What causes Engine Misfire – Symptoms, Diagnosing and Fixing

Engine misfire is one of the most frustrating things that you don’t want to face. But unfortunately, they are bound to occur in every engine for one reason or the other. What does an engine misfire feel like? What causes a cylinder to misfire?  What are engine misfire Symptoms? Can I avoid an engine misfire? And what does it mean for the safety and health of your vehicle? We’re going to answer all these questions and how you can tell that you have an engine misfire, diagnosis, and fixes of engine misfiring.

What causes Engine Misfire

Understanding misfiring engine causes is a step toward fixing the problem and avoiding them from happening. Let’s have a close look at the causes of a misfiring engine.

how to fix engine misfire

Lousy ignition coil or Distributor: Typically, once you suspect you have a misfiring engine, the first place you want to inspect is the ignition coil or distributor, or both. Older cars utilize distributors for igniting spark plugs. Inside the distributors are sensitive contact points. Suppose any of these points get rusted, worn, or damaged; it can lead to an engine misfire.

In addition to distributors, some older cars are equipped with both distributor and ignition coils. When the coils fail, it will not provide the required voltage to the spark plugs or distributor, leading to a misfire. If a thorough diagnosis proves that the ignition coil or distributors are long gone, you have to replace them to avoid severe cylinder misfire. Most newer cars are equipped with only ignition coils.

Lousy Spark Plugs: A lousy spark plug is one of the most common causes of a cylinder misfire. Do not forget that the spark plug is responsible for providing the spark that creates a little explosion that drives the piston up and down in the cylinder walls. If any spark plug is not functioning well or not functioning at all, it could leave excess fuel in the combustion chamber, resulting in an engine misfire at low rpm. Over time, spark plugs can damage or wear out, so it should be one of the first things to inspect when you experience a misfire. You should always hope for it to be the cause because they are pretty cheap and easy to replace.

Bad injectors: A lousy or’ stock close’ fuel injector may provide little or no fuel to the combustion chamber. We’ll explain how to diagnose and replace lousy fuel injectors soon.

Low fuel pressure: Low fuel pressure means not enough fuel is getting into the cylinder resulting in an engine running lean. That lean fuel mixture will cause a misfire. Many factors could be the culprit. An excellent place to start is from the fuel regulator ( if your vehicle is equipped with one). A defective or failing fuel pressure regulator will not allow adequate fuel pressure from getting through.

Another place you want to look into is your fuel filter. Most cars are equipped with one fuel filter that filters fuel coming from the gas tank to prolong the fuel injectors and carburetors’ lifespan. Some cars have an extra filter (making them two filters). If these filters get clogged due to the junk and grime they have been refusing access to pass through, it will lead to low fuel pressure.

Finally, a defective fuel pump will also cause the car misfires when accelerating or when running hot. If you diagnose your car and get all cylinder misfiring or random misfiring codes, you likely have fuel pump issues.

Low compression: Another common culprit of engine misfiring that you want to look into is low compression. However, many factors could lead to low compression. The first thing you want to check is the engine timing. A defective timing belt is usually the culprit for compression and timing issues. You have to check the timing first. The easiest way to check the timing is by utilizing a timing light. So you have to learn how to use a timing light to find a timing indicator on your main engine pulley. If your car utilizes a distributor, you can adjust the timing by turning the distributor to adjust the ignition points.

Intake manifold leaks: Vacuum leaks are one of the common causes of engine misfiring. And you would want to examine your intake manifold. An excellent place to start a vacuum leak test is the intake manifold. There is also a pretty and inexpensive way to diagnose a vacuum leak, and I’ll show you how long the passages of this article.

Symptoms of Engine misfiring

Before we move into diagnosing your car for a misfire, there are a few symptoms you need to watch out for.

Engine warning light: if your car develops a misfire as a result of bad Injectors, lousy spark plugs, low fuel pressure, or any other causes, it will cause the warning light to pop in the dashboard. The check light indicates problems in the engine system. Modern cars have an electronic module, also known as the car computer. If any part of the engine starts malfunctioning, these sensors will send a signal to the computer.

The computer will analyze how severe the problem is, and if the issues require urgent attention, it’ll light up the check engine light to notify the driver of the problem. You have to bear in mind that many other factors can also trigger the check light. You want to diagnose your car once the light pops up to ascertain the actual cause of the problem.

