I had the same problem on an Audi a3 2.0 tdi, as soon as I opened the hot air (without operating the climate, the water temperature of the engine is lowered, while now on g.p. 1.3 mjet if I open the hot air when the temperature of the liquid ? on 30?, to get the temperature to 90? it takes a nice p?..if I have the fan turned off ? much more? fast, if you instead want to turn the internal recycling (which in practice it does not get air from the outside) puts us even less! Do you think it may be the tap water? Why? what if I'm not mistaken, is the water hot (or cold) from the radiator inside. If it doesn't work well and makes cold water when the radiator is in need of hot water and then the temperature is lowered as it passes the cold water in the circuit. If you know any autodemolitore try to get a used one in order to understand if ? that or not...