It feels like a sudden, irregular, and repetitive jerk when driving or when your car is on idle. Although there are other signs of engine misfire, this is the most common telltale sign.
And this is where people begin to ask questions like “What was that?” “Is my car okay?” “Do I need to call an auto mechanic?” Followed by “Oh no, this is going to cost money…”
Frustrating as it may be, asking questions is going to be the first step to solving the problem. That way you’ll have your car back to serve you. Healthy and strong.
In this article, we are going to answer almost every question you may have about engine misfire. From the common signs to look out for to why it happens, and how you can solve the problem (however unique it maybe).
What is Engine Misfire?
We all know that in the last couple of decades there has been a very significant improvement in transportation technology. This improvement did not only affect the design and shape of vehicles, but it also affected the design of engines.
Engines used to be mechanically held together by nuts, screws, and bolts. This meant you needed the technical know-how of the vehicle before working when something goes wrong.
Recently diagnosing a problem in a vehicle relies so much on the result from an engine ECU. That is the Engine Control Unit that controls the engine to ensure optimum performance. This directs to where you should be looking for the problem.
A vehicle engine works on a principle of sequence. This sequence includes the intake of an air-fuel mixture, the compression of that mixture, ignition of that mixture through the spark plug and in the end, letting go of the burnt gas. This can simply be known as Suck, Squeeze, Bang and Blow.
Whenever something goes wrong in the sequence or when one of the four cylinders malfunctions, a misfire occurs. When your engine misfires, its performance drops. If you look inside the misfired cylinder, you would see that with every revolution of the piston and every controlled explosion the engine is going through a lot of stress.
An engine misfire, in simple terms, means that something is off in the sequence that keeps the engine running.
The Science of Engine Misfire
Before we go deeper into the matter, it’s best we understand the science of engine misfire. More like “how engine misfire works”.
Your car engine runs on 3 fundamental elements:
- Air (oxygen in the air)
If one or more of any of the 3 above are missing in the mixture, the engine misfires.
In technical terms, when something prevents the coil voltage from jumping the electrode gap at the end of the spark plug, when there is not enough gasoline in the air-to-fuel mixture, that is, when the air-to-fuel mixture ratio is too lean to burn, or the engine loses most or all of its air-fuel mixture before it is ignited (loss of compression) a cylinder in the engine misfires.
This misfiring cylinder can cause vigorous shaking of the steering or the vehicle as a whole. If you calculate it, you can also notice a loss of about 27% of the engine power. Which is like a cheetah trying to run on three legs.
You will also notice an increase in exhaust emissions and poor acceleration which makes your brakes and gas pedal respond slowly. There will be problems in starting your car, rough idle and a whole lot of other symptoms which we will talk about in detail below.
What are the Symptoms of Engine Misfire?
When your engine misfires, it is going to be obvious to you. There will be certain signs (or symptoms), you can spot and know for sure that your engine is misfiring.
1. Rough Acceleration
During a misfire, the cylinders in your engine don’t act right. There is always a certain amount of power they all have to produce together, at definite time durations. Everything else about the engine is timed in that order to a fraction of a second. If one or more cylinders are not producing power the way they are designed to, you will experience rough or poor acceleration.
Also, some engines are equipped with an oxygen sensor that regulates the air/fuel ratio. During a misfire, that sensor doesn’t get accurate info and hence, messes up that delicate ratio. You will sense this as a jerky kind of acceleration.
2. Rough Idle
Even when idle, your engine can misfire. This is because of the same reason we told you in number 1. The cylinders aren’t producing power the way they are supposed to. So, the RPM (Revolutions per minute: how fast your engine is running) drops lower than normal, and sometimes jerks back up. This goes on and on and you notice the engine isn’t running smoothly. If you take a look at your exhaust pipe, you will see that irregular movement when it jerks. The engine could go off during one of those dips in power output.
Manufacturers design smoother and smoother engines every year. Some are almost soundless with as little vibration as possible. These days, new cars just hum. If your engine is misfiring, there will be random and irregular explosions in the cylinders. This is not how the well-timed and rhythmic explosions in your engine’s cylinders were designed to go.
4. Check Engine Light
Newer vehicles have built-in sensors in the engine, so the engine can monitor itself through the Engine control unit (ECU). When the engine is running as it should, the “check engine light” stays off. But if the sensors notify the ECU that something is off in the engine, and this occurs over and over again, your ECU goes on to decide if it is a serious problem or not. If it is, then it turns on the check engine light.