Rough idle: A triggered check light might not take your mind to a misfire, but one noticeable sign of misfiring is rough idling. A rough idle can be a result of several things. It could result from bad spark plugs, vacuum leaks, clogged fuel filters, or a lean running engine. If the rough idling results from a lean running engine, it might cause severe engine damage.

Slow Acceleration: A misfiring engine will affect your air-fuel mixture ratio. When this happens – it will send false data to other components. One of these components is the oxygen sensor. If the oxygen sensors send false data because of the inappropriate air-fuel ratio, it will cause the car computer to slow acceleration. This symptom will be more noticeable in cars with turbochargers. The engine won’t receive the demanded air or boost, causing a bad driving experience.

Rough acceleration: A misfiring engine can also cause your car to experience rough acceleration. As the engine gets to higher rpm while accelerating, you may experience jerking and surging while driving. Engine misfire when accelerating is the surest way to tell you to have a misfire.

Change in Engine Sound: if you’re not an automobile engineer with field experience, you might find it hard to spot this symptom. The truth is, that engines make different sounds. A v8 engine sounds different from a V6 or 4cylider engine. One or more cylinders may stop working entirely during misfiring, and your 4cylinder engine becomes a 3 or 2cylinder. In most cases, only a trained ear will notice an engine misfire sound.

A loss of a single or more cylinders will produce a different engine sound, indicating a severe misfire. This will also result to slow acceleration. In a V8 engine, a loss of two cylinders will lose 25% of engine capacity. However, in a 4cylinder, a loss of two cylinders will lose 50% of the engine capacity. If you have a dead cylinder during misfire, you can easily track it down on a 4cylinder than the V8 engine.

Engine Vibrations: Car manufacturers undergo expensive procedures to ensure you have a balanced engine. When you have a misfire, that balance is lost. You’ll notice that your car vibrates while idling or accelerating. If the car is parked on even ground, you may open your hood and see how it’s vibrating. The vibration increases depending on how many misfiring cylinders are in the engine.

What causes a cylinder to Misfire

I will give you an overview of what causes cylinder misfires in a gasoline engine aside from the misfiring engine causes we outlined above. Some of these reasons might seem common, but you have to watch out for them. It could be an easy solution that is causing the misfire.

Lousy valve cover seals: Another reason for a misfire is due to lousy valve cover seals. A bad or shrink seal will cause engine oil to leak into a spark plug tube, resulting in a misfire. You have to get the oil out of the plug tubes and replace the defective seals.

To clean the oil, you have to loosen the spark plugs and take your suction tool to get the oil out of the tubes. ( The suction tool will work only when there’s too much oil inside the tubes) or take a giant screwdriver and a clean rag and get it into the spark plug tubes and clean the oil. You have to do this one at a time.

Water in fuel lines: If you have water in the fuel line, chances are, it will get to the combustion chamber and cause a misfire.

Bad PCV valve: If you have a bad PCV valve, it can cause a vacuum leak which will result in a cylinder misfire.

Water in the spark plug tubes: This is not common, but if you recently take your car for a steam wash, there is a possibility that water gets into where it’s not needed. A good example is the spark plug tubes. Not only will it cause a misfire, but it can also damage your ignition coils.

Engine Misfire and Spark plugs

Without functioning spark plugs, there will be nothing to ignite fuel in the combustion chamber. Spark plugs are essential components in an internal combustion engine. Spark plugs are designed to create sparks that ignite the air-fuel mixture in the combustion chamber by transmitting an electrical signal sent from a distributor or an ignition coil at a predetermined time. Every car requires specific spark plugs with a designated spark plug gap set by manufacturers during installation. Good spark plugs will ignite efficiently, while lousy spark plugs might not produce a single spark.

what is an engine misfire

Spark plugs are just like air filters, fuel filters, and motor oil that require maintenance and replacement to keep your engine running nice and smooth. Although, some modern SUVs and trucks with improved ignition systems do not need spark plugs replacement. Regardless of car manufacturers’ claims or warranties, there will be cases where spark plugs will show signs of failing or wear out. So, when your spark plugs go bad, they will profoundly affect your car’s performance. If you ask – how to tell if the spark plug is misfiring, watch out for the following signs.

  • Slow acceleration
  • Poor fuel economy
  • Engine misfiring
  • Hard starting.

