Oil on Spark Plug – What It Means & What To Do

When you pull a spark plug out of your engine you expect a clean piece of metal with a contact on the end. But if you pull out the spark plug and find oil on it, what exactly does it mean? More importantly, what should you do to ensure your engine stays in great working order?

We’ll answer both of those questions for you here, and we’ll even walk you through what to do to figure out what’s going on. But one thing’s for sure, if there’s oil on your spark plugs, you don’t want to ignore it.

5 Reasons There’s Oil on Spark Plugs

If there’s oil on your spark plugs, you need to know all about the potential causes before you can narrow down exactly what’s going on. That’s why we wanted to start with five of the most likely reasons there’s oil on the spark plugs in your engine.

1. Leaky O-Ring Seal

Each piston slides up and down in its own combustion chamber when the engine is running, and around each piston, there are a few different rings that help keep all the different fluids where they should be.

While these o-rings do a great job when they’re all intact when one starts to fail liquids like oil can start to seep their way into the combustion chamber. A tell-tale sign of this is when you only have oil on one spark plug.

The good news is that an O-ring seal doesn’t cost much. The bad news is that it’s a pretty labor-intensive process to get to the O-ring seal. If you’re doing the work yourself, it will take some time, and if you’re paying someone for it, the labor costs will add up.

2. Blown Head Gaskets

If you have oil on more than one spark plug, this is by far the most likely cause. Head gaskets keep oil and coolant right where it needs to be in the engine.

Typically, the gasket is simply cardboard, and over time the cardboard can wear out and start to leak. If the leak is around the oil passage, then it’s possible it can get into the combustion chamber. Oil in the combustion chamber can get on the spark plug.

Keep in mind that while a blown head gasket can lead to oil on all of the spark plugs, it still might only appear on some spark plugs or have more oil on one spark plug than another. It’s also possible that a blown head gasket will lead to coolant in the engine.

3. Worn/Leaking Valve Guides

Valve guides work in tandem with the rocker arms in your vehicle’s engine to keep everything moving in the right direction at the right time within your engine. The rocker arm pushes the pushrod, and the pushrod sits inside the valve guide.

To keep everything moving the way it should when the engine is running there’s oil flowing through all the different passages, and the valve guide prevents this oil from reaching the combustion chamber.

When they’re in great shape, it’s an outstanding system, but if the valve guides start to leak, then oil can quickly reach the combustion chamber and work its way onto the spark plugs.

4. Valve Cover Gasket Leaking

If you’re finding that you have oil on the top part of the spark plugs, then the problem might be a leaking valve cover gasket. To be clear, we’re talking about the part of the spark plug on the exterior of your engine.

That’s because the valve cover gasket helps keep all the fluids around the valve cover in the right place. But if the valve cover gasket starts to leak these fluids can work their way down the outside of the engine, which can easily lead to oil getting on the exterior part of the spark plug.

But if you have a blown valve cover gasket you should have oil on other parts of the engine too. That’s because the oil doesn’t just jump straight from the valve cover gasket to the spark plug, it runs down everything else in between.

5. Damaged Piston Compression Rings

Around each piston in the engine, there’s a compression ring that cleans off the sides of the compression chamber with each turn of the engine.

Not only do these compression rings help clean off small amounts of oil and debris that works its way into the combustion chamber, but they also help keep oil out in the first place.

If the compression rings start to give out, then you can expect oil and other debris to start accumulating inside the compression chamber. And when this happens it can easily start to work its way onto the spark plugs.

Like most of the reasons on our list, the cost of compression rings is pretty affordable, but it’s a labor-intensive process to reach them. This means if you’re taking your vehicle to a mechanic the cost to repair the problem can be pretty high.

6. Damaged Piston

While this is a pretty rare reason to find oil on your spark plugs, you can’t rule it out entirely. With a damaged piston, the compression rings won’t work as they should, and you’ll likely have a misfiring engine.

If you have a damaged piston, you’ll need to replace it, and unfortunately that’s not a cheap process. However, the alternative is having the damaged piston potentially destroy the entire engine, so it’s not something you’ll want to ignore.

Symptoms of Oil on Spark Plugs

Now that you know a little more about why there might be oil on your spark plugs, it’s time to learn about some signs it’s there in the first place.

Of course, you can always pull the spark plug and check it out yourself, but if you don’t know there’s a problem it’s not something you’re likely to do. But if you’re noticing any of these three symptoms then you might want to dive a little deeper to figure out what’s going on.

1. Low Oil

When you put oil in your engine it should stay there until the next oil change. So, if you’re finding that you need to top it off between oil changes then it’s a sign something is going wrong.

There’s either an external leak sending oil outside the engine, or it’s finding its way into the combustion chamber.

If it’s in the combustion chamber there’s likely oil on the spark plugs. Quickly pulling the spark plugs to check each one should give you a decent idea if the engine is burning oil.

2. Blue Exhaust Smoke

If there’s oil in the combustion chamber it won’t withstand the heat each cycle of the engine creates. Oil will burn, but it won’t burn like fuel.

When you burn oil it has a blue tinge, and that blue coloring can work its way out of the exhaust. But to really notice a blue tinge in the exhaust there needs to be a significant amount of oil in the exhaust.

If you’re noticing a blue tinge to the exhaust, your engine is burning quite a bit of oil.

3. Decreased Engine Performance

Once again, this isn’t something you’re likely to notice until you have a severe oil leak into the combustion chamber.

But if there’s not enough oil in the engine or an excessive amount of oil in the combustion chamber it’ll throw off the fine-tuned balance of the engine.

The engine will run rough, which leads to a decrease in performance. However, it takes a pretty significant oil leak into the combustion chamber to lead to a decrease in engine performance.

Can You Clean & Reuse Spark Plugs?

If you find oil on your spark plugs, nothing is stopping you from simply cleaning them off and using them again. But before you do this you really want to figure out how the oil got there in the first place.

Because while a bit of oil on the spark plugs really won’t hurt them all that much, the oil really shouldn’t be there. And if you clean off the oil and reinstall the spark plug the oil will find its way right back to the spark plug.

Finally, if you do clean off oil from a spark plug we highly recommend checking the spark plug gap before reinstalling it. It doesn’t take much to throw off a spark plug gap, and even a small change can make a big impact on performance.

Final Thoughts

If you find oil on your spark plugs, you don’t want to ignore it. Oil on spark plugs is a sign of a much deeper problem, and if left unattended, it can lead to severe engine problems.

Now that you know a little more about why it can happen, it’s up to you to determine if you want to troubleshoot the problem yourself or take your vehicle to a professional mechanic to figure out what’s going on. But one thing’s for sure, if you ignore the problem it’s only going to get worse.

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Rickard Cefalk

Rickard is the owner of Caraspect.com and a dedicated and avid do-it-yourselfer who has always enjoyed working on his own vehicles since childhood. He now devotes his time to sharing his expert knowledge of car maintenance and other car-related information through his website.

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