Where Does Blow by Come from?

Typically, blow-by is cylinder pressure leaking past the piston rings into the crankcase. Commonly, it starts as cylinder glazing or carbon forming in the top ring grooves, both of which allow cylinder leak down, and some oil burning. Oil burning produces further ring deposits, and more blow-by.

What causes Blowby?

During combustion, high pressure on the top side of the piston pushes combustion gasses, as well as droplets of oil and fuel, past the piston rings and into the crankcase. This mixture is known as “blow-by.”

What causes blow-by in a diesel?

"Blow-by" is a fairly common term across all types of engines—diesel, gas, etc. For diesels, it's when compressed air and fuel in the cylinder bore is greater than pressure in the oil pan, and gas leaks past piston rings and down into the crankcase.