This pipe connects to the bottom of the oil separator valve. An unfortunate and ongoing issue with modern BMWs is with the oil separator system, or crankcase ventilation 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. These plastic pipes are also notorious for cracking. Replace yours with this brand-new pipe from Febi-Bilstein.