overheating

i have e pretty hot 340 in a duster i am using a radiator from a ford scorpio (dont kill me for that one) and right now it is overheating alitle from time to time and i noticed that my electric fan doesnt start after som thinking i checked the sensor and thats not the strange part of the problem because the radiator only gets warm about halfway up and the sensor for my fan is at the top of the radiator but how is it possible that i am loosing water from my expansion tank that is over the top level of the radiator when the radiator is nowhere neare full?
any help apreciated
if my description of the problem needs some more information please let me know:)