A8 4.2 overheating when...
 

...I turn inside temperature under 26c (outside is +8c), then enigne temperature rises over 90c, when I turn heater to 26c temperature is going back to 90c in one minute or so, and everything is ok.I understand, when the heater is less then 26c then the coolant dose not go thru the radiator...
I checked coolant level, it's ok and warm air comes to from the heater...
What can be the issue?
I allso checked all the sensor and fans, they seems to work and allso the PDC computer dose not show anything...

Plase help! :)

Car is Audi A8 4.2 petrol 2003

Lack of coolant circulation in the radiator it sounds like. Either thermostat is stuck closed or the radiator is plugged. There is also a condition where if the timing belt is not tensioned correctly the water pump will slip and not cool the engine enough, you having the heater on may be giving it that little bit of extra cooling it needs.