Spark Plug Fouling – How To Detect And Fix A Fouled Spark Plug?

Spark Plug Fouling – How To Detect And Fix A Fouled Spark Plug?

July 30, 2021 3 By CarGuy

The So, you are experiencing certain problems with your car like engine misfires or rough idling? You shouldn’t worry, you probably have a spark plug fouling issue.

Spark plug fouling is one of the problems that torment drivers on a daily basis. It can be frustrating to deal with fouled spark plugs. Especially if your car bogs down in the middle of traffic.

Spark plugs are one of the most important aspects of the ignition system. If you have a bad spark or no spark at all, you may be in trouble. Your car will not start and potentially shut down in traffic. This is because the cylinders are not getting enough spark to keep the combustion process going.

In order to make sure that you will not going to deal with these issues it is important to do some preventive maintenance and inspect your spark plugs. Because only with close inspection you are going to notice a spark plug fouling. Although there are other symptoms that are a clear sign that you have fouled spark plugs.

In this article, we are going to cover all of the symptoms of fouled spark plugs. We are going to explain how to fix this issue and then we are going to answer all the common questions when it comes to the spark plug fouling issue. So, if you want to learn more about this problem and how to solve it, follow along till the end!

What Is Spark Plug Fouling?

Spark plug fouling is the inability of a spark plug to produce a spark inside the combustion chamber. Spark plug fouling occurs because there is damage to the electrode in the spark plug. The spark plug fouling can be attributed to a few factors. And these are all connected to some electrical problems in the ignition system or mechanical problems inside the engine.

Spark Plug Fouling

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Some of the electrical issues can include, bad starter solenoid. Bad distributor, coils, and wires. If these components do not work properly can damage the spark plug. And cause the spark plug to change color and to foul.

Mechanical problems that can cause the spark plug to foul are leaking valves. The oil can leak from the head down into the combustion chamber. And this leaking can cause some problems with the spark plugs. This can be attributed to increased carbon build up and can protentional damage your valves and cylinder.

Another mechanical aspect that causes spark plug fouling is a bad head gasket. The gasket can leak coolant inside the cylinder and cause the spark plugs to go bad. So, it is important to inspect the spark plugs to know what is going on inside of the cylinder.

We are going to talk about the color of the spark plugs later in this article, but first, let’s discuss the fouled spark plug symptoms.

Fouled Spark Plug Symptoms

Like with every part in your car, spark plugs also develop symptoms when they go bad. So, which are the fouled spark plug symptoms? Or symptoms of bad spark plugs?

Engine Misfires

Engine misfires are one of the most common problems that car owners face when they have bad or fouled spark plugs. This is the case because the spark is nonexistent or the spark is not strong enough or delayed. This can result in uneven work of the cylinders and cause the engine to misfire.

When the engine misfires, you can notice how the work of the engine is completely different and doesn’t run well. These misfires can also be heard from the exhaust and you will notice them when you apply the throttle. There should be some strong bangs when a cylinder is misfiring.

Difficulty To Start The Car

Another symptom of fouled spark plugs is the difficulty to start the car. If there is no spark inside the combustion chamber the fuel is not likely to ignite and start the engine.

This can be caused by fouled spark plugs, low compression, bad cables, bad coils, bad starter motor, or empty battery.

When you have some difficulties starting the car the problem can be caused by many things as we mentioned above and you need to know how to troubleshoot the issue.

First, you need to check the spark plugs and see their condition. If they are black from carbon or covered in oil it means that there is not enough spark.

If the spark plugs are in good condition, then you need to move to other components such as the cables and coils. It is also a good idea to check the starter and battery. You will have to get a multimeter in order to diagnose this problem.

The condition of the spark plug will tell you a lot about what is going on inside of the engine. And later we are going to explain how you can read these signs.

Lack Of Acceleration

Lack of acceleration can also be one of the fouled spark plug symptoms. If your car is not punchy as it used to be when you apply the throttle it can be because of bad spark plugs.

The cylinders need energy in order to ignite the fuel in the combustion process. If there is a lack of spark or uneven spark, the car will accelerate very slowly.

If you notice your car accelerates extremely slowly. It is a good idea to check the spark plugs and inspect their electrodes. If they are all blacked out, there is a lot of carbon inside the combustion chamber. And you will probably going to need some carbon cleaning.

Rough Idle

A rough idle accompanied with misfires is another of the fouled spark plug symptoms. When there is not enough spark in the cylinder the car will have uneven work. And it will be difficult for the engine to maintain the right rpm.

