The Impact of Oil on Spark Plugs: Causes, Symptoms, and Fixes

The Impact of Oil on Spark Plugs: Causes, Symptoms, and Fixes

February 19, 2024 0 By CarGuy

Spark plugs are extremely vital components. They create spark and ignite the air-fuel mixture in the gasoline internal combustion engines. However, when oil fouls the spark plug electrode, it can create problems in the engine and affect its reliability and longevity. 

In this article, we will cover the problem of oil on spark plugs and explore the possible causes, symptoms, and common fixes for this common car problem.

What is Oil on Spark Plug?

Oil on spark plugs refers to the problem when you have the presence of engine oil on the spark plug electrode. This problem is 100% because of underlying internal problems of the engine. Something is allowing the oil to enter the combustion chamber and foul the spark plugs with engine oil. So, what are the possible causes? Let’s elaborate more on that in the next chapters.

Causes of Oil on Spark Plugs

Now let’s take a look at the causes for this problem when you have oil on spark plugs. Let’s see what could be the reasons behind this issue before we learn how you can tackle the problem.

1. Poorly Sealing Piston Rings

The most common cause for this problem is definitely bad piston rings. These components are crucial in order for your engine to seal properly. 

Whenever the piston rings do not seal, you have problems with oil leaking passing through them and entering the combustion chamber.

This oil will eventually end up ignited and if the leak is too strong, it will foul the spark plug.

The electrode will become covered in engine oil and will not produce a spark anymore.

Piston rings often fail when the car reaches high mileage or if the engine is poorly maintained.

The rings simply start to collect sludge and this sludge starts to glue them in the ringlands and your engine starts to consume oil and you have a problem with oil on spark plugs.

2. Blown Valve Cover Gasket 

A blown valve cover gasket can also cause this problem when you experience oil on spark plugs.

Usually, this happens around the rings that seal the valve cover and the spark plugs.

These rubber seals can fail and they can fill the spark plug holes with engine oil.

But whenever this happens, the electrode of the spark plug is not damaged since in this case only the bottom portion of the spark plug is affected.

3. Leaking Valve Seals

Another very common cause for this problem is leaky valve seals. This is the case when you have problems with the cylinder head.

The cylinder head is the component that holds the injectors, valve cover, and spark plugs.

On each valve, there are rubber seals that can often fail and cause such problems to develop.

Usually, these seals rot out and oil finds its way into the cylinder. This will make the engine consume oil and foul the spark plug.

The solution to this problem is to replace the seals. A process that can require removal of the cylinder head and in some cases, this will also require proper machine work.

4. Overfilled Engine Oil

In some cases, owners can overfill the engine oil. Whenever this happens, you will have a problem.

In this case, engine oil can be sucked into the cylinders and this will potentially foul the spark plugs.

This is why you should beware of this possibility and if you overfilled the engine oil, make sure that you bring the level to normal spec.

5. Bad PCV Valve

A bad PCV valve can also cause the problem with oil on spark plugs. So, what is a PCV valve?

This is a special valve that has the purpose of removing the excess blowby gas from the crankcase and redirecting this gas to the intake manifold.

The PCV valve often fails in such a way that causes the engine to consume a lot of engine oil.

When you have oil consumption, you also have oil on spark plugs. So, it is worth checking this component in order to make sure that this is not causing your problem.

Symptoms of Oil on Spark Plugs

Now let’s take a look at the symptoms that your engine will produce when you are having such a problem in your car.

1. Engine Misfires

The most common symptom that you will get when it comes to this problem is engine misfires.

The engine will love to misfire whenever you have oil on the spark plug electrode.

The engine will trigger a check engine light with a P0300 code or a similar code that starts at 300 depending on the cylinder that is affected.

2. Poor Performance

Whenever the spark plug is misfiring and is fouled by the engine oil, the engine will also tend to run poorly.

The engine will run rough and there will be poor acceleration, as well as poor fuel economy.

So, if your car is slow to accelerate, you might want to check the spark plugs and see if they are fouled with oil.

3. Engine Damage

Engine damage is a common thing whenever this problem appears. But this does not happen immediately.

Even if you run your car for thousands of miles, there will be no problems. But as the miles accumulate, there will be damage to the cylinder walls, piston rings, and other integral components in the engine.

4. Poor Emissions

Emissions will increase if your engine is not operating as it should. This would be the case because the fuel is not ignited properly.

So, if you take your car to your local DMV for registration, you will probably be rejected until you fix the problem.

This is a clear sign that your car has a problem, most likely with the spark plugs and this needs further investigation by a professional mechanic.

5. Poor Fuel Economy

And last but not least is the poor fuel economy. The engine will inject more fuel if it deems that there is a problem with the spark plugs.

The computer will not be able to calculate the right air-to-fuel mixture and will definitely dump more fuel into the mix and this will lead to a rich air-to-fuel mixture.

So, if the exhaust smells of gas, you definitely have a car that runs rich and this needs to be addressed!

Possible Fixes to Oil on Spark Plugs

So, what you can do in this case in order to address the problem?

The first thing you need to do is to find the main cause of the problem. In the previous chapters, we listed the most common causes of this problem.

You need to inspect the PCV valve and see if this component is causing this problem.

If not, there should be done a leakdown test in your engine. Also, checking the compression is another great idea.

If there is a lack of compression, you have a problem with either the cylinder valve stem seals or with the oil ring of the specific cylinder.

This is why a leakdown test is more beneficial because it will clearly show whether the problem lies in the piston rings or the valve stem seals.

This process should be done for all of the cylinders because there might be more cylinders that are affected. You don’t want to pay twice for the same job.

When it comes to solving the issue, replacing valve stem seals is not something difficult.

But if the piston rings are damaged, there might be a need for more engine work to be done and this will understandably be more expensive.

If the engine is a major oil consumer, you will highly likely need a new short block or an engine rebuild if you can find a good shop to handle this type of work.

So, let’s sum up things real quick:

  1. Check or replace the PCV valve is faulty
  2. If PCV is OK, perform a compression test and leak-down test
  3. Then move on from there depending on the root cause
  4. If the valve stem seals are bad they have to be replaced
  5. If the piston rings are faulty you might need new rings if possible, or a new short block if the wear on the wall is too much

Cost To Fix The Problem

The cost to fix the problem might vary depending on the root cause of this issue.

As we noted, there can be a few main causes. The PCV valve is probably the cheapest solution, this component can cost $50 for most models.

The second most expensive is the valve stem seals, which can cost $300+ to get replaced.

And the most expensive thing that can go wrong is the piston rings. Replacing these rings is not possible if there is too much wear on the block.

And a new short block will cost you $1,500+ depending on the model we are talking about. Since some cars have very expensive engines.


Oil on spark plugs is a common problem that can have damaging effects on the engine performance as well as the longevity of the engine in total.

Before you do anything, make sure that you learn more about the causes and symptoms of this problem.

Then perform a compression test and a leakdown test in order to determine the root cause before moving forward to solving the issue.


What are the main causes of oil on the spark plug?

The main causes for this problem include a bad PCV valve, bad valve stem seals or bad piston rings.

What are the symptoms of oil on a spark plug?

Symptoms of this problem include oil-fouled spark plugs, engine misfires, rough idle, poor engine performance, and poor fuel economy.

If you want to read more on spark problems, you can check our article on spark plug fouling.