Radium Engineering valve for PCV (positive crankcase ventilation) systems. Do not mistake this for a simple one-way check valve. The PCV valve's internal plunger dynamically meters the appropriate amount of crankcase vent airflow based on current engine load. Proper PCV valves prevent the "vacuum leak" that a check valve would create at low loads. Also, in the event of a backfire, the PCV valve prevents combustible vapors from igniting in the crankcase.