This hose connects to the side of the upper radiator hose and runs back alongside the engine. Over time, rubber hoses can become hard, with the internal walls covered in sediment and the plastic fittings turning brittle or the internal o-rings simply leaking. A must replace hose for any X3 owner 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.