Ok, the new fan clutch has cured the overheating issue
After proving this to be the case I changed the cap for a new one....
I'm still getting air in the expansion tank. Its like when the car is turned off and cools the water level doesn’t balance out and find its natural, even, level as the air in the tank is not escaping, nor is it feeding back through the little pipe in the neck of the filler (which is not blocked, see posts above).
Again there is no loss of water as removing the cap when cold allows the level to balance at the cold mark. I have checked the level daily for nearly a week now over a distance of about 500 miles.
Any one got any further thoughts?
I can only think that the aftermarket radiator doesn’t have the tube in the right place and is obscured by the cap. Either that, or the head gaskets is going (gone) and blowing a small amount of gas into the cooling system, but not allowing water into the cylinders?
