zeaphod
Well-Known Member
"cracked the filler tank lid but there seems to be a lot of back pressure there and the water seems to 'chug' out of there" - lots of pressure in the expansion tank and the fact it happened at high revs (motorway) makes me think head gasket. Run it from cold with the expansion tank lid off, have a look in the tank when someone revs the engine - look for bubbles. Bubbles = head gasket.