temp gauge problem

i think my luck has ran out as it appears to be presurizing the coolant and has air in there but no signs of water or oil mixing but the coolant in expansion tank goes from mini to above maxi if i had a split pipe on back of the head /plenum it would let air in and water would piss out when hot logiclly???



Theoretically it would let water out yes if it's a coolant line.

Sounds like you have air from one of the combustion chambers lettering air into th coolant circuit I.E. through head gasket failure or a crack in the Head/block?

Next thing to do is a sniffer test on the coolant to confirm.