my vw golf mrk 4 runs fine on roads but on the motorway after about 1 hour it loses all its water and over heats can anyone tell me what the problem maybe?

sam, August 2008

Feel the radiator all over to verify it's the same temp.A blocked or clogged rad will operate in town ,but be unable to cope with high demand.Same could be said for partially inopp thermostat.In order.Have cooling system pressure tested.change the stat and flush the system.check rad temp variations.After that if same,it's check coolant pump,then head gasket.

caine, August 2008