Why Oil On Spark Plug Threads and How to Fix it

One morning you are doing regular car maintenance and suddenly see oil on spark plug threads after pulling it out. What could be the reason? Regardless of the causes, there should not be any oil on this part.

If you notice a spark plug covered in engine oil, it can happen for different reasons. The main reasons behind spark plug oil are O-rings leakage, worn-out head gasket, damaged valve guides, broken piston compression rings, and faulty piston.

In this post, we will share in-depth details of why oil on threads of spark plug and how to fix this problem.

oil on spark plug threads

What causes oil on spark plug threads?

Wondering why oil in my spark plugs? Here are the reasons:

  • O-rings leakage

O-rings are likely to present on spark plugs if there are no gaskets. The O-rings have a habit of keeping different things and lubricant on one side while keeping the other side dry.

O-rings leakage is one of the most common causes of oil on spark plugs threads. Quickly addressing the problem is necessary. Otherwise, different hazardous situations may pop up, such as engine misfire and oil leaking.

  • Worn-out Head Gasket

When a head gasket is worn-out, most people think coolant in the compression chamber is the main culprit. However, this can also happen due to oil leakage. Oil will enter inside the combustion chamber.

A huge smoke will come out. The plugs’ tip will have some lube too. Replace the worn-out head gasket fast. Or it could lead to extensive damage.

  • Damaged Valve Guides

You can’t find a perfect exhaust and intake valves position without valve guides. Additionally, it plays a key role in preventing the seal from wearing out, leading to oil entering the combustion chamber otherwise.

But when valve guides are damaged, the oil may enter the combustion chamber, followed by the spark plug threads. Though valve guides are affordable, they are time-consuming to replace and higher repair costs.

  • Leaky Valve Cover Gaskets

The primary job of valve gaskets is to supply engine oil where needed and prevent lube where it should not be. As a part of the function, it prevents oil from getting inside the plugs.

But when you will find leaking valve cover gasket issues, it fails to perform their functions, which lead to oil in spark plug threads.

  • Broken Piston Compression Rings

Several compression rings are installed in every piston of a car engine. Their main job is to stop the oil from getting into the combustion chamber.

But if piston rings are broken or damaged, they lose the capability to prevent lubricants from entering the combustion area. Since it plays a crucial role in running all components smoothly, repairing the bad piston rings soonest is necessary to prevent further damage.

  • Faulty Piston

Just because pistons are less prone to become faulty or damaged doesn’t mean they won’t have any problem. While running the engine, Combustion areas become highly hot.

Pistons’ capabilities to withstand this extreme heat reduce over time. They may become faulty after a specific time. Replacing a piston is a complicated task since re-building the engine is necessary.

What Happens If There Is Oil On Spark Plugs?

If you notice any is oil on the spark plugs, it will affect your vehicle performance. Let’s explain it with a simple example. Imagine you want to light the campfire.

But there is some dirt on the lighter tip, which prevents it from initiating. You may have to try multiple times to start the fire.

The same goes for plugs. If no spark is generated from car engine, no ignition will occur. Thus, no combustion will happen too.

Now, you might also be wondering about the chamber temperature. When your oily spark plugs leak lube, they will become weaker within a short time.

Plus, the oil will be seen in all combustion chamber areas. This will lead to many particles, which will burn along with the gasoline. As a result, your engine performance will be significantly compromised and you will notice blue smoke on exhaust.

How to Fix Oil on Spark Plug?

Though oil in spark plug well may occur for various reasons, a leaky valve cover gasket or O-ring is amongst the most usual reasons behind it.

Here we will share how to replace this part step by step. It is a good rule of thumb to go through the service/repair manual to understand the system better.

  • Step: 1 — Initial Inspection

Before starting the task, confirm the ignition is turned off. Also, don’t forget to detach the battery connection. You can disconnect the negative terminal.

  • Step: 2 — Take off Valve Covers

At first, take off each fancy valve cover from the engine. Untighten their nuts carefully, and then remove all the covers.

You may need to use handy tools like rubber mallets when the valve covers are too tight to unfasten.

  • Step: 3 — Take Off the Spark Plugs

Now, grab a spark plug socket ratchet and use it to take out each spark plug separately.

Look at their conditions whether they have any oil leakage or worn-out issues.

If the qualities of those are okay, clean the spark plugs and use again.

  • Step: 4 —O-Ring Seal And/Or The Old Gasket

The next task is removing the old gasket from the engine section. If the gasket was installed with silicone, you might struggle to detach it easily.

In such a case, you have to know the right way of removing the old gasket when it is jammed up. Finally, locate the cylinder areas and remove the spark plug o ring seal from each of them.

  • Step: 5 — Clean the Head and Valve Cover Surface

Your old gasket may have some oil traces. You should remove them using your plastic tool. Avoid using any metal tools as they might ruin the surface.

Clean the cylinder head as well as valve cover appropriately. You can use a degreaser for better cleaning.

However, be extra cautious while cleaning the cylinder heads since they are sensitive. Make sure no substance enters inside the engine system.

  • Step: 6 — Install an Intact Gasket And/Or O-Ring Seal

If your automotive engine literature is recommended to use silicone to install an original valve cover and spark plugs, you can use new valve cover gasket.

Make sure to choose the ideal size and model of spark plugs for your engine. Otherwise, you may fail to run the engine smoothly.


1. Is it bad if my spark plugs have oil?

Yes, it isn’t good if your spark plugs have oil because there should not be any grease on them.

2. Can oil leak through spark plug threads?

Yes, oil can leak through spark plug threads when excessive oil is mixed with gasoline.

Oil on spark plug threads

3. What causes oil in spark plug wells?

A spark plug may have oil for several reasons, such as O-rings leakage, worn-out head gasket, damaged valve guides, broken piston compression rings, and bad piston.

4. Will oil in spark plug well burn off?

Yes, oil on one spark plug or multiple will get into the compression chamber and lead to burn-off issues.

5. How do you clean oil off spark plugs?

At first, cover the spark plug holes with a rag. Then turn on the engine for a few seconds. This will force the oil and dust to quickly come out through the hole. Finally, use brake parts cleaner to clean the plug properly.


Regardless of the reason behind the oil on spark plug threads, it is crucial to treat this problem as quickly as possible. You can follow our step-by-step guidelines to learn the correct process of fixing oil on spark plug.


Shafiqule Islam is a graduated Mechanical Engineer. He has both on hand and managerial job experience of 15 years in automobile maintenance field. He served as Captain in Electrical and Mechanical Engineering Corp of Army, Technical Head in Taxi Cab business, Head of Plant in Car assembly plant. He is writing automotive contents to share his expertise knowledge to car lovers for solving their vehicle issues.

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