I've noticed that my coolant level was routinely just below the cold fill level on the expansion tank but each time I would remove the cap to top it up built up pressure within the tank would release and the coolant would come back to where it was supposed to be.

I know that modern coolant systems run at a pressure above atmospheric to increase the boiling point of water but for it to keep that pressure when cold seems odd.

Anyway today it was a bit more odd as after a 200 mile run in the car over the weekend I started it up this morning to a coolant low warning so off with the engine, popped the bonnet, removed the cap and sure enough a surge of pressure released and the coolant came up. Not as far though this time and I had to put in about 500ml at a guess of some 50/50 mix I make and keep for these times.

I have done some research and typically there are three main culprits to this. Firstly the coolant cap not releasing pressure correctly so I will order a replacement to rule that out.

Secondly a pocket of air trapped somewhere in the system that compresses under system pressure, draining coolant from the tank and then when the pressure is released the air decompresses and expands pushing the coolant back to the tank.

Lastly, and what I really don't want to consider... head gaskets. It is known for combustion gases to leak past the head gasket and pressurise the cooling system without evidence of oil in the coolant, or coolant in the oil (neither of which present). A CO2 sniffer test of the coolant expansion tank would help confirm this but it could only be happening under certain variables (temperature, load...)

Am I worrying too much? Any advice? please help 2010 Range Rover Autobiography 5.0 SC
2005 Lotus Elise 111R