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Oil on the Spark Plug Gives a Clue to Engine Troubles

If your doing maintenance on your vehicle and come across oil on the shaft or spark plug head, it can tell you a lot about your engine. You don’t have to be an expert mechanic to spot every issue with a vehicle. However, you must know what to look for when troubleshooting or doing maintenance. For instance, oil on a spark plug may not seem like a big deal to the untrained eye. Unfortunately, those who know a thing or two about engines know that it could be a clue to some severe engine troubles.

Today, we’re popping the hood and getting our hands dirty. Let’s see how serious oil on a spark plug can be for you and your engine.


How to Tell If There’s Oil on a Spark Plug

Removing a spark plug can be for replacement or troubleshooting engine issues.

You first must remove the spark plug to tell if there’s oil on it. To do this, you will need to locate the spark plugs. They are usually on the engine’s cylinder head and connected to the ignition system by wires. You’ll need to use a spark plug socket and remove each plug individually. Once you remove the plug, thoroughly inspect it. Is the oil on the top with the electrical connection or on the inside where the spark is? Inspect each plug and compare them to one another.

When talking about oil, it could either present on the shaft (or insulator, usually glass) or the firing end (where the electrodes are in the engine) Oil in either of these locations can indicate specific problems.

If you discover oil, you must find out why it’s there and where it’s coming from. Look in the surrounding area for any signs of oil leaks or issues that might cause the oil to get to the spark plug.

holding spark plug
Your spark plugs can show signs of other issues in your vehicle.

What Other Symptoms are Common With Oil On Spark Plugs

You may have oil on your spark plugs without even knowing it. There are a handful of common symptoms that you should watch for to keep your engine safe. Let’s take a look!

Decreased Engine Performance

An engine can experience performance issues for various reasons, including oil on the spark plugs. A spark plug working incorrectly will cause a lag in acceleration and not deliver the power you expect.

Having a layer of oil on your plugs can cause incomplete combustion. This decreased engine performance also lowers your fuel economy. Keeping an eye on your MPGs can help you spot any issues immediately.

Gas Smell from the Exhaust

Another significant sign that there could be oil on your spark plug is the smell of gas coming from your exhaust. This is because oil-fouled spark plugs can fail to ignite the fuel properly. The result is an increased amount of gas getting into the exhaust.

With your engine running slightly richer, you will smell the gasoline in your exhaust and use more fuel. Your MPGs will start nosediving, and you’ll find yourself at the pump more often.

Blue Smoke from the Tailpipe

If oil is making its way into the combustion chamber, it could be getting burned. Burning oil is usually blue in color.

Old spark plug with oil
If you discover oil on your spark plugs, you must find out why it’s there and where it’s coming from.

Engine Misfires

If you’re experiencing shaking while driving or idling, you could have oil on a spark plug or two. This is typically due to incomplete on nonexistent combustion in one of your engine’s cylinders. While there are several possibilities for engine misfires, oil on the spark plug will give you an idea of what could be causing it.

Increased Fuel Consumption

Have your MPGs tanked lately? If so, check your spark plugs for oil. One way your engine attempts to maintain pressure inside the combustion chamber is by using more fuel. This causes your motor to burn through gasoline and oil. You will have to top off your fuel tank, and you may need to add a quart or two of oil occasionally.

Engine Backfires

While some symptoms are easy to ignore, engine backfires are not one of them. These often sound like explosions or gunshots coming from your vehicle’s tailpipe. You’ll make a few enemies as you startle the elderly walking down the sidewalk and wake sleeping babies.

These backfires are typically the result of the spark plug not burning the fuel fast enough and the fuel dumps into the exhaust valves. Your internal combustion engine quickly becomes an external combustion engine.

Pro Tip: Avoid the mechanic. We uncovered How & Why to Change Your Differential Oil.

Mechanic replacing spark plug
There are multiple reasons why there may be oil on a spark plug.

What Would Cause Oil to Get on a Spark Plug?

Like before, there are two areas where oil could get on a spark plug. Depending on the area you can rule out some of the possible problems.

Defective or Worn Valve Guides

If you are finding oil on the firing end of the spark plug, then oil is getting into the cylinder and probably burning.

Defective or worn valve guides are one possible cause for oil getting to the combustion chamber. These are typically bronze or cast iron pressed or inserted into the cylinder head. They have two essential functions; guiding the valve stem and sealing the cylinder head and valve stem.

The valve could be worn or defective if you find oil on your spark plugs. These valves wear out over time from the movement and exposure to extreme temperatures. 

Faulty Head Gasket

It is possible for oil to enter the combustion chamber in areas other than the valve guides. The most common area of problem is the head gasket. This will cause oil on the firing end of the spark plug.

If you’re finding oil on your spark plugs, it could result from a faulty head gasket. Unfortunately, this type of repair isn’t fun or cheap. You’ll likely need to pay a professional to repair and pay for parts, spending upwards of $2,000.

Faulty head gaskets can cause several fluids to leak and mix. You’ll likely see drops of coolant under your vehicle and a mixture of coolant and oil on your spark plugs. If you notice your engine temperatures climbing while running your engine, check the head gasket.

