I have a 97 Outback that only overheats in cold weather. It has myself and other more knowledgable people quite flummoxed. I bought the car in early 2010 and it would intermitently overheat at idle. Then the warm weather arrived and nothing. This fall it started again much worse ( the overflow filled and leaked ) with the advent of cold weather. It was then I noticed exhaust fumes in the overflow. A friends mechanic suggested I try the Blue Devil treatment which I did and it seemed to clear up the bubbling in the (radiator which is new.) Sealed the head gasket issue. A new and tested thermostat also. After the treatment still problems. He suggested a purge. Parked the car on steep incline and ran the car a little while with the cap off. Helped for a little while and then came back. The weird part is while the temp gauge is way up the heat doesn't work. Is there a switch /sensor that shuts the heater? Would this make for bad circulation? One hose to the heater gets super hot while the other stays normal. Any help appreciated.