Sudden failure of the heater control valve cost me a head-gasket job once in a 745 turbo. The engine then quickly overheated, and cold tap water was put into the cooling system to limp home (won't make that mistake again). In that case, the head gasket failed into the combustion chamber and the car began emitting a nice hue of white smoke. Car had about 150K miles when HCV blew. Since then, changing that failure-prone valve is a first order of business after purchasing a wagon with the red-block engine. Chris in VT, '94 945, 153K.