Quote Originally Posted by pessling View Post
sounds like the valve, you need a black olive valve, think mentioned before that even with the aircon on there is too much heat coming through as the valve is leaking by even in the close position when cold is selected
Doesn't explain why it's fine after a short stop when the engine is still warm but can't rule it out either.

Marcus, also have a look here https://www.two-sixties.co.uk/mgoc26...nostics-checks