Coolant problem

[Carbon maniac]

So today's been a bad day! Was on route up the m5 to the national motorcycle museum doing about 95 ish :/ but all of a sudden the heater starts blowing cold air! I look at the temp gauge and it says the car is stone cold! So I slow down and pull up and as I pull up it cuts out and smoke pours out from under the bonnet
A few hours pass and we can't get it too cool down much so decide to risk driving it after a few miles I have to stop the gauge has risen to the top so I grab some water and anti freeze from the services and removed the thermostat just as I'm checking it over I find the tensioner on the aux belt is stuffed! So I jam it tight and get going again after at a steady pace of 55mph the temp stays steady and I get heat back inside the car but if you give the car any over throttle the temp shoots up! It's not burning the water and the oil is fine un mixed but it keeps boiling out the expansion tank :/ yet the cars been running at a steady 1/4 temp on the gauge what could be causing it to build the pressure/boil it?

 

[bloke]

belt slip and pump not turning as it should.

 

[Carbon maniac]

Yea I'm thinking it but even at low load or just idling it still boils the coolant with the tensioner held so it's on and the belts tight :/

 

[bashracing]

Have you opened the bleed nipple on the thermostat housing?
if thats ok possibly done the head gasket or cracked the head

 

[Carbon maniac]

Yea I've bleed it through not long put a new engine in so it was new stuff too! :/ hoping it's not a cracked head

 

[Ol’ Tarby]

Expansion bottle cap seal?

 

[bashracing]

do a leak down test when hot and cold

 

[Carbon maniac]

Um that I have no idea I just did it up tight and thought that it was a relieve valve built in to the cap?

 

[Carbon maniac]

So today after blessing out excess coolant I got the water level to hold steady but it rises if you give it any throttle but crack the bleed screw off and air has got in and the level goes back to where it was?? The cap is still passing vapor out still so I'm guessing it shouldn't be doing this?
Also if you stop the car after a short drive then start it again it also develops a air pocket at the thermostat is that the only bleed point on them? Or is there another I'm missing?