Oil Fouled Spark Plug: Popular Causes and Solutions

Oil fouled spark plug is one of the most common causes of decreased engine performance and fuel economy issues in a car. If oil fouls your car’s spark plug, it is usually an indication of a problem with the engine’s internal components or the ignition system.

Oil Fouled Spark Plug

In this post, you will learn the popular causes of this issue and how you can solve it with maximum efficiency.

What Is the Common Cause of Oil Fouled Spark Plug?

The common cause of oil-fouled spark plug is worn piston rings. The role of piston rings is to seal the combustion chamber to prevent the combustion gases and oil from entering each other’s respective spaces. If the piston rings wear down, it is impossible to maintain a proper seal.

When this happens, it allows oil to leak from the crankcase into the combustion chamber during the engine’s operation. As a result, the oil mixes with the air-fuel mixture. Once the engine oil gets into the combustion chamber, it coats the spark plug with an oily residue.

This explains the oil fouling on spark plugs which interferes with the spark plug’s ability to generate a strong spark. If you are wondering – can I drive with oil in my spark plugs? The answer is, it is not advisable since it can worsen the problem with further damage to the engine.

  • Cylinder Head Gasket Failure Leads to Spark Plug Oil Fouling

Another cause of oil-fouling spark plugs is a faulty cylinder head gasket. The cylinder head gasket sits between the engine block and the cylinder head. Its role is to seal the combustion chamber.

However, when the cylinder head gasket fails, it allows oil to leak from the oil passages into the combustion chamber. This leakage happens when the gasket is no longer able to maintain a proper seal. Often, this arises from factors such as age, engine overheating, or improper installation.

As oil enters the combustion chamber, it comes into contact with the spark plug. Once this happens, the oil coats the spark plug electrode and insulator. Ultimately interfering with the proper spark generation, which is necessary for efficient combustion. This results in carbon fouled spark plug.

  • Valve Seal Failure Contributes to Spark Plug Issues

If the valve seals in your engine start to leak, it can result in oil fouling the spark plugs. The valve seal is a tiny, rubbery cap that sits on top of each valve. The valve controls the flow of air and fuel into the engine’s cylinders.


These have an important mission of keeping oil and other fluids in their proper places. But with time, wear and tear or aging can make worn valve guide a little less effective. This results in a leaking valve cover since it has lost its ability to seal tightly around the valves.

When this happens, oil from the engine’s lubrication system can slip past the valve cover gasket. Instead of staying where it should be, the oil enters the combustion chamber, where the air and fuel mix together. This is the common cause of oil on spark plugs but no smoke. The oil interferes with the spark plugs.

To function effectively, spark plugs need a clean and dry environment. When oil enters the combustion chamber, it makes it all messy. It coats the spark plug, making it difficult for the spark to jump across the gap and ignite the air-fuel mixture properly. This results in increased fuel consumption.

  • Positive Crankcase Ventilation (PCV) System Malfunction

A malfunctioning Positive Crankcase Ventilation (PCV) system is another cause of this issue. The PCV system plays an important role in maintaining the health of the engine. It regulates the pressure and removes harmful gases from the crankcase, where oil is stored and circulated in the engine.

Fouled Spark Plug Problem

You can think of the PCV system as the engine’s respiratory system. It consists of various components, including the PCV valve and associated hoses. Together, they ensure the engine works properly by maintaining a balance between internal pressures.

However, when the PCV system malfunctions, it’s akin to having a blocked or congested nose. The system is unable to carry out its duties effectively. This causes several issues, including oil fouling of spark plugs. It is one of the reasons you may have good compression but oil on spark plugs.

The PCV system is responsible for diverting harmful gases and oil vapors from the crankcase back into the engine’s intake manifold. This recirculation process helps prevent the buildup of pressure and the accumulation of harmful substances in the crankcase.

When the PCV system is not functioning properly, the pressure inside the crankcase can increase. The increase in pressure may force oil to find alternative pathways to escape. In some cases, this means oil may be pushed into the combustion chamber and ultimately reach the spark plug.

Once the oil reaches the spark plug, it creates a coating or residue that interferes with the spark plug’s performance. This can lead to misfiring, reduced engine power, poor fuel efficiency, and difficulty starting the engine. The reason could be oil in spark plug well causing misfire.

  • Incorrect Oil Viscosity or Overfilling Can Cause This Issue

Another cause of this issue involves using the wrong oil with incorrect viscosity or overfilling the engine. Different engines require specific oil viscosities to ensure proper lubrication and optimal performance. If you use the wrong oil, it affects the oil’s ability to flow smoothly throughout the engine.

When the oil is too thick for the engine, it may have difficulty reaching all the necessary parts. This can result in overheating, which can damage cylinder walls and a broken piston. Conversely, if the oil viscosity is too low, it may not provide sufficient protection and lubrication for the engine components. This can result in increased oil consumption. Also, there is the potential for oil leaks, which can contaminate the spark plugs.

