HELP Audi A4 B6 1.9tdi losing Coolant

Hi

Wondering if anyone can point me in the right direction?

I have a 02 A4 1.9tdi, 164k miles, Recently it has started losing alot of coolant, I need to top it up every 2-3 days, The temp gauge fluctuates, mainly it stays around 70^oC but now its goes up to 90 and in the space of seconds goes down to 50/60.

I noticed today that going up hill causes it to rise rapidly but never going over the 90 mark.

The hose coming from the top of the Rad has high pressure in it and when I open expansion pack I get a hiss from the pressure being released.

Ive read a few forums from Coolant Temp sensor, Thermostat and possible Head gasket about to go.

Wondering has anyone experienced the same or anyone with experience working on this make know what the cause is?

Thanks for your time

After a journey,and also overnight,park on dry concrete and try to find if a leak exists first.
The tray would be best removed.
Coolant leak should be found easily enough if it is external.
The pressure if not exceptional may be normal.

do you get a hiss from the filler cap being removed if you remove it after the car has cooled down. is there still pressure in the top hose after it has cooled down, but before removing the filler cap. ?