I love hearing all these ideas of "What could it be?" sounds like you have made your mind what you want to do.But here are some things ever car has to have to run (and you should start here instead of guessing) compression (it's a big air pump)gotta have that timimg stuff with spark and fuel.Oh and on the subys,hondas,ect. When the headgasket is going or gone it allows combustion gasses into the cooling system,this pressure forces the coolant out and creates a air pocket which gets super heated forcing more coolant out and bang HOT! anyway if you guys ever have any car questions you can e-mail me and I will do what I can to help.I pop in here once in a while I need a new computer mine is really slow.Sumi