Oil On Spark Plug: How Did It Get There?

by Conner Mckay

Spark plugs are an essential component of modern engines. Since their introduction to the world of automotive by Jean L. Lenoir in the late 1800s, spark plugs received numerous design refinements. Today, they are cheaper, smaller, and for the most part, very dependable. Is your car displaying weird symptoms such as blue smoke from exhaust or engine misfires? The culprit may be oil on spark plug.

Having said that, despite all the design refinement, spark plugs aren’t completely foolproof. A number of things can make them fail. Oil on spark plugs can be a result of a bad valve cover gasket, faulty head gasket, worn valve guides or spark plug O-rings, or a broken piston ring. Signs of oil on spark plugs would be poor fuel economy, blue smoke emitted by the exhaust, and engine misfires.

In today’s article, we talk about all the different reasons there might be oil accumulated on your vehicle’s spark plug, how to troubleshoot and repair that, as well as some general information to keep in mind for the overall betterment of your precious ride.

What Is A Spark Plug?

Defined by their small sizes, spark plugs are electrical devices tasked with converting thousands of volts of electricity into powerful sparks that ignite the combination of air and fuel inside gasoline-using combustion engines. Thanks to the ignition, there is a controlled mini-explosion, producing energy required to run the engine.

Vehicle spark plugs generally do not have debris accumulation from working, but two-stroke engine spark plugs are a different story.

Function Of A Spark Plug

Arguably, spark plugs are the most valuable parts of the ignition process. Consider it like this: You are deep into the woods at night and need to make a campfire. The lighter to the fire is what is the spark plug to an ignition inside the vehicle, i.e., it starts the combustion. You can imagine the consequences of a faulty lighter.

But the importance of spark plugs doesn’t stop here. They are also responsible for maintaining the appropriate temperature inside the combustion chamber. The combustion process inside the engine needs a myriad of delicate variables to be kick-started. All of these factors affect the efficiency of the ignition.

If the temperature in the chamber is too low, the energy will not propagate as it should, resulting in a major loss of power. On the other hand, too high of power causes a pre-ignition, which is simply a fancy term for an unplanned ignition before the firing of the spark plug.

Since it’s such a relevant factor, spark plugs are categorized by their “heat range.” Automakers select the proper range for the engine after conducting various tests in multiple conditions. This is why it is best to only use spark plugs recommended by the manufacturer for your vehicle.

As you can tell already, spark plugs are an indispensable part of the internal combustion engine, making it crucial to ensure that they are kept in the best condition for the lifetime of the vehicle. Oil on spark plugs is something that can dramatically reduce the service time of spark plugs. Moreover, it can damage other parts of the engine.

How Oil On Spark Plug Impact Engine Performance

Let’s use a lighter example once more to simplify the answer. Imagine you are trying to light the campfire but there is engine oil on the lighter tip. Although you may be able to start the fire, it will take several tiring attempts. Now think like that but inside your vehicle.

Without a spark, there is no ignition or combustion. Fuel will be passed to the engine but will leave the cylinder unused.

On to chamber temperature! Let’s assume there was a weak spark. With oil particles constantly burning with gasoline, there isn’t much you can expect. As mentioned earlier, the process of combustion is a delicate and well-timed one. The oil has a significant effect on the performance of the engine.

Oil on spark plugs for prolonged periods can destroy the catalytic converter as the fuel remains unburnt, and it could also wear out the engine bearings faster due to the additional gasoline stored in the oil pan. A condition as severe as this has to be addressed as soon as possible.

Symptoms Of Oil On Spark Plug

How do you know that the problem with your vehicle is oil on the spark plugs? These telltale signs of an oil outbreak can help you identify the problem fast.

Oil On Spark Plug Symptoms: Blue Smoke From The Tailpipe

We mentioned already how oil accumulation on the spark plug tips adversely affects the efficiency of fuel utilization by the engine. Spark plugs are meant to create a spark at the right moment, in alignment with the engine valves’ timing. This has to be timed to happen between the closing and opening of the valves and precisely at the correct time and sequence.

If the quality or the timing of the spark isn’t up to the mark, the combustion process takes a hit, making it significantly inefficient as the fuel does not burn completely anymore.

