Mk11 GTi starts hot then runs cold.

[atlastaGTI]

The heater in my GTi reaches temprature in about 4 minutes which is great but when the engine reaches running temprature the heater suddenly runs cold and then stays cold untill the engine is left off for about an Hour. I have checked for loss of water or head gasket leakage into the water or water into the oil and there is nothing. It is a realy rapid cooling from hot to cold in 20 secs once the car is at running temp. Any one got any ideas an air lock would surely be there all the time???

[boschmann]

That is strange. Are you sure it isn't just diverting the air in a different direction, like from panel to defrost? I don't know of anything that could suddenly stop the water flow, usually it's an air flow issue. Most common problem (aside from a leaking heater core) is the flaps inside the air box will deteriorate, giving only luke warm to cool air. Have you notices black foam pieces coming out of the vents?