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

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:

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.

Oil on Spark Plugs Causes

1. Leaking valve cover gasket

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

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

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

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

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.