While performing routine maintenance, you keep an eye out for anything that looks odd or problematic. Having oil on the spark plugs or spark plug threads is one of those issues that should not be overlooked, especially with the increased risk of an engine fire. Your engine oil should remain in one place only—the engine—so if it is getting into other places, something is wrong. If you find yourself asking, “Why is there oil on my spark plug threads?” and scratching your head, you have come to the right place.

**Note** – A quick hello to anyone reading this, I’m Alastair and this is my site Synthetic oil.me. I started this site to help people with their oil questions, and hopefully what you’re about to read will help answer your questions. This page may include affiliate links to the likes of Amazon, which if you make a purchase I qualify to earn a (typically small) commission. Don’t worry as this won’t cost you anything, the likes of Amazon pay any commissions. Thank you in advance for your support as this helps bring you more (hopefully) helpful content.

Why Is There Oil On My Spark Plug Threads header image

What Are Spark Plugs?

First off, you need to know why oil on the spark plug threads is problematic. Spark plugs are needed to ignite the fuel for combustion. Any dirt, oil, or other debris around the spark plugs could hinder the spark needed to fire the engine.

Should oil be on your spark plug threads, you may notice other symptoms, including bluish smoke coming from the tailpipe. This happens because the engines’ electronic control unit (ECU) tries to correct the fuel combustion by adjusting the fuel-to-air ratio. However, this only causes the vehicle to run rich, due to unused fuel exiting through the exhaust valve. Thus, the exhaust gets a blue or white tinge.

Other symptoms include:

  • Poor fuel economy
  • Poor engine performance
  • Backfiring engine
  • Engine misfirings – when combustion is incomplete. This feels like hesitation when you press on the accelerator.
  • Gas smell in the exhaust

8 Reasons For Oil on Spark Plug Threads

Here are the most common problems that cause oil to end up on your spark plugs. Spoiler alert: You are going to get your hands a little dirty—not that you care.

Broken Head Gasket

The head gasket is one of the most crucial pieces in the engine. Positioned at the top of the engine block and cylinder head, the head gasket creates an airtight seal. Most of the time, head gaskets are crafted out of steel and elastomer, making them durable. However, with age and other issues, head gaskets can fail.

What happens when a head gasket breaks? Well, one thing is that the oil and coolant that it is meant to contain will start to leak out onto the spark plugs. From there, a whole list of problems could occur—alongside a hefty repair bill. The repair will typically include an engine rebuild, stripping the engine down to the block in order to replace the head gasket.

Some car brands and models are more susceptible to head gasket problems, including the Mini Cooper or Mini Clubman and Chevrolet Cruze. BMWs and Buicks between 2011 and 2014 also reported issues with head gaskets.

Leaky O-Rings

Spark plugs have O-rings that seal the space between the plugs and the cylinders, which are coated with oil. Should you remove the spark plugs and O-rings, you will notice that one side is dry while the other is slick with oil. Unfortunately, O-rings can be defective or become worn over time. Since they are made of materials like silicone or rubber, they also degrade faster when exposed to high heat. Aging can cause the O-rings to crack, as well.

Generally, any car, truck, or SUV that is older than 5 years is at an increased risk of leaky O-rings.

The good news is that leaky O-rings are easy to fix—not like a busted head gasket or broken piston. Start by removing the valve cover and remove the O-rings. You may need to pry them out. Fit the new O-rings into the holes. If you are having a hard time fitting the new ones in, wet them with a little water and dish soap. Make sure that the tapered edges are pointing in towards the engine, as that helps with fitting over and sealing the spark plugs.

Too Much Oil

If you recently changed your oil but over-poured it, the excess may begin leaking out of the reservoir. When that happens, the motor oil can gather around the spark plug holes. This could cause a set of symptoms that include engine misfiring, poor idling, reduced engine performance, and decreased gas mileage. Typically you really have to overfill the oil for this to happen.

Wondering how to know if you poured too much oil or not enough? Check the oil level in your car. That will tell you all you need to know.

If you find that there is too much oil in the engine, find the drain plug. This is the easiest way to remove some of the excess oil. You will need to raise your car up as if changing the oil and pouring the oil into a pan. Another method is an oil extraction pump, which can draw out excess oil through the oil cap or dipstick tube.

Broken Valve Cover Gasket

The valve cover gasket is found between the engine and valve cover. Its primary purpose is to seal the oil inside. The secondary purpose is to ensure that oil heads to the valve springs, camshaft, valve guides, and so on. Over time, the valve cover gasket can begin to weaken and break. Symptoms of a broken valve cover gasket include leaking oil onto the spark plug threads, a burning smell, rough idling, and engine misfiring, as well as the Oil Life Remaining indicator, switching on.

Remember that risk of fire mentioned in the introduction? Well, a broken valve cover gasket issue caused a mass recall of GM vehicles back in 2015. During hard braking, the oil drops leaking from the valve cover gasket were falling onto the engine manifold, where they caught fire.

Want to know how to replace a valve cover gasket? This video shows you how to do it on a 1999 Mazda Miata:

Worn or Broken Pistons

Pistons are another prime component of an operational engine, as they transform combustion into a rotational force. The air and fuel mix gets ignited inside the combustion chamber, forcing the piston down, thus rotating the crankshaft. Due to the positioning of pistons, they get exposed to combustion from the fuel burning. Should a piston get exposed to too much heat, it could crack. Pistons will fatigue over time too, additionally, foreign materials coming into contact with pistons can cause failure.

