Misfires are one of the most common problems when it comes to modern gas engines. The spark has a critical job inside the car engine.
There are a lot of different things that could cause a misfire, and it’s not easy to troubleshoot if you do not know where you should start to look.
In this guide, we will go through the engine misfire symptoms, different causes and how you should troubleshoot it the best way. We will start with the various symptoms!
Engine Misfire Symptoms
There are several different symptoms that you may notice when it comes to engine misfirings. Here are some of the most common symptoms.
1. Rough Acceleration
When a misfire occurs, you may feel like a light or strong jerk coming from the engine. These misfires do often come under load from the engine, and you have the most load on the engine when you are accelerating on higher RPMs and higher gears. Rough acceleration is a typical sign of that your engine is misfiring.
2. Rough Idle
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.
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 cylinder is not firing correctly, the engine will become unbalanced, and this can cause heavy vibrations inside your cabin on acceleration or at idle.
4. Check Engine Light
Modern cars have great monitoring for all the different car sensors on the engine. If one sensor has failed or if 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 serious 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.
5. Slow Acceleration
As we discussed before, misfires can cause the O2 sensors to receive faulty information and generate a mixture which is too rich or too lean. Overly lean or rich mixtures can cause lowered acceleration and even put your car into limp mode, which will cause the vehicle not to rev past 3500 rpm’s, and it will shut off the boost pressure from the turbocharger.
6. Engine Sound Changed
If you are a bit into cars, you have probably noticed that there is 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 strange, it’s most likely misfires that you can hear.
Common Engine Misfire Causes
So, when you suspect that your engine is misfiring, where should you start looking for the problem? Well, from my experience of over ten years in cars, here are the most common causes of engine misfires: Ranked from the most common to the least common.
1. Bad Ignition Coil/distributor if You Have an Old Car
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 spark 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 if you find one faulty or have a trouble code stored for one ignition coil.
2. Bad Spark Plug
The second most common cause of a misfire is bad spark plugs. The spark plugs fire up 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
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, check 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 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.
5. Injector Problem
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.
6. Low Compression/Damage Inside the Engine
If you have checked everything else, it’s most likely that you have low compression or other damage inside of your engine. A faulty timing belt could also cause low compression and you should make sure this one is correct first. If you want to learn a bit more about how to check this, scroll down in the article, and you will find more information about it.
Is it Safe to Drive When the Engine Misfires?
When a misfire occurs, the air-fuel mixture that entered the engine will come out from the engine unburnt. On modern cars, you have a catalytic converter that will reduce the emissions from the vehicle. The catalytic converter can have temperatures of 600 degrees when it’s working and what will happen when unburnt air-fuel mixture enters this thing? – Yes, it will explode inside the catalytic converter. Explosions inside of the catalytic converter can damage it, and it’s not very cheap to purchase a new catalytic converter.
Instead from that, misfires can damage other sensors in the engine like the O2 sensors. So, I would never recommend driving an engine that is misfiring for any long distances. Short distances to your mechanic workshop without putting a load on the engine is fine, but do not ignore the misfires and keep driving.
What is a Misfire?
To find out what a misfire really is, we have to go through the basics of the car engine 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.
- Piston goes down, filling the cylinder with an air-fuel mixture from the intake
- Piston goes up, compressing the air-fuel mixture to a high pressure
- The ignition from the spark plug is igniting the air-fuel mixture and the explosion is pushing the piston down and rotating the crankshaft
- The piston goes up, emptying the burned air-fuel mixture through the exhaust pipe.
- Repeat the process from step 1
That’s the function of a four-stroke engine that is fitted in almost all modern car engines. There are some older cars that use a 2-stroke engine but I will not go through that procedure in this article. If you are interested in how a two-stroke engine works, you can check out this article: Two-stroke engine
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
Now you know the basics of how a cylinder in a car engine works and in what steps the misfire would possibly occur. With that knowledge, it’s a lot easier to find the fault 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 are starting to diagnose your car, you will realize that it is not always as easy as it looks to find the problem. Continue reading to learn how to diagnose and how to fix your misfires at home.
