Blow-by

A small amount is normal in every engine. The crankcase ventilation system routes it back into the intake. Excessive blow-by is a sign of engine wear. Workshops can measure blow-by flow at the oil filler with a special gauge; a simple visual check looks for strong pulsing vapour from the filler opening with the engine running.

High cylinder pressure pushes gas through the tiny gaps around the rings.

Heavy blow-by causes oil mist from the breather, oil use and pressure in the crankcase. A blow-by test or compression test helps judge engine condition.