If this turns out to be the case, it could be
a bad radiator cap which is not containing the pressure as it should.
if you have
a bad radiator cap (One that doesn't seal correctly) and you are at high elevation coolant will totally boil away at lower than normal operating temperatures.
Not exact matches
It could be a few things: Low coolant Failing water pump (or slipping belt) Sticking thermostat Obstructed passages in the
radiator Cooling system not holding pressure (rad
cap not sealing, cracked rad, leak somewhere,
bad head gasket)
The water pump recently went
bad, so I'm not sure how much oil was in the
radiator because all the coolant leaked out; I just found some on the inside of the
cap.