This dual hose connects from the oil separator to the back of the valve/rocker cover. A common problem with modern BMWs is with the oil separator (or PCV) system. Over time, the oil vapours that the system is designed to capture and recirculate can cause clogged lines and other blockages, resulting in oil leaks, reduced engine performance, and other issues. Replace yours with this quality part, manufactured by Febi-Bilstein. 

OEM Part Number: 11151743386
Alternative Part Number: 11151247743
Alternative Part Number: 11151739231
Febi Part Number: 32246