This hose connects from the bottom of the expansion tank and runs along the front of the vehicle before going back to the hard plastic water pipe underneath the intake manifold. Over time, rubber hoses can become hard, and the plastic fittings turning brittle or the internal o-rings simply leaking. A must replace hose for any owner of an E60 5-Series looking to preventively maintain or restore their cooling system. We've found these Rein hoses to be the best quality, which is manufactured by the original equipment supplier and use BMW Genuine plastic hose connectors.