5. Slow Acceleration
Misfiring doesn’t let your engine to burn fuel the right way. The oxygen sensors (we mentioned above) get affected when this happens. If the air/fuel ratio isn’t right, the engine won’t produce enough power. This means slower acceleration for you, the driver.
6. Engine Sound is Different
If you’ve been driving your car for some time, you may be familiar with its normal sound. If you hear something that sounds remotely off, like you are not getting the full orchestra of your 4-, 6-, or 8- cylinder engine, then you should consider that it is misfiring.
What Does a Misfire Feel Like?
Since a misfire means your engine is not working as it should, it will almost always feel like a jolt or a jerk. A good engine runs steadily, with no sudden jerks or irregular vibrations. A misfire feels like your car jerking, sort of like it is coughing or sneezing.
What Does an Engine Misfire Sound Like?
A misfiring engine sounds like it is sneezing. This sneezing (or popping) sounds comes from the engine because of the increased wear and tear. Along with the jerking and sneezing sound, a misfiring engine has a particular smell. It is best not to drive your car with a misfiring engine. It causes damage that will cost you in repairs.
What Causes an Engine to Misfire?
Why would your engine misfire? What do you check for? When you can find the cause of the problem, it becomes easier to solve it. And also prevent it from happening later. Here are the common causes of an engine misfire.
1. Bad Ignition Coil/Distributor in an Old Car
The ignition coil is a transformer that helps your engine’s spark plug to create a spark. It converts and produces enough electricity to make that spark possible. You know how important the spark is in getting the internal combustion engine to work.
Some cars have one ignition coil to a spark plug. While others have one ignition coil serving all the spark plugs. Hence, the need for a distributor. If the ignition coil is not working as it should, the spark plug won’t spark. And this is the most common reason why engines misfire.
If you are using a car with separate ignition coils, you can find out which one is the culprit. What you have to do is unplug each coil and see if the corresponding cylinder responds. When it doesn’t respond, replace the ignition coil.
2. Bad Spark Plug
Sometimes the spark plug can be the problem. Spark plugs wear out over time and they are cheap, so you can get new ones with ease. If the spark plug is worn out, replace it.
3. Leaking Intake Manifold Gasket
Intake manifold gasket helps keep the air in the coolant chamber away from the air in the intake manifold. Without this, the internal combustion process will go haywire. The intake manifold gasket stops this from happening.
Then what happens when it leaks? You guessed right. Misfire. In newer cars, check for leakage around the gasket and vacuum hoses. But unlike in older cars that don’t have steel intake gaskets, this problem is not so common.
4. Low Fuel Pressure
When the fuel isn’t going into the cylinders as fast as they were designed to, you’ll have a problem. The cylinders won’t have the right amount of fuel to burn in each cycle.
What would make the fuel pressure low? Look out for the fuel pressure regulator, the fuel pump or the fuel filter. Either one or more of them might be malfunctioning. The fuel pressure regulator may be bad, or the fuel filter may be clogged.
5. Injector Problem
This is not a usual problem you’ll find in newer cars. They’re less common these days than they were some years back. A fuel injector that’s faulty will cause your engine to jerk and sneeze. To diagnose this problem, you have to flow test the injectors. Before you go into this, it’s best to eliminate the more common causes first.
How to Diagnose and Fix an Engine Misfire
To properly diagnose an engine misfire, you need to understand how your engine works, the different parts and their functions. We will summarize it a little bit so our tips on diagnosing and fixing a misfiring engine will make sense.
How does your engine work?
In a typical four-stroke engine, you will find two valves, a spark plug and a piston in a cylinder. The piston will be connected to a crankshaft that rotates when the piston goes up and down.
It is called a four-stroke internal combustion engine because the combustion of the air and fuel mixture happens inside the engine and it works in a 4-step cycle.
The air-fuel mixture enters into the cylinder through one of the valves. The piston goes up and compresses this mixture and causes the spark plug to spark and ignite the cylinder. This leads to an explosion that pushes the piston down and it turns the crankshaft. Then the piston goes back up and pushes the burnt mixture out through the second valve. This repeats as a cycle and gets your motor running. Literally.
There is a less common type of engine called a two-stroke engine. But let’s stick to the 4-stroke for now and figure out how and why it misfires, so we can fix it.
Why does misfire happen?
