I have had this on my 1.8 turbo but i swopped out the expansion tank cap and the hiss ceased - so maybe a fault in your cap Basically a cold coolant system should not have any pressure in it :shrug:

There are gaskets and there are gaskets. Get the latest (black) Viton one. If necessary go to Rimmers.
Rimmer: https://rimmerbros.com/Item--i-LKJ101110


Read up about slackening and tightening the plastic manifold. A new gasket won't seal a warped joint.


TC