If you are wondering why are my spark plugs covered in oil, you are not the only one wondering this. Oil on spark plug often means there could be a possible problem with your engine. If you notice oil on your spark plugs, it means you might have a severe engine problem. Now, the causes of oil on spark plug threads can be different for different situations. Therefore, we will be discussing those causes and signs today to fix the problem.

Causes of the oil on the spark plug

1. Worn out valve guides

The valve guides ensure that the valves are in the right position for a proper air intake into the combustion chamber when the engine gets started. Spark plugs can be found in between the intake and exhaust valves. When the guides leak or get damaged, the spark plug lets the oil leak through, causing the oil deposition.

2. Leaking O-ring seal

The O-ring seal sits at the lower part of the spark plug and stops oil from getting to the spark plugs. Hence when O-ring seals get worn out or damaged, it’s highly likely to create a way for the oil to pass through the valves, eventually causing oily spark plugs. A battered seal can cause engine misfiring or more serious issues to the engine.

3. Defective piston

Another common cause of having traces of oil on the spark plug is a broken piston. The heat pressure in which a piston works can become excessive, causing the piston to crack or break down. These cracks or holes make way for oil to seep in and enter the combustion chamber and eventually smear the spark plug. If you hear rattling noise or if the engine misfires, that’s how to know you have your engine’s piston broken or damaged. Delay in replacing the piston can result in permanent damage to the vehicle.

4. Faulty piston compression rings

A damaged piston compression ring can also cause oil on the spark plug. The compression rings stay at the higher and lower part of the engine. The role of the rings is to prevent excess oil from reaching the combustion chamber via cylinder walls. When the rings get damaged and stop working accordingly, this causes the oil to make its way through the piston to the chamber before appearing on the spark plugs.

5. Blown head gasket

A blown head gasket is yet another reason for oil deposits on the plug. This issue can worsen when the owners ignore that a head gasket is blown and needs repairing. The head gasket is part of the combustion chamber. It sits at the top of the chamber, sealing the head of the cylinder. If the head gasket is blown, it will give oil access to rush and get deposited on the plug. People generally ignore this issue because they think it doesn’t make any difference. However, it does; if you keep ignoring or forgetting it, you might end up damaging your car. So, whenever you find issues with the head gasket, talk to your car’s mechanic.

How to spot oil on a spark plug

How to spot oil on a spark plug - AutopartsZ

1. Slower engine performance

If you suddenly notice that your engine has slowed down or doesn’t start at all, there might be a problem with the spark plugs. It happens because spark plugs make the engine components of your car run smoothly. So, basically, an oil leakage on the plugs can affect the performance of your vehicle.

2. Misfiring engine

Without proper combustion in the cylinder, the engine will surely misfire. As I have explained earlier, a broken piston can make the engine fail and can even cause serious damage the engine. It’s best to check your engine once in a while to ensure the plugs are all in perfect condition.

3. Odd smell

Have you noticed a strange odor of gas coming from the exhaust pipe? If you have, then that’s another sign your car’s spark plug has oil on them. The gas emerges when oil and gas mix and get released into the air through tailpipes. The smell is powerful and lingers for a longer time.

4. High fuel consumption

The spark plugs with oil on them can have reduced efficiency and can use up more fuel than required. When you notice your fuel meter shows that the fuel is frequently draining, that’s when you know that there is something wrong with your vehicle. High fuel consumption is not only related to spark plug problems but many other complications. Therefore, you should take your vehicle to the mechanic for spark plug troubleshooting.


These are the causes and signs that your vehicle’s spark plug has oil on it. If you observe one or more signs of complications, the spark plug is likely to be faulty. However, don’t just rely on guesswork. For surety, take your vehicle to car maintenance and get it checked thoroughly.