The RPMs may start to slowly drop or they can climb and drop. If you have a rough idle issue, you will notice the tachometer. The needle will rise and drop all the time.

This is not good. If you have this problem, it’s best to check the spark plugs and see their condition. If they are ok, then you need to further investigate and check the coils and wires. And hopefully there you can find your culprit and you will hopefully fix your problem.

Reduced Gas Mileage

It is no question that your gas mileage is going to suffer if you have bad or fouled spark plugs. If you notice some of these issues then it is a good idea to inspect the problem.

This is the case because when your car runs. The exhaust gasses are measured by the O2 sensor that is mounted in the front part of the exhaust.

And if your fuel does not burn well enough the O2 sensor will check the readings that your exhaust gasses are not ok. And then will tell the engine to dump more fuel inside of the combustion chamber. More fuel means more unburnt fuel and more frequent visits to the gas station.

Your car will run poorly and also you will have to pay a fortune for gas. The good thing is that when you have one of these issues you will get a check engine light.

The check engine light will tell you that it’s time to inspect the car and scan it with a computer. Then you will find the real issue for your problem and that are the fouled spark plugs.

You can get these scanners for very cheap on Amazon. But I would recommend getting a decent scanner that will tell you a lot when it comes to your car’s overall health and will help you in diagnosing your issues quickly and effectively.

So, if you have a reduced gas mileage, the best idea is to get a good scan of your car to investigate the problem in-depth.

How To Fix Fouled Spark Plugs?

The easiest way to fix it by replacing them, but this is not always a long-term solution. If your spark plugs are fouled, it is a good idea to inspect them. And then draw some conclusions from the evidence you have gathered.

Carbon Fouled Spark Plugs

What causes carbon fouling on spark plugs? If you detect a lot of carbon buildup on your spark plug, it means that your engine is frequently misfiring or your engine is running too rich. So, the spark plug is not able to ignite all the fuel that is dumped inside of the cylinder. And this causes the carbon buildup on the spark plug.

In order to solve this issue, it is a good idea to replace the fouled spark plug. And also make sure that the other ignition components are working properly. It is also recommended to check the carburetor if you are running a carbureted car or the fuel injectors if you are running a fuel injection car.

If you are running a direct injection vehicle like a car with the Hyundai GDI engines, this is a normal occurrence. The carbon buildup occurs because of their engineering and the way they are built.

Coolant Fouled Spark Plug

If your engine has a bad head gasket it means that the car is burning coolant. The coolant in the combustion chamber may cause vapors. And these vapors can go inside the spark plug and cause some rust to develop on the threads and it can also damage the electrode.

In order to get this problem solved you will have to replace the head gasket and also replace the spark plugs with brand new ones.

This will make sure that your problem is solved and you have no other issues with the ignition system and the spark plugs.

Oil Fouled Spark Plug

If the spark plugs have a lot of oil on them it means that you have leaky valves. The valve seals do not seal properly and the oil comes down inside of the cylinder and ignites. When oil ignites it creates carbon in the combustion chamber and then it’s fouling the spark plugs.

Another reason you can have oil on your spark plugs is a bad PCV valve. When this valve gets clogged, it will not going to allow the crankcase pressure to build up. Which will eventually allow the oil vapors to go past the piston rings and make an oily mess on your spark plug.

The simple fix for this problem is to replace the spark plugs and call it a day. But this is not a long-term solution because your spark plugs will again start to foul and become bad. The solution for this problem is to remove the head and inspect the valves. See their condition and then move to fix this issue permanently. Also not forget to inspect the PCV valve.

Ash Deposits On Spark Plugs or Ash Fouled Spark Plug

Ash deposit in spark plugs is a pretty common occurrence. These ash deposits can build up over time and cause your spark plug not to work properly.

What causes ash deposits on spark plug? Ash deposits may be caused by the burning of oil inside of the combustion chamber or by fuel additives.

If these deposits create a lot of buildup. They can mask the spark and prevent the spark plug to work properly. This can result in developing misfires. In order to solve this issue, you will have to remove the spark plug and clean off the ash deposits with sandpaper. Make sure that there are no other contaminants and your spark plug should be working properly in no time.

Blistered Spark Plug

Spark Plugs may start to blister. If your spark plug is blistered you will notice some bubbles on the spark plug. And you will also see a light brownish color on the spark plug.

