Blowby occurs when an effective seal between the piston rings, pistons and cylinder walls becomes overwhelmed or otherwise fails, allowing an above normal volume of combustion gasses to leak past or "blow by" the piston rings into the crankcase, thereby pressurizing the crankcase. Although, the primary purpose of the rings is to seal the combustion chambers around the interface between the pistons and cylinder walls, some leakage past the rings is unavoidable since moving parts require relative clearance to function without seizing. Even when new, piston rings cannot provide a perfect seal between the pistons and the cylinder walls, so allow a "controlled" amount of combustion gasses to enter the crankcase. Normally, the PCV system compensates and purges combustion gasses from the crankcase. But when accumulated wear (or poor fit) become excessive, uncontrolled blowby develops that can overwhelm the PCV system.
Blowby is influenced dominantly by the degree of cylinder wall glazing, cylinder wear, piston ring clearances (fit) and wear, and abnormal rocking of the pistons. Evidence for blowby is abnormal puffing of vapors from the crankcase or valve cover breather, oil accumulation in the air cleaner, and in some cases, chronic recurring oil leaks past passive gaskets and seals that resist routine correction.
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