Just above we talked about the 4 stages of the 4-stroke engine. If just one of those stages don’t go right or is mistimed, your engine will misfire. Here are other suspects:
- The inlet of the air-fuel mixture is mistimed and/or the outlet of the burnt gases is mistimed
- The ignition spark is mistimed or the spark plug is not working
- The air-fuel mixture is leaking
- The air-fuel mixture is either too rich or too lean
Now that you understand how the engine works, diagnosing and fixing it when it misfires is a small leap from here. You will have to go through those stages and parts that make the engine work. Then you will pinpoint the cause and fix it. It will save you a lot of time to check the most common causes first before going deeper.
Below we will show you the steps you can take to diagnose the problem and solve it.
1. Read the DTC Code Memory
The first step in diagnosing your car when you suspect it’s misfiring is to read the Diagnostic Trouble Codes (DTC) Memory. You can find tools that’ll help you read these codes if you want to do it yourself. The tool you get must be the one for your vehicle. Compatibility is crucial.
Take note of the codes you find in the DTC code memory. You need them to move on to the next step. Clear the memory when you’re done.
2. Use the Information You Got
With the DTC codes you got, you have to interpret what the DTC is reporting. There are two scenarios to look out for.
The first scenario: The codes you get are pointing to multiple cylinders misfiring at the same time plus trouble code about the air-fuel mixture.
In this case, check for leakages. Look around the intake and be sure no hoses are cracked or broken. A good way is to spray start gas or brake cleaner fluid around the intake when the engine is idle. Be careful with this and have a fire extinguisher nearby because this is a highly flammable situation. If the RPM goes up when you spray, there’s a leakage problem in that area.
The second scenario: One particular cylinder creates more and more trouble codes even after you clear the memory.
When a particular cylinder is a reason more troubles are coming up over and over again, you can fix the problem in the following steps we’ll share. It is a much simpler situation.
3. Check the Ignition
In this step, you are going to find out if the problem is coming from the ignition, which is a very common cause of the misfire. There are 2 main things we will look at:
- Distributor ignition (on older cars) or ignition coil
- Spark plugs
The logic we will use here is this: If the problem is from one item, then when you remove that item, the engine is going to run without any changes. The changes you will observe are the sound or vibrations.
On older cars, this should be the first thing you check if you suspect the problem is coming from ignition. Distributors do get worn out.
Put the engine on idle. Then you can unplug the ignition cable for each cylinder and listen for changes in the way the engine is running.
Warning: Be very careful and use the right tools when handling ignition cables while the engine is running. It is best to use safety equipment and/or unplug the ignition cables while the engine is off.
If the engine doesn’t sound different, then that’s the misfiring cylinder. It means that is the cylinder that was not contributing to the engine power.
On newer vehicles, you will find separate ignition coils. They carry high voltage, so be careful with them. Unplug one ignition coil and listen to the engine while it is running, just the way you’d do for the distributor cables. Use the same logic.
Now you’ve found the problem cylinder, try swapping ignition coils between this cylinder and another one. If the new cylinder shows the same problem, then the coil (or ignition cable) needs changing.
If you tried swapping and the problem stayed on the same cylinder, it’s time to check the spark plugs. Swap the spark plug of this cylinder with another one. If the problem goes with the spark plug, change it. If the problem stays then you have to check if there’s a spark.
This is an electrical problem and you will need the engine wiring diagram of your vehicle to fix a more complicated issue.
4. Check for Intake Gasket Leaks or Other Intake Leaks
This is pretty easy to do, but you still have to be cautious. To check for leakages in the intake gasket, you can first try to listen for any unusual sound there. If you don’t hear anything, then use a brake cleaner or starter spray to check.
Warning: Get a fire extinguisher because these fluids are very flammable.
Here’s how: Spray some fluid around the intake gasket. If the engine revs up like you’re stepping on the gas, no matter how slightly, you have a leakage. And that spot you just sprayed is the problem area.
5. Check the Compression
When the spark plug is okay, but the cylinder is still not working right, then you have to check the compression. You have to do a compression test on the cylinder. If the compression on the misfiring cylinder is too low, put some oil on low pressure into it. Then test it again. If the compression is still low, you have to go deeper into cylinder repairs.
6. Check the Fuel
If you have confirmed that everything else is fine, then there’s one last reason left. The cylinder is not getting enough fuel. Do you remember how important the air/fuel ratio is?
This problem is more unique to newer models with injectors instead of a carburetor.
