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Blowby is combustion gases that leak past the piston rings into the crankcase during normal engine operation. These gases are composed of water vapor and unburned fuel and are removed by the Positive Crankcase Ventilation (PCV) system to prevent the buildup of harmful acids in the crankcase. Blowby can turn into harmful crankcase sludge if the PCV system is not maintained and the engine oil is changed on a regular basis.