Bought a 94 Astra 1.7 TD for my Daughter, what a cracking motor it is. However, it keeps boiling up after about 4-5 miles driving. No heat in internal compartment from heater, at all. Expansion tank bubbling. Replaced thermostat, no joy. Took thermostat out, no joy. Flushed radiator through with a hose, no joy. I am about to replace the head gasket, just in case there is a 'leak' into the jacket. Water pump 'appears' to be circulating water. Can anyone else think of something that it may be?