Oil On Spark Plug Threads | Diagnose and Fix

There is nothing pleasant about having oil on your spark plug. As we know, spark plugs play a crucial role in your petrol-powered vehicle’s ignition system. On the other hand, oil is a lubricant that prevents friction amongst the car’s moving parts. The lack of oil in your vehicle can lead to engine failure.

Despite the importance of oil in your vehicle, it is not expected to seep out through the plug well, as it can affect the performance of your car.

Today, I will go into detail to explain the various causes of oil on spark plug threads & tip to maintain your car.

spark plug

Oil on spark plug threads symptoms

There are six clear signs that oil has seeped through your car’s plug well.

  • Blue smoke from the tailpipe
  • Poor fuel economy
  • Decreased engine performance
  • Engine backfires
  • Gas smells from exhaust pipes
  • Engine misfires.

I will further explain each of these symptoms below.

Blue smoke from the tailpipe

One of the results of having oil on the thread of your spark plug is poor fuel efficiency. Spark plugs play the role of providing prompt sparks exactly when needed in perfect timing with the engine valves. This occurs between the opening and closing of the valves. When the timing of the spark plugs is off, it alters your engine’s combustion process, rendering it less inefficient as fuel does not burn completely.

If the spark plugs no longer usually work because of the oil present, the engine ECU will reduce fuel consumption efficiency. It does this by changing the cylinders’ fuel to air ratio. These alterations increase the fuel consumed by the engines and release unburnt fuel through the exhaust. The release of this unburnt fuel causes blue or white smoke to escape from the tailpipe.

Therefore, if you observe the discharge of blue smoke from your vehicle’s tailpipe, then there might be oil on your spark plugs.

Reduced Fuel Efficiency

Oil on your spark plug thread will decrease the fuel efficiency of your vehicle. When new, spark plug tips function at temperatures well over 500 degrees Celsius. The oil that finds its way to the spark plug threads quickly burns out at such temperatures leaving a burnt oxidized coating at the tip, and this coating reduces the effectiveness of the plugs in generating a spark.

Reduction in Engine Performance.

Once spark plugs come into contact with engine oil, their ability to generate a prolonged and well-timed spark is reduced. The contamination of the spark plug tip halts the generation of a spark that can ignite the mixture of fuel and air in your car’s combustion chamber.

If the mixture in the chamber does not burn efficiently, it will be incapable of generating the right amount of pressure needed to force the piston to move when desired. Therefore, it is not uncommon to experience a lag in acceleration due to the other spark plugs and ECU providing the necessary power to the wheels.

Engine misfiring

If your engine has started misfiring, that could be a sign that oil has seeped into one or more of your spark plugs. Inefficient combustion of the fuel and air mixture in your engine’s cylinder(s) can cause your engine to misfire. It feels like hesitation or shakes when the accelerator is pressed down.

Oil in the electrode tip or the well of the spark plug affects your engine’s ability to generate the right spark needed to ignite the gas/air mixture. The result is often an engine misfire.

The gas smell from the exhaust pipe

A clear symptom of poor internal combustion is the smell of gas from your vehicle’s exhaust. This smell usually occurs when the engine is still cold or right after the car has been started. The reason is that the engine uses a richer mixture of fuel and air.

Since cold fuel is harder to vaporize, the engine will require more fuel for a combustible mix. Oil on the spark plugs will further halt the efficient combustion of the oil and fuel mixture, thereby causing the engine to dump fuel directly into your vehicle’s exhaust.

Engine backfiring

Fuel that is not adequately burnt in the combustion chamber will escape the exhaust pipe through the exhaust valves with a high energy charge. This results in mini explosions in your exhaust, otherwise known as backfires.

Therefore, engine backfires occur when your fuel and oil mix combust after leaving your engine cylinder’s combustion chamber. Such backfires are harmful to your car’s exhaust or intake if not promptly fixed, and they can also lead to an overall drop in engine performance and decrease fuel economy.

What causes oil on spark plug threads?

There are five major causes of oil on spark plug threads.

