if it's not caused by what's mentioned, it could be plugged from mineral deposits. i've had two dubs that overheated due to it. a lubro moly flush with a refill of distilled water and g12 solved the problem on one. the other had to have the radiator replaced from buildup. from experience, waterwetter always left a chalky film. i think it's mainly meant to be used with straight water with religious intervals.