While the ignition is on, get a multimeter and check the voltage on one of the wires of the injector connector. This injector connector would be the one from the malfunctioning cylinder. The other connector will be grounded by the ECU, so you won’t be able to work with it. But you can just test with a diode to see if there’s any electricity passing through it.
If you’re getting 12 volts on the connector, you can try swapping here just as we did before. This time switch the injector with that of another cylinder. If the problem is still on the cylinder you were working with, use an oscilloscope to check the ground signal from the ECU.
If none of this still solves the problem, please go over the steps again.
Answers to Frequent Questions About Engine Misfire
We are going to answer some questions many people ask about engine misfire.
How do you know if your engine is misfiring?
There are common telltale signs that your engine is misfiring. The top ones to look out for are:
- Is the acceleration rough?
- Are you experiencing rough idle?
- Does the car jerk or vibrate suspiciously?
- Is the check engine light on?
- Is the car accelerating slowly?
- Has the engine sound changed?
If you answered ‘yes’ to any of the above, you are most likely experiencing engine misfire in your vehicle.
Can a misfire ruin an engine?
Yes, but this differs. If it is caused by an ignition problem, you might incur costs in repairs if you run the vehicle with a misfiring engine for too long. In the beginning, you will only experience performance and fuel consumption issues.
But for an injector problem, this affects how much fuel is in the cylinder at any given time. This one can cause serious damage. If it is too lean, it may overheat and damage a valve or piston. Too rich? It will wash off some lubrication in the cylinder causing wear and tear.
How do you fix an engine misfire?
First, you have to find out the reason the engine is misfiring. This could be a number of reasons like bad ignition coil, bad spark plugs, leaking intake manifold gasket, injector problem or bad fuel pump. Only after finding the cause can you figure out the next steps, whether repairs or replacement.
How long can I drive with a misfiring cylinder?
It is unsafe to drive with a misfiring cylinder. Many people think it isn’t such a big deal to go 1 or 2 miles with a misfiring cylinder. But that is just 1 or 2 miles of damaging your engine. A misfiring cylinder causes damage to your engine, sometimes severe damages that will cost a lot of money to repair. Not to mention the poor performance of the engine while driving with a misfiring cylinder.
Will bad gas cause misfire?
It goes without saying that bad gas is bad for your engine. When it comes to misfiring, yes, a bad gas will cause your engine to misfire. The gas helps keep the engine clean. An engine that isn’t clean enough because of the bad gas leads to varnish deposits clogging the injector. When the injector is clogged, the right amount of fuel doesn’t get into the cylinder. Hence, giving rise to a lean air/fuel ratio that causes a misfire.
Can a bad fuel pump cause a misfire?
Misfire happens when the air-fuel mixture isn’t in the right ratio, or the spark plugs aren’t working well. If you have a bad fuel pump and it leads to a lean air/fuel ratio in the cylinder, the engine will misfire.
Will driving with misfire damage my engine?
Yes, it will. A misfire can cause wear in the engine and cause you to repair or replace some parts that get affected by the wear.
Can a misfire go away on its own?
Yes. Although this sometimes happens if the cause of the misfire is the fuel supply. If the cause of the misfire is mechanical, it will not go away with higher RPMs.
Will changing spark plugs fix a misfire?
In those cases when the spark plugs are the only things causing the misfire, yes. But if the spark plugs aren’t the problem, you will have to check other options. The problem could be from a faulty distributor or ignition coil, leaks in the intake manifold gasket, or even the fuel injector.
How do I know if my engine is misfiring?
The usual signs are poor or slow acceleration, rough idle or irregular vibrations coming from the engine on load. The car can also tell you about this by turning on the check engine light.
Why does my car misfire when accelerating?
When your car is accelerating, it needs a certain ratio of air and fuel mixture. If this ratio isn’t rich enough, the engine won’t give out as much power as needed. And you will sense this as a misfire. It makes the car jerk, or stumble.
When your car misfires during acceleration, it is called hesitation or stumbling.
Is an engine misfire expensive to fix?
The cost varies. It depends on what you have to repair or replace.
Spotting and diagnosing an engine misfire as early as possible will save you a lot of money. Some people still drive their cars not knowing that the signs they’re dismissing as “my car is just getting old” are actually the engine misfiring. It’s not surprising that it only takes a matter of time before they have to pay a lot of money to fix their engine. Because the misfire causes serious wearing if you drive the car with it.
We’ve talked about how you can avoid this by being more sensitive to the signs of the engine misfiring and how you can spot the cause and fix it while answering common questions you may have on the subject.