the sensor is problably bad....
BMW part # 5185718571857
actually thats not the part #......
when the engine is running the thermostat will open and close as needed and the collant will be drained out of the resevoir.... when its empty and sucking like that.... add distilled water. or better yet Redline water weter
<img src='http://www.satchmo.com/cashmoney/btymers.jpg' border='0' alt='user posted image' />
<span style='font-family:Impact'><span style='font-size:21pt;line-height:100%'><span style='color:red'> GANGSTA Tape</span></span></span>