Hello!

About a month ago i was passing some traffic on highway and suddenly hear a beep, check the display and it says the engine is too hot, i stop asap and sure enough its spilling coolant out of the overflow pipe from the pressuretank. I fill her up with water and no more problems.
So week before the last week i got the coolant changed. And day before yesterday it suddenly overheated again, this time i have coolant with me so i fill the tank and its ok untill this morning when it did it again
And when i took off the pressuretank cap, the hose that lets coolant to it was kind of spittin the coolant, like it was mixed with air (it made growly sounds too).

The overheating is very very sudden, like in few seconds it goes from dead center to just before the red, or atleast it seems like that to me, i keep my eye on the meters constatly.

Common sense tells that air in the system causes the problem and i will bleed it but can there be anything else, like faulty thermostat or maybe it sucks in air from somewhere? I just want to be sure that it wont happen again, head/gasket are just way too costly to experiment with.

Coolant does not seem to have oil in it, the oil does not seem to have coolant in it, she wont smoke with when its warm and dry outside and the large radiator hose has pressure in it put you can still squeeze it, when the engine is warm and running.

PS When i bleed the system can the pressuretank cap be open when engine is running so i can fill it/check coolant level on the fly?

Thanks for reading!

Toomas