An oil catch can is used in turbo applications, or high-performance race applications where excessive blow-by (leakage past the piston rings) of air and fuel vapor occurs. This creates a positive pressure in the crankcase.
Engine manufacturers have placed a valve on the engine block which releases this pressure. This valve is known as a PCV (Positive Crankcase Ventilation) valve. During engine operation, blow-by gases, as well as oil mist from the rotating components of the engine, pass through the PCV valve and route back into the intake for the engine to burn off. However, some of the oil mist and other products settle along the engine intake and over time form a “gunk.” The oil catch can collect the oil mist and condenses the fuel vapors while allowing “cleaner” gases to be passed back into the intake.