Why are My Spark Plugs Wet with Oil?

Spark plugs can tell you a lot about the health of your car’s engine. For example, you might find that the plugs are fuel soaked, indicating a fault with the fuel or ignition system, or that the plugs are carbon fouled, pointing to a rich running condition. 

Or you might find that the spark plugs are wet with oil. What does this condition mean, and is it something you should be worried about? 

Why are My Spark Plugs Wet with Oil? 

Are your engine’s spark plugs covered in oil? Don’t panic yet—while oily spark plugs can indeed point to an engine problem, the issue can also result from oil leaking into the spark plug wells. It’s fairly easy to differentiate between these two concerns:

• Oil pooled in the spark plug well indicates a leaking component (e.g., tube seal) allowing oil to enter the well and contaminate the plug. The corresponding ignition coil (or spark plug wire) will also be wet. 

The video below illustrates a scenario where oil is leaking down into the spark plug wells:

• If the tip of the removed plug is oil-fouled and/or covered in ash deposits—but the plug well is dry—you’re likely dealing with an engine issue allowing oil to enter the combustion chamber. 

The video below demonstrates oil fouled plugs due to an engine problem:

Oil Leaking Down Into The Spark Plug Wells 

Oil leaking down into the spark plug wells can lead to an accumulation of wet oil on top of the plugs. There will also be oil on the ignition coils (or spark plug wires). 

Common causes for oil leaking down into the spark plug wells include:

Worn Spark Plug Tube Seals or O-rings 

Many engines have valve cover(s) with holes for the spark plugs and ignition coils to pass through. The spark plug tube seals seal the holes in the valve cover(s) to prevent oil from leaking into the spark plug tubes in the cylinder head(s). 

Depending on the engine design, there may also be a set of spark plug tube seals or o-rings somewhere in the engine’s cylinder head(s).

Anytime the tube seals or o-rings fail, the spark plugs can quickly become oil-soaked. Oiled spark plugs deliver reduced performance and might fail earlier.

Damaged Spark Plug Tubes

Although somewhat rare, it’s possible for the spark plug tubes in the cylinder head to crack, allowing oil to contaminate the plugs. In this case, replacing the oiled spark plugs won’t resolve the issue. Track down the root cause of the oil leak and fix that part. Otherwise, you’ll keep finding oil on spark plugs.

Certain General Motors (GM) vehicles equipped with the 3.6L V6 engine are prone to cracked spark plug tubes. GM has issued a technical service bulletin (TSB) advising dealership technicians of the issue. The remedy is to replace the entire cylinder head—a repair that’s both labor-intensive and costly. 

External Oil Leaks 

In some cases, an external leak can cause oil to migrate down into the spark plug wells. For example, a leaking valve cover could potentially allow oil to wick down into the spark plug holes, resulting in oil-contaminated plugs.  

Before you replace any oiled spark plugs that you come across, check the valve cover for any signs of leaking. If you don’t fix the faulty cover, you’re going to keep finding oil on the spark plug.

Oil Entering the Combustion Chamber 

Oil entering the combustion chamber typically results in wet oil and/or ash accumulation on the tip of the spark plug. You may also notice the spark plug has oil on the threads. 

Common causes for oil entering the combustion chamber include: 

Worn Piston Rings 

Each piston inside of your car’s engine has two compression rings and an oil control ring. The rings seal the small space between the piston and the cylinder wall. The bottommost ring, which is the oil control ring, is primarily responsible for keeping oil from entering the combustion chamber (where the tip of the spark plug resides).

If the rings are worn or stuck, oil will be able to enter the combustion chamber, resulting in wet oil and/or ash deposits on the tip of the spark plug. There’s also a good chance you’ll notice that the engine starts consuming oil, and you may see smoke coming from the tailpipe. 

Worn Pistons/Cylinder Walls

When the pistons and/or cylinder walls are worn, the rings can no longer seal the cylinder properly. As a result, oil can enter the combustion chamber, causing wet oil and/or ash build-up to accumulate on the tips of the spark plugs. 

Worn Valve Guides or Seals 

The engine’s valves, which are located in the cylinder head(s), allow air to enter the engine and exhaust gases to exit. Each valve has a stem that fits into a guide in the cylinder head. Dedicated valve seals prevent engine oil from seeping past the guides and into the combustion chamber. 

If the guides or seals are worn, oil can enter the combustion chamber and foul the spark plugs. The engine will also consume oil and you may notice smoke from the tailpipe (often when the engine is first started). 

Positive Crankcase Ventilation (PCV) System Issues 

All modern cars use a positive crankcase ventilation (PCV) system to reduce emissions. The PCV system vents and reroutes engine crankcase vapors to the air induction system so that the vapors can be burned inside the engine.

Issues with the PCV system can lead to oil siphoning, resulting in excessive vapors and oil droplets fouling the spark plugs.

Cylinder Deactivation and Variable Valve Timing Problems 

Nearly all modern engines have variable valve timing (VVT) and quite a few have cylinder deactivation. 

VVT adjusts the angle of the engine’s camshaft(s) to alter valve timing, thereby improving fuel efficiency, engine performance, or both. Meanwhile, cylinder deactivation shuts down half of the engine’s cylinders to improve fuel economy.

In some cases, the abnormally high vacuum created by these systems can lead to oil getting pulled past the piston rings, resulting in oil-fouled spark plugs and increased oil consumption. If a spark plug has oil on it, you must replace it.

What Can Happen if Your Spark Plugs are Oil-contaminated? 

If your car’s plugs are oil-contaminated, you’ll likely notice one or more of the following symptoms: 

Engine Misfiring and Running Rough 

An engine is said to be misfiring when there’s incomplete combustion inside one or more of the cylinders. Anything that disrupts the combustion process—including low compression, an unbalanced air/fuel ratio, or inadequate spark—can cause an engine to misfire and run rough. 

Spark plugs that are covered in oil and/or ash deposits can prevent the engine from getting proper spark, resulting in a misfire. 

Illuminated Check Engine Light

An engine misfire can result in incomplete combustion that leads to an overall increase in hydrocarbon emissions. Your car’s engine computer continuously monitors the engine for misfires. If the device detects a misfire, it turns on the check engine light and stores a diagnostic trouble code in memory. 

Increase Tailpipe Emissions and Fuel Consumption  

An engine misfire (caused by oily spark plugs) can lead to an increase in tailpipe emissions and fuel consumption. 

Can I Drive with Oil on My Spark Plugs?

Spark plugs that are significantly oil-contaminated can result in an engine misfire. Because a misfire can cause additional damage to other parts of the vehicle, such as the catalytic converter, it’s a good idea to address oily spark plugs right away. 

What to do if You Have Oil on Your Spark Plugs

If you find your car’s spark plugs are contaminated with oil, you (or your mechanic) will first need to determine the root cause of the problem. You can often limit the number of possibilities by noting where the majority of the oil is accumulated.

Oil pooled in the spark plug well indicates a leaking component (e.g., tube seal) allowing oil to enter the well and contaminate the plug. The corresponding ignition coil (or spark plug wire) will also be wet.

On the other hand, If the tip of the removed plug is oil-fouled and/or covered in ash deposits—but the plug well is dry—you’re likely dealing with an engine issue allowing oil to enter the combustion chamber.

Once you’ve determined the root cause of the oily plugs, you can proceed with the necessary repairs.