How Does the Engine Control Module Know When a Misfire Occurs?
The engine control unit can detect a misfire in various different ways, depending on what car model and engine you have. The engine control module uses a lot of sensors so that it knows when to ignite the spark plug and when to inject the fuel into the cylinder and to optimize the air-fuel mixture. To detect a misfire, the engine control module is often using the crankshaft sensor.
The crankshaft sensor measures the position of the crankshaft and calculates the rpm (Revolutions per minute). The crankshaft sensor uses the camshaft sensor to detect which of the cylinders are up in the top and are ready to be ignited. When the pistons are pushed down, there will be a slight speed increase of the crankshaft. If the crankshaft sensor is not recognizing the increase of speed, the engine control unit will save a trouble code on the cylinder which the misfire occurred on.
Sometimes, the engine control unit cannot detect which cylinder the misfire occurs on and it will display the DTC code P0300 (random misfire). Some engine control modules use the resistance of the ignition coil, and when an ignition does not occur, the engine control unit measures it through the wirings, and it would result in a DTC code. This function is not as regular as the crankshaft sensor detecting though.
How to Diagnose & Fix an Engine Misfire
If you have read the article from the beginning, you now know what could cause a misfire. If you jumped directly down here, I will write the possible causes fast. A misfire occurs if one of these are missing/wrong timing:
- Air-fuel mixture
To diagnose a misfire or several misfires, we have to go through all steps to check all the parts that could affect anything. But some faults are more common than others and it could save you a lot of time to start in the right direction. I work as a car technician, and I will write a short guide on what I do when I’m trying to solve misfire problems.
1. Read the DTC code memory
When you think that your engine is suffering from misfires, the first thing you should always do is to read the DTC trouble code memory. If you want to do it yourself, you can check out this tool. It’s not too expensive and works with most car brands. Always make sure that it’s compatible with your car before you purchase it! Check it out on Amazon
If you find any DTC codes in the DTC code memory. Write them down and clear the DTC memory; you are going to need them in the next step.
2. Use the information you got
Now that you know what DTC codes you have, search for some information about what the DTC code is telling to you. There are different ways you should start your diagnosis, depending on what trouble codes you got.
Case 1: You get a lot of DTC codes of misfires on different cylinders together with an air/fuel mixture related trouble code.
For example, if you have two or more DTC codes that seem to be related to each other:
- P0171 – System Too Lean ( Bank 1 )
- P0300 – Random misfire
- P0301 & P0302 & P0303 & P0304 etc. Several misfire codes
In this case, if the engine is running roughly or badly on a specific RPM, you should start to fix the air/fuel mixture related trouble code, and that will probably cause the misfires too. If you have an air/fuel mixture related code together with the misfire code, always search for information about the code related to the air/fuel mixture trouble code first. There are cases where you get air/fuel mixture related trouble codes due to misfires as well, but it’s not as common.
There are some general common causes that you could check for in this case. Start by checking for air leaks around the intake, and check for cracked hoses. You can use brake cleaner or start gas to spray carefully around the intake when the engine is at idle. If the RPM is rising, you have an intake leak somewhere there. Remember that those sprays burn very easily so be prepared with a fire extinguisher in case it ignites.
If you can’t find any intake leaks, the next step is to start researching the air-fuel mixture related trouble code. You could use Google to find the possible causes and solutions to these trouble codes, or you can ask us at our homepage
Case 2: You get a trouble code on a specific cylinder over and over after you clear the memory
If you are getting a trouble code on a particular cylinder all the time or just trouble codes without another related trouble code, you can continue with this guide. This problem is much easier to fix, and I will talk more about it in the next step.
3. Check the ignition
Now it’s time to start to check the ignition. Because in most cases, the problem is caused by a failing ignition, that’s the step we are going to begin with. If you have an older car with distributor ignition, always start by checking inside the distributor; it’s very common that the distributor is worn out.
If you feel that the engine is running rough at idle and if you have a newer motor with separate ignition coils, you could let the engine idle and then unplug one coil at a time. Remember that these ignition coils have a very high voltage and you should be really careful.