During this, the engine’s ECU tries to correct the combustion efficiency situation by altering the air to fuel ratio in the cylinders. Often, this means a rise in the amount of fuel consumed by the engine, making it “run rich.” When too much fuel enters the combustion chambers, unburnt fuel can leave through the exhaust valve and enter the exhaust. This explains the blue or white smoke unnaturally being emitted by your vehicle.

Oil On Spark Plug Symptoms: Poor Fuel Economy

A common sign of oil on spark plugs is a visible decrease in fuel economy. A vehicle consumes more oil when the spark plug is unable to create a high-quality spark. When new, spark plug tips can comfortably operate at temperatures higher than 500°C. If oil gets onto the spark plugs, it will form a burnt oxidized coating over the tips.

A burnt spark plug cannot operate efficiently since it will not produce a good spark. The ECU attempts to supplement the underperformance by increasing the amount of fuel. If more than one spark plug has oil on it, the fuel economy will drop noticeably. In fact, some studies have shown a 30% decrease in fuel efficiency due to poor spark plugs. The increase is huge – only comparable to faulty fuel injectors.

Oil On Spark Plug Symptoms: A Drop In Engine Performance

As mentioned above, a spark plug that has been in contact with engine oil will not be able to produce a spark for too long. As the engine oil contaminates the plug tip, it prevents the spark plug from generating a spark powerful enough to ignite the fuel/air combination in the combustion chamber.

If the gas blend is not burning completely or efficiently in the combustion chamber, it will not produce the level of pressure required to push the piston when the engine needs it. If you push the gas pedal hoping the car will accelerate, you are sure to experience a noticeable lag in acceleration.

Oil On Spark Plug Symptoms: Possible Engine Misfires

Engine misfires are one of the dangerous and common symptoms of oil on spark plugs. An engine misfire is triggered by incomplete combustion of the air/gas mix inside the cylinder(s) of the engine. As a result, the driver experiences hesitation or stuttering when they press down on the accelerator.

Oil on the electrode tip or in the spark plug well interferes with its power to produce sparks hot enough to ignite the mixture. The cylinder pistons cannot generate enough power thus, causing an engine misfire.

Oil On Spark Plug Symptoms: Engine Backfires

If the fuel inside the chamber does not burn up entirely, it will escape through the exhaust valve into the exhaust pipe. What is an engine backfire? It is an instance when the air/gas mixture in the vehicle combusts after it leaves the chamber of the engine cylinder.

The fuel particles have to burn up entirely while they are inside the combustion chamber. This reaction creates an energy that is released in the form of backfires or mini-explosions. We don’t have to explicitly mention how engine backfires are bad for your vehicle. It also leads to a drop in engine performance as well as poor fuel economy.

Oil On Spark Plug Symptoms: Gas Smell From Exhaust Pipe

When the gas does not burn up entirely in the combustion chamber, there is a noticeable smell of gas coming from the exhaust. A gas smell is emitted from the exhaust fumes, particularly when the engine has just been started or is cold.

The reason behind this is that at startup, the engine uses a slightly richer air/gas mix. Cold fuel is harder to vaporize, leading to an increased demand for fuel to make a combustible fuel-air mixture. Oil on the spark plugs adds to this problem and more fuel is dumped straight into the exhaust, creating that strong gas smell (for more insight, check out our guide on how to get gas smell off hands).

Reasons Behind Oil On Spark Plugs Of Your Vehicle

Once you have deduced that there is, in fact, oil on your vehicle’s spark plugs, it’s time to answer why it’s there. We have mentioned some of the most common reasons your vehicle’s spark plugs may be filled with oil.

Oil On Spark Plug

Leaky O-Ring Seal

Spark plugs do not have a gasket, but they do have O-rings. O-rings contain the spread of oil and keep everything off the spark plug on one side, keeping the other side dry.

If there is only oil on one spark plug, the problem might be a leaky O-ring seal. Fortunately, this is one of the easier problems to solve. That, however, does not mean this situation should not be addressed immediately. Leaky O-ring seals lead to engine misfires and more extensive destruction.

