Why Is There Oil On My Spark Plug Threads?


If you’re performing some routine maintenance on your car and you pull out your spark plugs only to find that the threads are covered in engine oil, you might be (and should rightfully be) somewhat concerned. Spark plugs aren’t supposed to have oil on them, so if you find oil on your plugs then there’s something wrong with some other component of your engine.

The presence of oil on your spark plug threads can be caused by several possible issues. Leaky O-rings, a broken valve cover gasket or head gasket, a broken piston or faulty piston compression rings, and worn-out valve guides can all be sources of oil on your spark plugs.

In this article, we’ll be going over the various issues that can leave your spark plugs in this state. We’ll explain what these issues can mean for your engine as a whole, and we’ll show you how to fix these issues.

Related: What Happens If You Put Too Much Oil In Your Car

Too Much Oil

Spark plugs are a vital component of your engine, for without them, your car wouldn’t have the means to ignite the fuel in its cylinders. Spark plugs need to be clean in order to work properly. When spark plugs are really dirty, the spark is unable to jump between the two electrodes of the plug.

If you put too much oil in your engine, it can leak out and cover your spark plugs, leaving them fouled. Fouled spark plugs have their own set of symptoms, including reduced gas mileage, engine misfiring and rough idling, and reduced engine power.

How Do I Fix it?

There are two ways you can remove excess oil from your engine.

  • Drain Plug: The first way is to simply open the drain plug on your engine’s oil pan and let the excess oil drain out. This way is pretty simple, but you’ll have to raise your car up in order to access the oil pan, and draining the oil this way can be a bit messy.
  • Oil Extractor Pump: The second way is to use an oil extractor pump to remove the excess oil. Oil extractor pumps let you remove excess oil through either the dipstick tube or the oil cap. This way is a little cleaner, and you also don’t have to raise the car up to do it. 

Leaky O-Rings

The spark plugs in your engine are sealed in place through the use of O-rings, which are intended to prevent oil and other undesirable substances from reaching the parts of the spark plug where they aren’t supposed to be. 

You’ll find the O-rings for your spark plugs located within the valve cover. Depending on the make and model of your car, the O-rings may be separate components from the valve cover, or they may be molded into the valve cover and permanently attached.

The O-rings in your engine are usually made of materials like rubber or silicone, which means they’re susceptible to degradation if they’re exposed to high heat or if they just get too old. 

How Do I Fix It?

In general, fixing a leaky O-ring is a pretty quick and inexpensive fix. All you’ll need is a replacement O-ring and a tool you can use for prying, like a small crowbar or a flathead screwdriver.

  • If you’ve determined that one or more of your O-rings is leaking, start by removing the valve cover. The O-rings will come off with it.
  • When you have the valve cover off of the engine, use your prying tool to pry the O-rings out from the underside of the valve cover.
  • With the old O-rings out of the valve cover, you can install the new O-rings by pushing them into their holes.
  • One trick you can use to make the O-rings fit into place more easily is to wet them with some water mixed with a little bit of dish soap, which will give them a bit of extra lubrication.
  • If you don’t have any soapy water on hand, you can still push the O-rings into place, but you’ll probably need a special tool, namely a driver, to make sure the O-rings are properly seated. Using this driver will help prevent you from damaging the O-rings during the installation process.
  • The last thing to remember is to install the O-rings the right way around inside their holes. Make sure you install them with the tapers pointing towards the engine, so that they can easily slip over the spark plugs.

Broken Valve Cover Gasket

There are a ton of different gaskets in an engine, with one of the most important ones being the valve cover gasket. This gasket sits in between the valve cover and the top of the cylinder head, and is used to create a seal between these two components of the engine.

Within the cylinder head there are tons of components that need oil in order to work properly, like the camshafts, the valve guides, the valve springs, and more. Oil is therefore pumped up into the cylinder head and provides these components with the lubrication they need. The valve cover gasket is there to keep all of this oil in where it belongs.

Like the O-rings we just mentioned, the valve cover gasket in your engine is likely made out of rubber or silicone, which makes it more likely to fail due to old age or high temperatures. If your valve cover gasket is leaking, it should be pretty obvious; you’ll likely see oil dripping from under the valve cover. 

How Do I Fix It?

Luckily, fixing a broken valve cover gasket is pretty cheap, especially if you do it yourself. It’ll be a little more expensive if you take your car to a shop to have this done, since you’ll have to add labor costs onto your repair bill, but overall this is one of the easiest repairs to perform on an engine.

  • Start by removing the valve cover. The valve cover gasket will be a strip of rubber that runs along the edge of the valve cover.
  • When the valve cover is off, you can remove the old gasket by simply pulling it off the cover.
  • Now that the old gasket is off, you can install the new one. Make sure you order a replacement gasket that actually fits your car’s valve cover; these gaskets are always designed to fit a specific engine, so it’s important to get one that works with the car you have.
  • When you have the correct gasket on hand, line the gasket up with the edge of the valve cover, and install it by pressing it into place.
  • One thing you don’t want to do, however, is run your finger over the gasket as you’re installing it; the friction this generates can actually stretch the gasket out, leaving it unable to properly fit on the valve cover.
  • With the gasket in place, you can now reinstall the valve cover back onto the cylinder head.
  • You may also want to apply some sealant to the valve cover so that it really sticks to the cylinder head. Many cars come from the factory with their valve covers sealed in this way, so take note if this is the case with your car. 

