Mk4 Golf GTI 1.8T 20V

I’ve refently bought a 1.8T Golf and the water keeps boiling over but the engine temp won’t go over a touch over 90 degrees, car can be sat there for ages on tick over and it will not go over 90 degrees but when you turn the engine off you can hear the water boiling over and pressure through the system from the expansion tank. But undoing the watter bottle to release the pressure and the water will spit out too. Water pump has just been changed on it too. Just wondering whether they run with a presurized water system or is this a posssibe head gasket job? There’s no milky gunk in the water bottle and no **** in the oil. Hence why I don’t think it’s the head gasket. No oil leaks anywhere either

It sounds like no circulation even though the pump is new. Ignore what the dash gauge says, it is completely irrelevant to what is going on except that at one point it will be right, a bit like a broken clock is right twice a day. The only way to know what is going on is to be able to see what the temp sender is saying using a diagnostic system with live data viewing and even then that could be wrong as the sensors are a common failure. The thermostats on these tend to stick open, the one on my Octavia VRS is stuck open but I have not had the time or energy to change it. Where did this water pump come from and do you have any paperwork or the box? Do the fans come on? If not, do they come on when you put the AC on.