Worn out valve guide seals

Valve guides are responsible for air intake into your car’s engine, securing the valves in the process. Although safeguarding the valves is their primary responsibility, they also prevent oil from reaching the spark plug well. Over time, these seals wear out and require replacement after a period of prolonged use, and failure to replace them will cause oil to seep into the spark plug wells.

Malfunctioning O-rings

Underneath the spark plug tubes is an O-ring. These rings are seals that provide cover for the spark plug wells. As is expected, these seals deteriorate over prolonged use, and once they are faulty, oil leaks into the spark plug well. When changing the O-rings, the spark plug will need replacement because its oil will render it unusable.

Bad Piston

Bad pistons allow oil to accumulate in the spark plug well. Damaged pistons also give engine oil access to the cylinder, allowing it to saturate the spark plug itself. Pistons crack under excessive heat, causing an engine misfire, rattling noises, and inefficient oil/ gas combustion.

Faulty Piston Compression Rings

Above and beneath the piston are compression rings that prevent oil from entering the car’s combustion chambers. They play a crucial role in removing excess oil from the walls of the cylinder. Damaged piston rings will be unable to prevent oil flow into the spark plugs. A clear symptom of broken piston rings is the sight of blue smoke from the exhaust or the smell of engine oil in your car cabinet.

Aged Valve Cover Gaskets

Your car engine has a metal cover that sits atop the valve cover. It is responsible for preventing leaks from the engine of your vehicle. Although they have a lifespan that lasts years, they fail due to improper use or normal wear and tear. Such failure is more evident when the temperature rises and the cover becomes brittle. The result of this is leakage into the spark plug well.

How to fix oil on spark plug threads?

Here are the steps to fixing oil on spark plug threads:

Step 1: Carry out the preliminary checks.

First, carry out a visual inspection of the exterior of your OHV engine cylinder heads. Check for signs of oil emanating from the valve gaskets. Any signal of oil emanating is sufficient reason to change the gaskets.

The best preliminary check for DOHC and SOHC engines involves the same procedure for OHV engines and an inspection of the spark plug exterior. If there is any sign of oil on the spark plug wires, ceramic coating, or coil-over plug, all spark plug O-ring seals need replacement.

Before commencing the replacement procedure, switch off the ignition and disconnect your car battery. You may detach your battery’s negative terminal.

Step 2: Remove the valve covers.

Take out the engine’s decorative cover bolts before gently pulling out the valve cover. If the cover is tightly affixed to the cylinder head, use a non-metal tool like a rubber mallet to pull it out. Never use screwdrivers or other metal objects to pull the cover out, leading to permanent damage.

Step 3: Detach the spark plugs

After removing the valve cover, the next step is to unplug the spark plugs carefully. Use a removal socket to do this. Remember to inspect each plug being removed for leakage or wear and tear.

Step 4: Detach the O-ring seal and the old gasket

Next, remove the O-ring seal from all cylinders and the old gasket. Removing the old gasket can be cumbersome if silicon is applied during installation.

Step 5: Clean the surface of the head and valve cover

With the aid of plastic tools, peel off any trace of the former gasket. Once done, use a degreaser to clean the surface of your cylinder head and valve cover. Carefully perform this act to prevent any substance from entering the inside of your engine.

Step 6: Change the gasket or O-ring seal.

After cleaning is completed, it is time to install a new spark plug and valve cover. Ensure that the spark plugs are used to match your car’s make and model.


The presence of oil on spark plug threads is one of the key reasons for poor engine performance. If left unchecked, this can cause permanent damage to your engine and your exhaust. That is why any of the symptoms highlighted above should be addressed immediately. This guide should help you check for the signs of oil leakage, discover the causes and fix them.

Photo Attribution

Image by [2], CC BY 2.0, via Wikimedia Commons

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Author: Donald Casey
Donald Casey is a car enthusiast and the Editor-in-Chief of DC Car Care. He has 24 years of experience in Car Care & Car Maintenance. He can start with something that’s neglected and will not stop until it’s perfect. Currently, he focuses his time on sharing his knowledge & expertise. You can read more of Donald's articles on his website.

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