As well, overfilling the engine with oil can contribute to oil fouling of spark plugs. Each engine has a recommended oil level specified by the manufacturer. When the oil level exceeds this recommended limit, it can cause excessive oil splashing or foaming within the crankcase.

Excessive oil splashing or foaming can force oil into areas where it shouldn’t be, such as the combustion chamber. It can happen if you have a damaged piston. This can result in an oily spark plug since the excess oil can coat the spark plug surfaces. When the oil burns in the combustion chamber, it results in blue smoke.

  • Faulty Turbocharger Can Cause Oil-Foiled Spark Plugs

A faulty turbocharger can also contribute to the oil fouling of spark plugs. A turbocharger increases your vehicle’s engine power output by forcing more air into the combustion chamber. However, when the turbocharger malfunctions, it can result in oil-related issues that affect the spark plugs.

If the turbocharger develops a fault, such as worn-out bearings, it can lead to oil leakage. The turbocharger relies on a supply of oil to lubricate its moving parts and maintain their proper functioning. When seals or bearings fail, oil can escape from the turbocharger and enter the intake or exhaust side of the engine.

Now, this problem arises when oil finds its way into the combustion chamber. When it enters the combustion chamber, it coats the spark plugs. The oil-fouled spark plugs can result in an engine misfire.

Additionally, the presence of oil in the combustion chamber can also affect the air-fuel mixture’s proper combustion. The oil can mix with the fuel and disrupt the combustion process. This leads to incomplete combustion, carbon buildup, and further fouling of the spark plugs.

What Is the Solution for an Oil Fouled Spark Plug?

The solution for an oil-fouled spark plug is solving the underlying cause with the help of a qualified mechanic. Mechanics have the necessary knowledge and experience to effectively diagnose and address the issue. Moreover, mechanics understand automotive systems, including components that contribute to the oil fouling of spark plugs.

Through expertise, a mechanic can accurately identify the root cause of the problem and recommend appropriate solutions. Once a professional diagnoses the problem, they can carry out the necessary repairs with precision and efficiency.

Solution for Oil Fouled Plug

As well, mechanics have the expertise to disassemble and reassemble engine components properly, ensuring that repairs meet manufacturer specifications. This helps restore the engine’s optimal performance and prevents further issues related to oil fouling.

Also, they can source the right components to replace worn-out or faulty parts contributing to the oil fouling. Using genuine or quality parts helps maintain the integrity and reliability of the engine. A qualified mechanic may resolve this issue by:

  • Replacing worn compression rings
  • Repairing or replacing a faulty cylinder head gasket
  • Fixing valve guides
  • Resolving PCV system issues
  • Correcting oil viscosity or overfilling
  • Repairing a faulty turbocharger

The oil in spark plugs repair cost will vary depending on the cause. In addition, a qualified mechanic will provide you with guidance on preventive maintenance practices to prevent future occurrences of oil fouling. You will get to know recommendations on proper oil viscosity, oil change intervals, PCV system maintenance, and other maintenance tasks to keep the engine in good condition. Regular maintenance by a qualified mechanic can also help identify and address potential issues before they result in oil fouling or other engine problems.

  • Cleaning Oil Fouled Spark Plug: Quick Solution to This Issue

If the oil fouling is not too severe, you can clean the spark plug with a wire brush or a spark plug cleaner like carb cleaner. Be sure to follow the instructions on the cleaner carefully. Cleaning an oil-fouled spark plug is going to restore its functionality and alleviate the issues caused by oil fouling.

By cleaning the spark plug, you remove the oil deposits and improve its performance. Cleaning restores the optimum spark plug gap. This is especially true if you have a fuel fouled spark plug.

Faulty Piston Rings

What Are the Common Causes of Oil Fouled Spark Plugs?

Oil fouled spark plugs can occur due to various reasons. One common cause is engine oil leakage, which can lead to oil reaching the spark plugs. Incorrectly installed or worn out spark plugs can also contribute to the issue. Another factor to consider is engine oil flammability misconceptions cleared completely, as some may mistakenly believe that engine oil is highly flammable and leads to fouling. However, the truth is that engine oil itself does not ignite easily and usually requires high temperatures or an open flame.


Throughout this post, you have learned various causes of spark plug fouling and how to solve them.

Below is a quick recap of what we covered in this article:

  • The top causes of this issue include faulty piston rings, failed cylinder head gasket, failed valve seal, and malfunctioning PCV.
  • Other causes are a faulty turbocharger and using incorrect engine oil.
  • The solution to this issue requires a thorough diagnosis by a qualified mechanic to establish the source and fix it.
  • After fixing the cause, you will have to clean or replace the spark plugs in your car.

With this understanding, the next time you have bad spark plugs because of oil getting into the combustion chamber, you know where to check.

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