1. Worn or damaged valve stem seals: When valve stem seals become worn or damaged, they can allow oil to leak into the combustion chamber, where it can accumulate on top of the spark plugs.
2. Worn or damaged piston rings: Piston rings help to seal the combustion chamber and prevent oil from entering. When piston rings become worn or damaged, they can allow oil to leak into the combustion chamber, resulting in oil on top of the spark plugs.
3. Excessive blow-by: Blow-by refers to the leakage of combustion gases past the piston rings. When excessive blow-by occurs, it can force oil from the crankcase into the combustion chamber, leading to oil on top of the spark plugs.
4. Overfilling the oil: Overfilling the oil can cause the oil level to rise and come into contact with the spark plugs. This can result in oil being deposited on top of the spark plugs.
5. Clogged PCV valve: The positive crankcase ventilation (PCV) valve helps to control the flow of blow-by gases back into the intake manifold. When the PCV valve becomes clogged, it can cause excessive blow-by, resulting in oil on top of the spark plugs.
