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Yes that's of course another fault - a lose hose. The expansion bottle cap has 2 functions, and therefore sections - 1) to keep it pressurised to a set level then release the "over pressure" value, which should just be air if the bottle is not over filled. Hence the value marked on it, possibly say 1.1 bar. 2) then allow air back into the bottle as the system cools and the pressure drops below its' set value, and as this happens the coolant level drops back into the system. So if the bottle cap isn't controlling the pressure at all properly, the water will expand when heated and piddle out of the top of the bottle - like my Galaxy just did. The expansion of water from 20 deg to 80 deg is 10%, a fair bit, so the pressure goes up and the air in the bottle is at pressure. On a house, "sealed" central heating systems, no tanks in the loft, as in combi boilers etc, we have expansion vessels, either usually in the boiler, or if they fail and can't be sensibly replaced (as on older boilers) an external red vessel connected somewhere on the heating pipework. A radiator cap as on my 3.3 GV performs a similar function, but the over pressure relief is coolant into the expansion bottle, then the smaller metal disc in the cap pulls down when the pressure drops to allow coolant back into the rad.

When mine was boiling, it pushed out the lower radiator hose, so assuming is boiling, there is still a loose connection where the water is escaping from. Mind that there is pressure in the system when the driving temperature is reached. In the expansion bottle cap fault would be that it does not allow the water to be sucked back in the system while is cooling, my opinion at least.

Hi Argee, I've recently had a similar experience on our 1.8 diesel Galaxy, at this point I've swapped the expansion bottle cap for a new one and it seems to be hopefully behaving itself. Because if the cap fails the coolant isn't pressurised so can boil up in the first instance - like yours and mine getting very hot - then will leak out, but we would be keeping a close eye on the temp hey!! Not sure what side the bottle is on yours?

Hi, I assume rear right is car's point of view and not by looking at it( just to clarify is not the gearbox side). Behind there is the original thermostat and oil cooler which has water hoses as well. Slightly behind is the water pump too, more to the side but I guess a leak would be pushed behind the engine. Plus, by topping only a litre every once in a while means it looses only when the engine is on otherwise would loose more and you will see it on the floor where the car is parked. Any of those connections could be loose. Leaving the car running on idle, you could spot it by removing the wiper blades, wiper motor cover and wiper motor enclosure and you can see exactly if is from there. Hope it helps.

Thank you @@bignev and @@mikebh8 it just means she gets to boss me around officially now! ;-)