Then it seems to me that it is burning water through a bad head gasket . Now i could be wrong so the best bet is to take it to a mechanic and have the coolant system pressure tested to see if the leak can be penpointed. Also is there any coolant in the overflo tank , if so a blown head gasket could be pressurizing the cooling system and forcing it out the over flow tank if it is still full . There are so many varaiables that need to be checked , Then it can still wind up something as simple as a therostat.