Chrispb The coolant does come out by the cap and there is still pressure when cold.not tried running the engine with the cap off. Heater doesn't go cold. No water in oil, no oil in water (just some rust where we've had no antifreeze in it), no white smoke. It's been pressure tested which was fine and a mechanic I know said he doesn't think it is the head gasket. He had it idling for 3 hours and it didn't overheat once or loose water. We traveled 120 or so miles on Monday at a steady speed 65/70 mph. No overheating or coolant loss. However on the way home Huby decided to do 80mph and almost instantly the car overheated and when we pulled over you could see where coolant had come out by the cap and when we removed the cap loads bubbled out. I don't believe the thermostat was changed and I have brought 1 to put in the car. I am really hoping it's not something as drastic as the head but is there a definitive way to check?