Hack Mechanic: I own and drive a BMW 525i, a 2005 model, E51. I am having a recurrent problem of the coolant in the tank drying up. I work rotational assignment in the offshore oil industry and so I am away from home for 28 days, every other month. No one drives the car while I am at work for 28 days and when I return home, I notice each time I start the car upon my return, the coolant in the tank has dried up completely. There is no sign of a leakage. I don't know what may be causing this problem. Can you help?
I would suggest getting a coolant pressure test to make sure the system has no leaks it could be a very small leak over time turns into a big one.
Check the lower radiator hose. The lower right side of the radiator has a sensor that sits in the plastic hose connection. I had a similar problem and found the seal leaking at the sensor. I used a fuel injector "O" ring and it solved the leak. So I went and looked at My E90, turns out it too was showing signs of leakage at this location.
