptoms

1. Rough Acceleration

Car Acceleration

When a misfire occurs, you may feel like light or strong jerk coming from the engine.

These misfires do often come under load from the engine, like when you are accelerating hard. The most common situation to notice misfires is on high gears, low RPM, and the accelerator to the floor. Rough acceleration is a typical sign that your engine is misfiring.

2. Rough Idle

Car Engine Rough Idle E1609793094987

Sometimes, the engine will misfire on idle also; your engine sensors will get faulty values, and the air-fuel mixture will get messed up. This can cause a very uneven idle, which can jump up and down, and the engine may also shut off on idle.

The car engine is most sensitive for small air-fuel mixture problems at idle, so this is probably one of the first places you notice misfires.

3. Vibrations

Car Engine Movements E1609791582981

A car engine is very balanced when manufactured and often has balance axles and different tricks to get as few vibrations from it as possible.

When one or more cylinder is not firing properly, the engine will become unbalanced, and this can cause heavy vibrations inside your cabin on acceleration or at idle.

4. Check Engine Light

Check Engine Light Reset Itself

Modern cars have great monitoring for all the different car sensors on the engine. If one sensor has failed or one sensor picks up that something is not quite right with the engine, it will send the information to the engine control unit.

When the engine control unit receives the data, it will decide if the problem is severe or not. If the problem is occurring several times, the engine control unit will light up the check engine light to notify you that something is not right so you can get it repaired.

When the ECU notices misfires, it is very common to light up the engine light and store a trouble code on the cylinder on which the engine misfired. Check the trouble codes with a diagnostic scanner.

5. Slow Acceleration

Slow Acceleration

As we discussed before, misfires can cause the O2 sensors to receive faulty information and generate a too rich or too lean mixture.

Overly lean or rich mixtures can cause lowered acceleration and even put your car into limp mode, which will cause the vehicle to not rev over 3500 rpm’s, and it will shut off the boost pressure from the turbocharger.

6. Engine Sound Changed

Car Sound

If you know a little bit about cars, you have probably noticed a difference in sound from different engines. V8 engines have a very different tone than a four-cylinder engine.

If your 4-cylinder engine is misfiring on one cylinder, it may sound like a three-cylinder engine. If your car’s sound is extraordinary, it’s most likely misfires on every cycle that you can hear.

7 Common Causes of an Engine Misfire

The most common cause of misfires is a bad ignition coil or a bad spark plug. It can also be caused by fuel-related issues such as a faulty fuel injector or a bad fuel pump. In rare cases, it can also happen due to low engine compression.

Here is a more detailed list of the most common causes I have collected through the years of working with cars. Let’s begin with the most common reason:

1. Bad Ignition Coil/Distributor if You Have an Old Car

Three Ignition Coils Vag

The most common problem when it comes to misfires is the ignition coil. Some vehicles have a separate ignition coil on each spark plug, while some cars have one coil with a sparking cable to each spark plug.

Older cars have a distributor and, in some cases, also an ignition coil. If you have separated spark plugs, unplug each coil to see if you can find out if any cylinders are not responding.

Replace the ignition coil if you find one faulty or have a trouble code stored for one ignition coil.

2. Bad Spark Plug

Spark Plug

The second most common cause of a misfire is bad spark plugs. The spark plugs fire up your cylinders, and they can get worn over time. Spark plugs are often very cheap and, in most cases, easy to replace.

If you can’t remember the last time you replaced your spark plugs, it’s probably time to replace them. If you want to learn a bit more about spark plugs, check this out: Spark Plugs symptoms.

3. Intake Manifold Gasket Leaks

Bad Intake Manifold Gasket

Intake leaks near the cylinder heads are also very common when it comes to spark plugs. This problem was a lot more common in older cars without steel gaskets for the intake.

So, if you have an older engine, you might want to check this. If you have a newer car, it is probably worth checking for any other signs of leaks around the intake manifold gasket or the intake. Check for broken vacuum hoses.

4. Low Fuel Pressure

Low Fuel Pressure

Low fuel pressure could be caused by a faulty fuel pressure regulator, a defective fuel pump, or a clogged fuel filter. Low fuel pressure will cause a lean mixture in your engine, which will result in misfires on all cylinders.

If you have trouble codes for misfires on all cylinders, you will want to check your fuel pressure. You can find the low fuel pressure causes more in detail in this article: Low fuel pressure causes

5. Injector Problem

Fuel Injector Spray

Another problem, which was more common five years ago, is injector problems. A faulty fuel injector will cause your engine to misfire, and these can be pretty difficult to diagnose without flow testing them.

Injector problems are not very common on newer cars, and because of this, you want to check out the other possible causes first, but it is absolutely worth checking.

6. Low Compression/Damage Inside the Engine

Car Compression Test

If you have checked everything else, there is a big risk that your engine has low compression or other damage inside your engine.

A faulty timing belt adjustment could also cause low compression, resulting in misfires. If you know that the timing belt was replaced recently, you should double-check so this person installed it correctly.

Wrong Air-fuel Mixture

Misfires can also be caused by a faulty air-fuel mixture sometimes. Many different sensors can cause a faulty air-fuel mixture in your car, like the MAF sensor, O2 Sensor, Coolant Temperature sensor, and more.

What is a Misfire?

Misfire

To find out what a misfire really is, we have to go through the car engine basics first. Here you will see a good picture of how your pistons and crankshaft are moving inside the cylinder when your engine is running. The pistons are pushed down by an explosion inside the cylinder.

When the piston is pushed down, the crankshaft is spinning. The engine is working in four steps; that’s why this engine type is called a four-stroke engine.

  1.  The piston goes down, filling the cylinder with an air-fuel mixture from the intake
  2.  The piston goes up, compressing the air-fuel mixture to a high pressure
  3.  The ignition from the spark plug is igniting the air-fuel mixture and the explosion is pushing the piston down and rotating the crankshaft
  4.  The piston goes up, emptying the burned air-fuel mixture through the exhaust pipe.
  5.  Repeat the process from step 1

That’s the function of a four-stroke engine that is fitted in almost all modern car engines.

A misfire occurs when ONE or more of these stages is wrong or missing

  • An overly lean or overly rich air-fuel mixture
  • Bad ignition spark / Wrong timing of the ignition spark
  • Low compression / Air-fuel mixture is leaking out
  • The timing of the inlet/outlet of the air-fuel mixture is wrong

With that knowledge, it’s a lot easier to find the problem that is causing your misfires. As you can see, in theory, there are not a lot of things that could cause a misfire.

But when you start to diagnose your car, you will realize that it is not always as easy as it looks to find the problem.

Written by:

Magnus is the owner and main author of MechanicBase. He has been working as a mechanic for over 10 years, and the majority of them specialized in advanced diagnostics and troubleshooting. Certified Automotive Diagnostic Technician.