Odd Volvo 2020 coolant level behaviour

[Lucky Duck]

Yesterday I noticed that the coolant level in the header tank looked rather too low, so I topped it up.

Given they way this season has been going I suspect cylinder head gasket problems so
I brought the engine up to temperature with the cap open only for the coolant to overflow the cap!

Certainly felt like most of the liquid I had put in needed to be mopped up.

Any ideas what might be causing this.

Engine seems to run OK and there is no sign of water in the oil.

 

[bigwow]

Thermostat stuck or air lock.

 

[wiggy]

Check the exhaust elbow, mine did the same and blocked elbow was forcing salt water back into the fresh system, eventually the thermostat corroded, failed shut causing overheat, seize- complete re-build and re-bore, new pistons etc very expensive (pistons are £190 each to fit re-bored cylinder).