Hi Jimmy,

I had the same issue in my wifes AUII Falcon. No cracked head, no leaking hoses. (Replaced them all anyway) new radiator, no water/coolant in the oil. Baffled me for months.

Then a mate of mine (who is a mechanic) said "Replace the radiator cap" - It couldnt be that simple....?!?!

I replaced the radiator cap and bingo, no more disapearing act for the coolant!!

Basically (as I understand it) is that preasure builds up in the system and any excess coolant flows into the overflow bottle. When the system cools it creats a vacum and sucks the coolant back into the radiator. If you have a doggy seal on the radiator cap, air gets in and it doesnt suck the coolant back.