Soon, I’ll show you how to diagnose and replace lousy spark plugs.

Diagnosing and fixing Engine Misfiring

Now that we have discussed several engine misfiring causes, I’ll walk you through how to fix engine misfire issues. If you don’t know the cause of the misfire, I recommend following the steps below.

engine misfire symptoms

Pull the trouble codes: If a warning light appears due to a misfire, the car computer should be able to give trouble codes. Most auto mechanic workshops and auto parts shops have scan tools that can pull and interpret the codes for you. There are also cheap code readers that you can purchase, but you need to research google to find the codes’ meaning.

Analyze the codes: Most high-end scan tools will display the codes’ meaning alongside the fault codes. If you have one of those cheap scan tools, you might only receive alphanumeric codes that you have to search the meaning on the OBDII manual or plug into Google search to get the misfire’s cause. Some of the misfire codes are;

  • All cylinders Misfiring or random misfiring – P0300
  • Cylinder 1 misfire – P0301
  • Cylinder 2 misfire – P0302
  • Cylinder 3 misfire – P0303
  • Cylinder 4 misfire – P0304
  • Cylinder 5 misfire – P0305
  • Cylinder 6 misfire – P0306. The codes continue in that sequence depending on how many cylinders you have.

Fault codes usually point you to misfiring cylinders. Suppose you get a random misfire code; you will need to check all possible causes. Without further ado, let’s look at each possible cause.

Check ignition: You should know that ignition problems usually cause most misfires. In as much as your scan tool did not point to ignition as the source of the misfire, it’s an excellent place to start. If you’re driving an old car that has a distributor, you want to check it first. Take off the distributor cover and check for any corroded or worn-out connections and points inside. If the connectors are the culprit, You can decide to replace the distributor cover for a few bucks. You’ll spend higher to replace the distributor itself, but it’s a quick and inexpensive fix.

If your vehicle is equipped with an ignition coil, you can easily track down the misfiring cylinder(s). Start your car and pay attention to how the car engine sounds. Then, shut off the car and unplug one of the ignition coils. Restart the car. The engine sound should change. If it doesn’t, you have found the misfiring cylinder. You have to repeat this process one at a time until you track down all the misfiring cylinders. You can also carry out this method while the engine is idling, but we don’t recommend it because you might give yourself a jolt of electricity.

Once you get a misfiring cylinder, swap the ignition coil or cable and repeat the test. If the misfiring transfers to the swapped cylinder, it tells you to have a lousy ignition coil or wire. Get a new one and replace it. However, if the misfire only occurs on one cylinder, you have to check your spark plugs.

You can quickly check your spark plugs the same way you test the ignition coils. Swap out spark plugs between two cylinders and see if the misfire is isolated to the same cylinder or travels with the spark plug. If the misfire travels, then you have a lousy spark plug. Get new plugs and replace all the plugs.  You have successfully fixed the misfire issues. If all the cylinders are giving good sparks and are still having a misfire, you want to inspect your intake manifold for leaks.

Check for intake manifold leaks: Diagnosing vacuum leaks around your intake manifold is pretty straightforward. However, you have to take the necessary precautions because there’s an element of danger. Ensure you have a fire extinguisher handy before diving into this process, just in case of fire. Start your car and allow it to run for one or two minutes.

Get a starter fluid and spray around any suspected areas. Ensure you spray around the intake manifold gasket. If you’re not confident enough to do this, you can contact your mechanic. If your engine rev when you spray on a particular area, chances are you have a vacuum leak right there. You have to fix the leak. If you found that the vacuum leak is not the cause, you have to move on to the compression test.

Compression Test: You are going to need a unique tool to carry out a compression test. I recommend getting Innova 3612 compression tool. If you have a compression tool already, you can use it for this test as well. There are more robust and expensive tools in the market. If you’re not doing lots of engine repairs, there’s no need to spend much money instead of using Innova 3612. Innova 3612 is pretty affordable and will work with almost all cars regardless of the make and model.

It comes with many fittings that enable it to work on any spark plug tube. To carry out this test, you want to start by loosening and removing the spark plugs from the misfiring cylinders. Plugin the compression tester hose into the spark plug tubes. Ensure you hook the other side of the hose on the compression tester. The next thing you want to do is unplug your ignition coil so that it won’t fire while performing the test.

