Tuned 850 T5 - Split liner or HG?

[LeeT5]

You won't split a liner with an 81mm block with standard rods, as already said it's most likely the head gasket. Get the head skimmed and also change the coolant cap.

Make sure you've got the right coolant in her. Head gasket symptoms can be caused by the wrong coolant. Also as stated already try changing the coolant cap. Before doing the head obviously. Hope it's not the head mate

What they both said! If the cap is leaking (ie, not holding pressure) then the system cannot pressurise, so the coolant will just expand with heat and then fill the expansion tank too much and leak out. Then when its cold, it will almost disappear because your losing a slight amount of fluid each time it gets very hot.
The split tank will cause identical symptoms to a leaking cap, so change them both!!!

If, after replacing the tank and cap, your still losing fluid and the level rises too much (ie, same symptoms, then it's most likely your HG leaking)

You will not be able to perform a Cooling system pressure test or Sniffer test properly until you've replaced the split header tank, so change these first.

Thanks for the comment. I have done some further investigation and found that there is a tiny split in the tank so I'm going to replace that and the cap at the same time. With regard to coolant I wish I could find a definitive answer on what type the car takes, even the Volvo dealer couldn't tell me - I have currently used the blue silicate type from Halfrauds?

The Volvo fluid is 'Pentosin' and is a blue colour. i'd advise you use the correct fluid as using incorrect fluid can have a detrimental affect on the engines internals long term and more importanatly, it can affect running temps.

Just buy it from Volvo! You don't need much and it's not that expensive.