Blown Head Gasket

Most people associate coolant in the compression chamber with a blown head gasket but another leaking fluid can be oil. When this happens, oil seeps into the combustion chamber with ease. Drivers start noticing excessive smoke as well as oil accumulation on the spark plugs’ tips.

You have to replace the head gasket before it causes more severe issues.  Good news for you: head gaskets are inexpensive. Bad news: replacing them is a labor-intensive procedure.

Leaking/Worn Valve Guides

Valve guides keep the exhaust and intake valves in place at all times. In addition, they feature seals that prevent the entrance of oil into the combustion chamber.

Once those seals deteriorate, the oil finds an easy gateway into the combustion chamber. A moment later there is oil on the spark plugs. Similar to other components mentioned, valve guides aren’t expensive but do require a lot of time and work to replace. Don’t be surprised when the repair shop hands you a lengthy bill.

Leaking Valve Cover Gasket

The valve cover gaskets in your vehicle are there to keep the oil near parts that actually need it. One of the components that do not need oil is spark plugs, and the valve cover gasket ensures oil stays away from them. But with a leaky cover gasket, oil can enter the spark plugs easily.

Similar to head gaskets, valve cover gaskets are inexpensive but labor-intensive repair process. Here’s how you can DIY a valve cover gasket replacement.

Damaged Piston Compression Rings

Compression rings exist around each piston in the vehicle’s engine. They will keep the oil outside the combustion chamber. However, when the compression rings are faulty, nothing stops the oil from entering the combustion chamber.

Not only do these rings keep the engine running smoothly, but they also prevent more problems inside. If you suspect destroyed compression rings, get them checked immediately.

Damaged Piston

Although cracked or damaged pistons are uncommon, they aren’t entirely unheard of. Due to rising heat in the combustion chambers and the increasing age of the pistons, the heat becomes too much to bear. If the piston is cracked, the engine will have to be rebuilt, which is a pricey process.

On the other hand, if you choose to ignore the problem, you risk having your vehicle break down on the side of the road with a potentially destroyed engine. Damaged or cracked pistons aren’t an issue you can ignore.

Diagnosing The Cause Of Oil On Your Vehicle’s Spark Plugs

Before anything, make sure you have followed basic safety guidelines. You don’t want to incur bodily harm while working on your vehicle. Refer to the correct OEM literature (if available). Original manufacturer guidelines for diagnostic procedures should be prioritized over a generic stream.

Now let’s dive into the diagnostic process.

1. Visual Inspection

OHV Engines

Conduct a thorough visual inspection of the exterior of the cylinder heads. If there is any oil coming out of the valve cover gaskets, replace them.


Do the same visual inspection as you would do on an OHV engine and replace the valve cover gasket if needed. Furthermore, examine the exterior situation of all spark plugs. If you notice any sign of engine oil on them, the spark plug ceramic coating, or coil-over-plug, then you need to replace the O-ring seals (all of them).

2. Engine Tests

In about 80% of the cases of oil on spark plug, the reason is a bad spark plug O-ring or faulty valve cover gasket. The rest 20% is usually related to issues with the engine.

Only conduct the given tests after the visual inspection talked about earlier has been done.

Engine Compression Test

Warm up the engine till it reaches its usual operating temperature. Take out all the spark plugs and rotate the crankshaft in its normal rotational direction until the #1 piston reaches its compression stroke TDC (top dead center). Use the appropriate adapter to connect cylinder #1 to the engine compression gauge.

Disable the ignition and fuel system and turn the engine six revolutions with the pedal on a wide-open throttle. Note the gauge reading and do this process for all the cylinders. Check the OEM literature to know if the compression is situated within the acceptable range (normally more than 100psi is okay).

A difference greater than 10% between the cylinders is a red flag, especially if there is oil on one of the spark plugs.

Engine Differential Pressure Test

This test might not be as widely used as the one mentioned prior to this as it involves cleaning the compressed air source. However, in the end, it saves a lot of time in understanding the main reason behind the compression issue. The process is the same for the most part.

You have to warm up the engine, turn the crankshaft until TDC is reached by piston #1, and disable the fuel and ignition systems. Afterwards, connect cylinder #1 to the special differential pressure gauge. Now, use an integrated air regulator to adjust the air pressure – it has to be between 80 to 90psi.

