What is blowby in an Internal Combustion engine and what can happen if it increases above normal level?

Blowby is the result of the piston rings not sealing properly against the cylinder walls any more allowing exhaust gasses to enter the crank case. This results in reduced compression which translates into reduced performance and fuel economy. It also results in more frequent oil changes as it leads to increased oil contamination. Left unchecked it can further reduce overall engine life due to contamination of the oil.

Repairing an engine with excessive blowby will generally require a complete rebuild along with machine work on the engine block.