Now, let an assistant turn the ignition key for about 10seconds while you read the gauge. Write down the readings. Put a drop of engine oil in the spark plug tubes. You can pour half the cover of the oil container. Plug back the compression tester kit and turn the ignition key again. Write down this reading as well. The later test should read higher than the former. Now, you have to repeat this process with other cylinders that are functioning well. If you find that one or more cylinders are lower than the others, it’s a good indication that you have low compression. If low compression is the cause of the misfire, it could be caused by workout cylinder walls or defective Piston rings.

Unfortunately, this is neither a cheap nor easy fix. It requires rebuilding the engine or getting a new engine if you intend to keep the car. If all the test above including the compression test looks good, there’s one last thing you have to check to track down the cause of your engine misfire.

Check fuel delivery: kindly note that it’s likely low fuel pressure issues if you have a misfire on all cylinders. If that’s the case, ensure the fuel regulator is appropriately set. Make sure you don’t have a clogged filter. Finally, ensure your fuel pump is exporting fuel at the standard PSI and replace any faulty parts.

However, if you have one or two cylinders that aren’t functioning well and follow all the steps outlined above, you probably have a defective fuel injector. First, ensure you’re getting voltage to the said injector. You can do this by using a voltmeter. If you’re not getting power on the ignition, you have to call for professional assistance.

If power reaches the injector, you have to test the injector by following the same procedure you used on spark plugs and ignition coils. Swap the ignition coil and check if the misfire will switch to the new cylinder. If it does, you have a defective fuel injector. Kindly replace it with a new one.

Avoiding Engine Misfire

  • One of the surest ways to prevent cylinder misfires is by going for scheduled maintenance in your owner’s booklet.
  • Ensure your engine is tuned to factory specifications. An annual diagnosis will help put out any baby problem that could have led to a misfire.
  • Always ensure you torque down your gas cover after refueling. This will help avoid EVAP system problems.
  • Apply adequate rust protection to help keep system line corrosion to a minimum.
  • Always pay attention to any illuminated warning light. If you ignore it, a small issue might expand to a major problem.
  • Don’t be penny-wise and pound-foolish when it comes to maintaining your car’s health and safety.
  • Ensure you keep your engine snuff with scheduled system maintenance and maintenance tune-ups as recommended in your owner’s booklet. It’s a sure way to avoid premature system failures.
  • Ensure you carry out professional fuel system maintenance every 30,000 miles. This will keep the EGR system’s greatest enemy – carbon buildups – away.


Q: Can a Misfire go away on its own?

If a car’s misfire is caused by little water in the gas tank or contaminated gas, it will go away on its own. If not, it is likely to return.

Q: Can a misfire ruin an engine?

Driving with a misfiring engine can cause severe problems if left unchecked. Not only will it cause rough idle, slow acceleration, rough acceleration, changes in sound, and vibrations, it can wind up expanding issues that may require rebuilding or replacing your entire engine.

Q: How much can it cause to fix an engine misfire?

The cost of fixing a misfiring engine depends on various factors. It is affected by your car make and model, and the cause of the misfire. However, you should expect to pay $70-$100 for diagnostics to pull out the fault codes and the actual cause of the problem. The most common repair is replacing spark plugs which could cause around $200-$300 for a 4cylinder engine. V6 and V8 engines will be around $400-$500.

Q: How long can you drive with a misfiring engine?

It depends on the level and cause of the misfire. However, most vehicles can run for 40,000-50,000 miles with a misfiring engine. I know you wouldn’t want to stress yourself this long with a misfiring engine. I recommend you diagnose and fix cylinder misfires once you notice any symptoms of a misfire to avoid causing catastrophic engine damage.

Q: Will the engine cleaner fix a misfire?

Yes and No. It solely depends on the cause of the misfire. If the cause of the misfire is a defective ignition coil or lousy spark plugs, then it is not going to do anything. But if the cause is a result of a clogged fuel filter, the cleaner may fix the cylinder misfire.

Final word

You have seen several factors that could lead to engine misfire – most at times, it is caused by cylinder issues. Nonetheless, you have also seen several easy-to-do steps to diagnose and fix misfire issues. The key is to pay attention and listen closely as you swap the ignition coils, wires, and spark plugs. If you are not confident enough to diagnose and fix engine misfiring issues yourself, your best bet is to seek professional assistance.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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