Connecting from the thermostat to the bottom of the radiator. 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 X5 owner looking to preventive maintain or restore their cooling system. We've found these Rein hoses to be the best quality, which are manufactured by the original equipment supplier and use BMW Genuine plastic hose connectors.