In your first post you said the coolant is bubbling out the top of the overflow bottle - is this with the lid on or off. My L series bubbles away quite happily but never out the top but has no issues. Also the L series will actually cool the engine quite well even without it being pressurised - accidentally left the cap off for a couple of weeks - never heated - didn't loose coolant. The other thing is the top of the overflow bottles craz and crack - doesn't seem to affect the system at all - new bottles will craz within a few weeks and as a result I have given up replacing mine.
If yours is bubbling out the filler then it can only be one of three things.
Air entering the system (check all connections etc), the system being pressurised (I agree that it is unlikely that the head gasket has gone - always a first though) (is the thermostat sticking allowing coolant next to the cylinders to get hot - when it opens steam exits the bottle - I do not think so), lastly is there a blockage in the coolant return pipe from the overflow bottle.
I agree with the others that it is unlikely that the headgasket is blown and the issue is most likely an overall cooling system issue - get it cleaned out and maybe a new thermostat.
If there is a leak or blown headgasket the chemiweld is most likely going ot hide that for a little while - I have a leaky heater matrix that I am too lazy to fix and chemiweld works well in fixing it for about a year and it will plug small head gasket issues for a little while.
Is the problem still there after you have used chemiweld?
Oh the obvious just occurred to me - have you replaced the overflow bottle cap - is it simply not holding pressure - doesn't seem to matter on mine though but may on yours.
Garry
REMLR 243
2007 Range Rover Sport TDV6
1977 FC 101
1976 Jaguar XJ12C
1973 Haflinger AP700
1971 Jaguar V12 E-Type Series 3 Roadster
1957 Series 1 88"
1957 Series 1 88" Station Wagon
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