6 Causes of Oil on Spark Plugs (on Threads & in Wells)

Oil in the combustion chamber is never a good sign. Here're the causes of why you may find oil on the spark plugs

Causes Of Oil On Spark Plugs

Oil is critical in the engine for lubrication. It helps keep the moving parts from friction and excessive heat.

Oil, water, and fuel are three vital engine components that should not mix. The moment this happens, you can start experiencing misfires and further engine damage.

It happens that oil leaks into the combustion chamber, and therefore the oil gets stuck on the spark plugs, but why does it happen? Let’s take a look at the most common causes of oil on spark plugs.

Oil on Spark Plugs Causes

The most common reason for oil on spark plugs is a leaking valve cover gasket. If you find oil on the spark plug electrode, it may be caused by clogged crankcase ventilation, faulty turbocharger, bad intake valve seals, or bad piston rings.

Here is a more detailed list of the most common causes of oil on spark plugs.

1. Leaking valve cover gasket

Engine Oil On Spark Plug

If your spark plugs look like the picture above, fully covered with oil when you remove them, you can feel lucky.

When the oil on your spark plug looks like this, and there is oil on the spark plug threads, it is not coming from the engine’s inside; it is actually coming from the outside from the valve cover, filling up the wells.

Around the spark plug holes, there are O-rings, which can be external or integrated with the valve cover gasket. When these get bad because of heat, they will start to leak, and oil will pour into the spark plug holes.

This is bad for both the ignition coils and spark plugs as it can cause misfires, and it can also cause these parts to fail.

If your whole spark plug is covered in oil, you should definitely replace the valve cover gasket and the O-rings around the spark plugs.

2. Clogged Crankcase Ventilation

Oil On Spark Plug Tip

If you have oil on the tip of the spark plugs like above, it means that it is coming from the combustion chamber, and it now time to be a little bit more worried. Oil in the combustion chamber is never a good sign.

The most common cause of oil on the spark plugs is that the crankcase ventilation is clogged or faulty. This creates an overpressure inside the crankcase, which forces oil to be pressed into the combustion chamber from the crankcase ventilation, valve sealings, turbocharger, or piston rings.

Check the crankcase ventilation breathing to ensure nothing is clogged, and the one-way breathing valves work fine – if equipped on your car model.

3. Faulty turbocharger


A faulty turbocharger is also pretty common when it comes to oil on spark plugs. When the oil leaks from the turbo inlet compressor sealings, it goes through the combustion chamber and gets stuck on the spark plug.

This can also be caused by bad crankcase ventilation, so make sure to check it first.

Check your intercooler in your car, and if the intercooler and intake pipes are full of oil, it is a big chance that your turbocharger failed.

4. Worn out intake valve seals

Valve Seal Around Valve Shaft

The Valves play a vital role in the combustion chamber by regulating the inflow of air and fuel while controlling exhaust gases’ outflow. Fuel, oil, and coolant should not mix at any time in the engine. If the valve seals wear out, the oil can find its way into the crankcase combustion chamber.

The first symptoms of this are your car emitting blue smoke. You should have the valves fixed as soon as possible to prevent further engine trouble. You can learn more about this here: Bad valve seal symptoms.

5. Faulty piston rings

Piston Ring Function

A closer observation of piston rings will reveal compression rings at the top and bottom. As the piston moves up and down along the wall, these rings will collect oil.

The rings prevent the oil beneath the pistons from reaching the combustion chambers. However, the rings wear out, and oil finds its way to the spark plugs. This can cause oil from reaching the moving parts.

You can learn more about the symptoms here: Bad Piston Rings symptoms.

6. Faulty pistons

Car Pistons

Oil can also leak to the spark plugs through a faulty piston. Pistons operate at high speeds, and they are subject to wear and tear.

Likely the piston rings, if there is a crack, damage, or a melted piston, the oil can find its way there onto the spark plug.

The best way to find faulty piston rings and piston rings are to make a leak-down-test or a compression test. A leak-down-test is strongly recommended because you will find these problems very easily with this equipment.

Oil on Spark Plugs FAQs

Is a little oil on the spark plugs bad?

If a little oil on the spark plugs is bad or not depends on where you found the oil. If you find oil in the spark plug wells, it’s likely a bad valve cover gasket, which is fairly easy to fix. If you find oil on the spark plug electrodes, it could be internal oil combustion, which could be caused by a much more serious problem.

Can I drive with oil in my spark plugs?

It is not recommended to drive your car with oil on the spark plugs, whether the oil sits on top of the spark plugs or in the combustion chamber. If the oil is in the wells, it can cause damage to the ignition coil. If you have oil in the combustion chamber, it must be addressed immediately to prevent further damage.

Can a misfire cause oil on spark plugs?

No, a misfire will not cause oil on your spark plugs. However, a misfire can cause damage to your catalytic converter or other parts, so it is important to fix the misfires as soon as possible.

How do you fix oil on spark plugs?

If you notice oil in the spark plug wells, you need to replace the valve cover gasket or the O-rings around the wells. If you notice oil on the spark plug electrode, a more serious problem like a bad turbocharger or bad piston rings could be causing it.

Magnus Sellén
Written by:

Magnus is the owner and main author of Mechanicbase. He has been working as a car mechanic for over 10 years, and the majority of them specialized in advanced car diagnostics and troubleshooting. Certified Automotive Diagnostic Technician.

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