This hose connects from the oil separator to the oil return on the engine - feeding back into the sump. An unfortunate and ongoing issue 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 or reduced engine performance. We offer this replacement hose from Rein, an OE quality replacement.