Broken Head Gasket

The head gasket is, perhaps, the most important gasket in your whole car. The head gasket is located between the bottom of the cylinder head and the engine block.

Its purpose is to maintain a seal between these two parts, which allows oil and coolant to circulate through the engine without leaking out and also keeps the combustion chamber airtight. Both of these things are incredibly necessary for your engine to work properly.

Related: Temperature Gauge High But Car Is Not Overheating

Unlike the valve cover gasket, the head gasket in modern cars is usually made from a combination of steel and elastomer, which is a type of rubber. Modern head gaskets are a lot more durable than head gaskets from previous decades, but they do still occasionally fail, usually as a result of overheating.

How Do I Fix It?

We’ll warn you right now that replacing a head gasket is one of the more difficult repairs to perform on any engine. This is because to get at the head gasket, you need to entirely remove the cylinder head from the engine block, which takes a lot of time and is tricky to do correctly.

You could also take your car to a repair shop to have your head gasket fixed, but be aware that you’ll likely wind up with a pretty insane repair bill. Once again, this is due to the complicated and time-consuming nature of this repair.

If you really want to try repairing a broken head gasket yourself, we’d recommend that you check out this online guide. It provides a detailed walkthrough of how to correctly replace a head gasket, which you’ll likely need as there’s a lot to remember when it comes to disassembling your engine correctly.

Broken Piston

Pistons are what allows your engine to turn combustion into rotational force. When the air/fuel mixture is ignited in the combustion chamber, the resulting force pushes the piston down, which in turn rotates the crankshaft. 

Being that the pistons are right up in the combustion chamber, they inevitably end up being exposed to a lot of heat. If a piston gets overheated, it may end up cracking. When a piston is cracked, it can allow oil to make its way up past the piston and into the combustion chamber, where it will eventually cover the spark plugs.

How Do I Fix It?

Replacing a broken piston is another fairly hard repair to perform by yourself, but it can be done. You’ll need some special tools to complete this repair, though; these include a piston ring compressor and a dead blow hammer.

  • First, you’ll need to disassemble the engine to gain full access to the pistons and cylinders.
  • Wipe out the cylinder bore to remove any dirt from inside the engine, as this can potentially damage your new pistons.
  • Next, lubricate the piston and the ring compressor, and then install the compression rings on the piston.
  • With the compression rings in place, put the ring compressor on the piston and tighten it so that it has a firm hold on the piston.
  • Then, oil the cylinder bore and install the piston bearing at the end of the conrod, and insert the piston into the cylinder.
  • Use the dead blow hammer to drive the piston into the cylinder, but not too far; you don’t want to accidentally damage the crankshaft.
  • Once the piston is far enough into the cylinder, you can remove the ring compressor and push the piston in the rest of the way.
  • Next, connect the piston to the crankshaft, tighten the piston rod bolts, and reassemble the engine.

Faulty Piston Compression Rings

We already briefly talked about piston compression rings in the previous section, but they deserve their own section because these rings can fail independently of the piston itself. These rings create a seal around the sides of the piston, which prevents combustion gases from leaving the chamber early and also prevents oil from getting into the combustion chamber.

If the piston rings break, however, then the seal around the piston is lost, which obviously does not bode well for engine performance.

How Do I Fix It?

We’ve already covered how to replace a whole piston, and since replacing a piston ring involves basically the same steps as it does to replace a piston entirely, we won’t go into too much detail here.

  • In short, what you need to do is remove the pistons from your engine, remove the old rings with a pair of pliers or a piston ring expander, install the new rings, and reassemble the pistons back into the engine. 

Worn-Out Valve Guides 

The valve guides within your engine have two functions. One of these is to protect the valve stems from being worn out, and the other function is to ensure that the valves are properly lined up with their valve seats. 

If the valve guides wear out, the valves won’t be able to create a proper seal within the combustion chamber, which can cause a vacuum that can draw oil into the spark plug wells.

How Do I Fix It?

Replacing your valve guides is another somewhat tricky repair that involves partial disassembly of your engine. If you’re not much of a handyman (and there’s no shame in that), then you should probably take your car to a shop to get this fixed.

If you want to try it yourself, disassemble your engine until you can access the valve guides, which are located in the cylinder head. Removing the guides from the head can be tough, and it may help to get yourself a valve guide removal tool for this step.

  • Once the old valve guides are out of the engine, use a valve guide driver to push the new guides into the space where the old ones were. When you have the new valve guides properly installed, you can go ahead and put the engine back together again.

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