Morning all, after getting the 850 T5 to where I wanted it last year it has been quietly working away for me for the last 12 months or so without any complaint. It currently rungs a 16T, downpipe, map etc.
In the last couple of weeks however I have noted that the coolant level has become extremely unstable, when cold it can all but disappear from the tank and when driven hard it fills the tank to the brim, I suggest very close to blowing! Gentle driving does also get some expansion, but only within normal bounds. It's using a small amount but nothing excessive, it seems more like the system is being pressurised based on how hard the car is driven.
If the system is pressurised to the brim of the tank and I switch the engine off it immediately begins to drain with a gurgling sound and the tank returns to normal levels, this will also happen if I crack the tank cap when the car is hot.
Last year I replaced the water pump and thermostat as part of a overall service and of course replaced the coolant. I did the coolant again on Friday paying extra attention to bleeding and ensuring the thermostat was opening correctly (which it was).
I guess at this point I am looking at either a failing HG or even worse a split liner. Can anyone suggest some diagnostic methods to determine this either way?