This hose connects from the oil separator to the valve/rocker cover. A common problem with modern BMWs is with the oil separator or CVV system. Over time, the oil vapours the system that 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 one of our high-quality hoses, made by Germany's Febi-Bilstein.