As a rule of thumb, the cylinder manometer shows the same pressure. A variance difference greater than 15% is indicative of a leakage.

The advantage of this process over the conventional compression test is that you can find out the location of the leak.

6 Steps To Fix Oil On Spark Plugs

Luckily, oil on spark plugs is not an issue to tear your engine apart over. It’s certainly a serious issue but once you find the reason and location of the leak, it’s easy to fix it. When you find and repair the bad component, the vehicle should be good to go. However, if you are fretting over the excess oil, spray a little carburetor cleaner inside the spark plug holes and allow the oil to run into the combustion chamber. Reattach the spark plugs and let the excess oil be burned off.

Oil On Spark Plug

As a leaking spark plug O-ring or valve cover gasket is associated with finding oil on spark plugs, here’s a guide on how to replace them.

Step 1: Preliminary Checks

Make sure the ignition switch isn’t on and disconnect the battery of the vehicle. Disconnecting the negative terminal is generally enough.

Step 2: Take Off Valve Covers

Take off all decorative covers along with the valve cover bolts and pull them out gently. If the valve cover is tightly secured to the head, use the right tool (for instance: a rubber mallet) to remove it. Refrain from using metal tools such as screwdrivers as you risk permanently damaging the cylinder head.

Step 3: Remove The Spark Plugs

Use a spark plug removal socket to remove the spark plugs once by one. Check for oil leakage on each one as you go.

Step 4: Take Out The Old Gasket And O-Ring Seal (If Any)

Take out the old gasket and the O-ring seal from all the cylinders. The gasket removal may be difficult if there was silicone used during installation.

Step 5: Clean The Valve And Head Cover Surface

​Peel off traces of the old gasket with a plastic tool. Clean the cylinder head and valve cover with a degreaser. Be extremely careful while doing so.

Step 6: Install The New Gasket And O-Ring Seal (If Any)

Apply the appropriate silicone (check if the engine OEM literature permits it) and install the spark plugs and valve covers. It’s essential to fit the proper spark plugs post-installation.

Can I Drive With Oil On the Spark Plugs?

Yes, technically you can still drive with oil on spark plugs of your vehicle. But, you cannot forget that you must figure out how the oil there in the first place. Oil on the spark plugs can be indicative of more serious issues.

Keep in mind that the oil isn’t the problem – be more concerned about how the oil ended up there.

Can I Clean And Reuse Spark Plugs?

Yes! Once the necessary repairs have been conducted and you have ensured the oil will not return, spray down the threads and electrode with a carb cleaner and let everything dry. Reinstall the spark plugs once dry and that’s all!

Facts: Symptoms, Causes, and Fixes of Oil on Spark Plugs

  • Oil on spark plugs can lead to serious engine problems, and common signs of excess oil on spark plugs include blue smoke from the tailpipe, gas smell from the exhaust, increased fuel consumption, decreased engine performance, engine misfire, and engine backfires.
  • Common causes of oil on spark plugs include leaky O-ring seal, faulty piston compression rings, worn-out intake valve seals, blown head gasket, leaking valve cover gasket, worn valve guide, clogged crankcase ventilation, and broken piston or worn cylinder wall.
  • To fix oil on spark plugs, it’s important to address the root cause of the problem first and seek the help of an automotive professional if needed. The process involves removing the valve cover, detaching each spark plug wire, cleaning the cylinder head and valve cover, installing new spark plugs, and replacing the gasket and O-ring seals.
  • It’s possible to drive with oil on spark plugs, but ignoring the problem can lead to serious engine damage and increased fuel consumption. To prevent oil from leaking onto spark plugs, regular maintenance and inspection of parts like the head gasket, spark plug boot, piston rings, valve cover gasket, valve stem seal, and PCV valve is recommended.


Oil on spark plugs may not sound like the most severe problem your automobile can show, but it is certainly one of the worst ones. Your vehicle will start breaking down due to lack of repair and soon the bills will rack up beyond your imagination. Stay safe and look out for the signs of oil leakage on spark plugs.

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