Spark plug blistering is caused by bad fuel to air mixture. More precisely lean fuel to air mixture. If your car runs too lean, then blisters on your spark plugs are one of the outcomes. You need to make sure that your fuel to air mixture is correct and your engine is working properly. You can do this by adjusting the carburetor if you have a carbureted car. If you are running a fuel injection then a simple tune on the computer will do the trick.

Fuel On Spark Plug

If your spark plug is soaked in fuel, it means that your car is running too rich. The air to fuel mixture needs to be adjusted and you need to fix this problem before it develops into something worse.

If you are facing this problem, it is a good idea to check the carburetor if you are running a carbureted car. Maybe the carb is dumping too much fuel inside of the combustion chamber. This results in fuel on the spark plug. Another thing to check are the ignition wires. If they are not good it means that they are not doing their job right and cause a malfunction in the cylinder.

For fuel-injected vehicles it is a good idea to check the condition of the coils and see if they are in proper shape. If the coil is bad, it can also lead to misfires and fuel on spark plug problems.

Another thing are the injectors. The injectors have to drop the right amount of fuel inside of the cylinder every time. If they are not doing that it means that there is something wrong with your fuel injectors.

They can be inspected by using an OBD2 scanner tool that you can connect to your OBD2 port. But pay attention that this troubleshooting may require some more capable scanner to investigate the output of each injector. More advanced scanners a pricier, so if you don’t have the money, you can take your car to a shop and they will see what is the problem with the injectors.

Detonation Fouled Spark Plug

A detonation in the cylinder can also damage the spark plug and cause it to foul. If a detonation happens there is damage to the spark plug. The insulator can crack and this piece of the insulator will fall down on the top of the piston and can damage the piston.

In order to avoid this issue is to always try to get good and quality gasoline that will work well with your car. Some of the gasoline on the market doesn’t work very well with certain engines and can potentially damage them. A damaged spark plug needs replacing, you should do this as soon as possible.

Gap Bridging Spark Plugs

Gap bridging in spark plugs occurs when the deposits are too large that connect the electrodes and thus preventing a spark to occur inside of the engine cylinder. This can be caused by oil or ash buildup.

The solution to this problem is simple. You just need to replace the spark plug. Or if you don’t want to replace the spark plug you can just pull it out and give it a good clean with sandpaper. This will make sure that the carbon deposits on the spark plug are gone.


What Happens When A Spark Plug Fouled?

When a spark plug fouled most frequently can cause the engine to misfire and cause uneven work. They can also slow your car down and drop your power ratings.

In more serious cases of fouled spark plugs, it may cause the car not to be able to start or to have a rough idle. You will notice this if the needle on the tachometer is jumping up and down. This is a clear sign that your combustion process is not complete and your engine needs checking.

How Do You Stop A Spark Plug From Fouling?

You can stop this problem by addressing the real issue that causes your spark plug to foul and that will require some deeper troubleshooting of the engine and ignition system of your car. But remember, the spark plug is going to tell you everything you need to know.

What Causes Spark Plugs To Foul Quickly?

Spark plugs foul quickly because there are other issues inside your engine that make them wear off prematurely. Make sure that your valves seal properly, make sure that your ignition system is working properly.

Check the PCV valve if you have oil-fouled spark plugs. A bad PCV valve is going to let the bad vapors pass by the piston rings and throw a bunch of oil on your spark plugs.

You should also make sure that your car does not run too rich and the fuel to air mixture inside of the combustion chamber is good. This will guarantee that the life of your spark plugs will be good and you will not have to worry about replacing them.

One Spark Plug Keeps Fouling?

If you have already replaced your spark plug and it keeps fouling then you need to deeper troubleshoot the problem and see what causes the spark plug to foul. We have listed above some of the most frequent types of fouled spark plugs. Usually when a spark plug foul there is some buildup on it.

If it’s gas you can smell it, if it’s oil then it should be black and tacky. If it’s blistered then your fuel to air mixture is too lean and you should adjust that on your car. So, before jumping to conclusions, make sure that you inspect the color of the spark plug and its condition.


In this article, we have covered a lot when it comes to the spark plug fouling problem. We have listed the main symptoms when a spark plug is fouled. As we said when the spark plug is fouled you will notice some misfires, drop in power, increased fuel mileage. All these are clear symptoms that your spark plug is not working properly.

How to fix fouled spark plugs? You can fix them by replacing them. Even though you should further investigate the issue and see what causes the spark plug to foul and go bad. This can be caused by oil in the combustion chamber, bad PCV valve, lean combustion, rich combustion, bad ignition system. All of these things can be possible culprits for this problem to appear.