When you unplug one ignition coil, and you can’t feel any difference in the way the engine is running, you have found which cylinder that is misfiring.
You can use the same procedure with older cars that are using a distributor ignition by unplugging the ignition cables until you find out what cylinder the car is not running on. When you have ignition cables, don’t use this method while the car is running, unless you are very careful, and use the right tools. If you are unplugging the wires with your hand without any safety, you will get a lot of voltage through your body. Always use either the right tools or unplug them with the engine off.
Video of how to test the spark
Now when you know for sure which cylinder the car is not running on. You could try to move the ignition coil/ignition cables between two cylinders. If the problem is moving to the other cylinder when carrying out the same procedure as above when you are unplugging the coils, then you have found the problem. Replace the ignition coil or ignition cable.
If the problem is still on the same cylinder, you could do the same with the spark plugs. Move the spark plugs between two cylinders and see if the problem is moving. If the problem is moving, replace the spark plugs. Now if you have tried to move the spark plugs and coils between the cylinders but the problem is still there in the same cylinder, you have to check if you have a spark.
If you don’t have a spark if you are moving coils and spark plugs between the cylinders, you have to get a wiring diagram for your engine to start to check if you have power and ground etc. to your coil. This step is more complicated and I will not write about it here.
If your spark is OK, but the engine is not running on that cylinder, go to the next step.
4. Check for intake gasket leaks or other intake leaks
To do this, you can let your engine idle and listen around the intake gasket to find out if you can hear any strange noises from leaks. You can use some starter spray or brake cleaner and spray around the intake gasket if the engine revs up while you are doing that. You have got an intake leak there, and you have to fix that.
Always bring a fire extinguisher because if the spray or brake cleaner ignites, it will burn really well. Trust me, I’ve tried that too 😉
Intake leaks often lead the engine to not run at all on that cylinder; it would have run roughly on that cylinder or other cylinders instead. But it’s easy to check, and it could be a possible cause. If you didn’t find any intake leaks, go to the next step.
5. Check the compression
Now when you know that the spark is okay but it’s not running on that cylinder, it’s time to check the compression on all the cylinders. To test the compression, you can use a tool like this: Check it out on Amazon.
If you see that you have very low compression on the cylinder that the car is misfiring on, put some oil in the cylinder with low pressure and test again. If you still have low compression, there is not much more to do, you have to go deeper into your engine. You have to check the valves, the valve shims, and the piston rings. You can also check the camshaft timing, but in that case, you have often got low compression on all cylinders. If your compression is OK, it’s time to go to the next step.
I will not go into details here of how to carry out a compression test. Watch this video instead; he will explain in details of how to do a compression test.
6. Check the fuel
Now there is only one possible cause left, your engine is not injecting any fuel into that cylinder.
In this case, where you are not getting any fuel to only one cylinder, you have a new car with injectors. Because if you had an old engine with a carburettor, it would not cause the engine not to run on just one cylinder.
Start by unplugging one connector to the injector on the cylinder that is not running. Check if you have 12-volt power on one of the wires with the ignition on. Check with a multimeter. The other wire is grounded by the engine control unit, and not that easy to check if it’s working properly. You could use a diode light on that wire when the engine is running, but I recommend that you use an oscilloscope and you will probably not have one at home. You can also check the fuel pressure, but in that case, it would probably run badly on several cylinders.
If you have 12 volts to the injector and if you are reaching the injectors easily, you can use the same procedure here as with the ignition coils. Switch between another cylinder to see if the problem is moving. If your problem is still there, you have to get access to an oscilloscope and check the ground signal from the engine control unit.
Now, I hope that you have found your problem. If not then you have to go through this guide step by step one more time and if you really can’t find the problem, contact us, and we will try to solve your problem and see if there is a step that you have missed.
- Ignition coils and bad spark plugs are the most common problem if you have a misfire
- Intake leaks is a common issue if you have an air mixture related trouble code also
- If you have an air mixture trouble code, always begin checking that code first
I hope that you have learned something and that you solved your misfire problems. If you have more questions or want us to add or edit anything in this article, you can comment down below and I will answer your questions as fast as possible.