Leaky O-Ring Seal On Spark Plug Tube

If you are finding oil on the shaft or insulator part of the spark plug, then oil is leaking in the head area but not making its way into the combustion chamber. These leaks are similar to external leaks.

Leaky O-ring seals are one of the main reasons you’ll find oil on spark plugs. The purpose of the O-rings is to keep oil and coolant from getting to your spark plugs as they penetrate the engine head. If the O-rings are failing, you’ll need to replace them. Replacing these O-rings isn’t challenging, and you’ll be in and out of the shop quickly.

Pro Tip: If you’re an RV owner, get the inside scoop on RV Oil Changes: What Every RV Owner Needs to Know.

Broken Piston Compression Rings

There’s a small space between the piston and the cylinder wall. The piston compression rings close off this space and prevent oil from getting into the cylinder from the oil pan below.

Oil ingress of this type typically gets burned or causes a misfire. Its possible the find oil on the firing end if a spark plug in this situation.

Clogged Crankcase Ventilation

Clogs in your crankcase ventilation system can cause an increase in engine oil consumption, oil leaks, and misfire. You’ll likely have a check engine light that won’t disappear until you fix the issue.

The crankcase ventilation system is responsible for the engine’s emission control process. It removes contaminants from the crankcase and recirculates them into the combustion chamber to finish the combustion process. However, when it clogs, it’s unable to release the pressure and can cause pressure buildup in the crack case.

If the pressure increases on the oil it can cause it to blow out seals all over the place. We once had our rear main seal fail due to this problem.

Leaking Valve Cover Gasket

Another possible cause of oil on top of your spark plugs is a leaking valve cover gasket. This seal keeps oil from leaking from the valve cover onto the engine and other components. Oil fouling is typical on ignition coils or spark plug wires when the gasket fails. 

Old spark plugs with oil
Remove the spark plug from your vehicle before attempting to remove oil from it.

How to Remove the Oil From a Spark Plug

To remove oil from a spark plug, you must first remove the spark plug. You’ll need a spark plug socket and ratchet to remove the plug. If you find a significant amount of oil fouling on the plug, it’s better to replace the plug than clean it.

Grab a wire brush or spark plug cleaner to remove the oil and carbon buildup on the plug. However, you want to be careful around the electrodes and insulators. Don’t get carried away and cause damage to them, or you’ll have no choice but to replace them.

You’ll want to clean the spark plug well. Use a rag and screwdriver to clean the spark plug as much as possible. Just be careful not to let dirt or debris fall into the engine after removing the spark plug from the well.

Can You Drive With Oil on a Spark Plug?

Oil on a spark plug is an indication that something is leaking. If your vehicle continues to function, it’s not a catastrophic leak. However, if you procrastinate or ignore the issue, you can cause severe damage.

While a trip to the mechanic isn’t usually enjoyable, it will only get worse the longer you wait. You’ll start to notice poor engine performance and increased oil consumption. However, it can get worse fast, lead to engine damage, and become a safety issue. Trust us; you don’t want to wait!

Pro Tip: Ditch the mechanic and care for your car yourself. Uncover the truth about How Often You Should Really Check Your Oil?

Replacing spark plug
If you find oil on your spark plugs, address the issue as soon as possible.

Can You Clean a Spark Plug and Reuse It?

It is possible to clean a spark plug and reuse it. However, it’s typically not a good idea. The spark plugs on your vehicle will naturally develop fouling from carbon deposits, oil, and other contaminants. While you may clean some impurities from the spark plug, it won’t be as good as new.

Spark plugs typically last anywhere from 30,000 to 100,000 miles. You’ll likely notice some of the issues we’ve mentioned, like poor engine performance, misfires, and damage to your engine.

If you discover fouling on your spark plug, you must do more than clean it. You also need to identify the cause of the fouling. If not, you’ll find yourself in the same situation down the road. Follow the manufacturer’s recommendations specific to your vehicle.

How To Change Spark Plugs in Your Car with Scotty Kilmer

Take Care of Your Engine Health

There are many parts of your car that you want to protect. However, there aren’t many that are more crucial than your engine. If you discover oil on a spark plug, it’s not something you can afford to ignore. Failing to address the issue will only end in a heftier and more costly repair bill. Preventative maintenance is critical to avoiding expensive repairs and extending the life of your vehicle.

When was the last time you checked the condition of your spark plugs? Tell us in the comments!

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About Tom Morton

Tom, a Pacific Northwest native, is our technical genius. Born in Washington and raised in Alaska before settling in Michigan. He's the man who keeps our operation running, both figuratively and literally.

With a background in Electrical Engineering, Tom specializes in RV solar systems and lithium batteries. He made history as the first documented individual to use a Tesla battery module as an RV battery. Tom has personally assisted countless RVers with system installations and has educated thousands more through his videos and articles.

Cinematography is another of Tom's passions, showcased in his work on the Go North series. You can see his camera skills on display in The RVers TV show on Discovery Channel and PBS where he also stars as a co-host.

Tom's mechanical expertise extends beyond RVs to boats, planes, and all things mechanical. He's renowned for taking on maintenance and repair projects single-handedly and is often spotted underneath RVs, making him the technical backbone of our endeavors.

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