Piston damage can also be caused by cooling system malfunctions, impacts, incorrect head gaskets, incorrect valve recesses or clearance, faulty injection nozzles, ignition delays, and many other engine problems.

Worn or broken pistons open up pathways for oil inside the engine to leak out. The oil will travel up from the piston and towards the combustion chamber, where it will also coat your spark plugs.

Repairing worn or broken pistons requires specialized equipment, such as a dead blow hammer and piston ring compressor. Without these, DIY repairs are rather challenging and are more likely to damage engine parts. In that case, it is best to take your vehicle to a professional mechanic.

Damaged Piston Compression Rings

While checking your pistons for damage, you may have noticed the thin compression rings that surround each piston. The bottom compression ring is called the oil control ring, and it is designed to prevent engine oil from leaking into the combustion chamber—where the end of the spark plug is found.

Piston rings get worn down over time. Some factors can contribute to a shorter lifespan, including bad ignition timing, lack of lubrication or contaminated oil, and overheating. Should the piston compression rings get damaged, your car is going to perform poorly, and its acceleration will leave much to be desired. Furthermore, damaged piston compression rings put pressure on the crankcase, deteriorating the engine seal and allowing oil to seep out.

Say hello to oil around the spark plug threads.

If you notice that the piston rings are damaged and that your car has lost efficiency, then it is time to replace the rings.

Misaligned Piston Rings

Perhaps in the past, you have installed a new set of piston rings. Did you notice that these piston rings have gaps? The gaps allow for the rings to expand and contract as the temperature in the engine increases and decreases as well as being able to be fitted to the piston. Additionally, the positioning of each ring, as well as the gaps, is important. You do not want to align the gaps, nor do you want to set these rings too far apart from one another.

The rings also prevent gases created by combustion from escaping. When the setup is faulty, oil and gases can break free. With the seal broken, engine performance suffers. You also end up with the symptoms of oil-fouled spark plugs.

Not sure how to properly install piston rings or fix a misalignment? This video will tell you everything you need to know:

Worn Valve Guide

You already know what could happen when a broken valve cover gasket leads to oil on the spark plugs, but what about valve guides? The function of the valve guide is to ensure the exhaust and intake valves remain in the correct position. Valve guides are also sealed to keep oil away from the combustion chamber. Should the seal around the valve guides begin to fail, oil droplets can entire the combustion chamber. The vacuum created by the worn-out valve guides also shuttles oil towards the spark plug threads.

Valve guides are not expensive parts, but they tend to be complicated to replace. Heading to the repair shop for worn valve guides means your bill is going to be massive. Now, you could try this yourself, but you need to be confident in dissembling your engine and getting it back together. Even removing the valve guides can be frustrating, so it is recommended that you purchase a valve guide removal tool.

Once you have the oil valve guides removed, you need a valve guide driver to accurately push the new guides into the places where the old ones had been.

Other Problems That Could Lead to Oil on Spark Plug Threads

While broken valve cover gaskets, too much oil, and piston problems are some of the leading causes of fouled spark plugs, there are some less common issues that cause the same result. If you open the hood of your car and cannot detect any of the eight issues previously mentioned, then it may be one of the following:

Positive Crankcase Ventilation (PCV) System Problems

Did you know that the first positive crankcase ventilation system was developed in the early 1960s? Basically, the PCV system reroutes crankcase vapors towards the air induction system, where those vapors are burned off by the engine. The PCV valve has been a prime component of fuel economy since then. You can find it located at the top of the engine’s valve corner.

If your PCV system is malfunctioning, it can cause oil siphoning, which increases the volume of vapors. Oil droplets can also mess up your spark plugs.

This video explains the PCV system in your car:

Variable Valve Timing (VVT) and Cylinder Deactivation Problems

Unless you are still driving around an old 1999 Toyota Camry, then you may have a vehicle modern enough to have variable valve timing (VVT). Unfortunately, some models in recent years have had cylinder deactivation problems, including Mazda, which had to recall 262,000 cars and SUVs. When there are issues with the VVT and cylinder deactivation systems, exponentially high vacuum forms, causing oil to get sucked through the piston rings. The end result is higher oil consumption and oil on the spark plug threads.

Here is a list of some automakers who use VVT (some with proprietary names):

  • Alfa Romeo (Twinspark)
  • Audi (VVT)
  • BMW (VANOS, Double VANOS, or Valvetronic)
  • GM (DCVCP, VVT, and Alloytec)
  • Hyundai (MPI CVVT)
  • Honda (VTEC, VTEC-E, and iVTEC)
  • Ford (Variable Cam Timing)
  • Nissan (VVL, CVTC, VVEL, and N-VCT)
  • Volkswagen (VVT)
  • Toyota (VVT, VVT-i, and VVTL-i)
  • Volvo (CVVT)
  • Mazda (S-VT)
  • Lexus (VVT-iE)

Conclusion

Spark plugs are an important part of the engine, so any oil on spark plug threads is bound to be disconcerting. Oil-fouled spark plugs point to other problems within the engine that will adversely affect your vehicle. Should those issues, such as a broken head gasket, leaky O-rings, excess oil, or worn pistons or piston rings, continue you will end up with a car, truck, or SUV that stops running. Worse, you could end up with an engine fire. Now that you know the risks, it may be time to pop the hood and get to work!


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