I have 2 1997 A6 Quattros and both of them suffer from a slow to heat engine and consequently poor heat in the interior. One of the beasts got a t-stat last year and still about the same result, a slow to heat engine, it does eventually get to the "thicker" mark on the gauge which suppose represents normal, however it takes 20-25 minutes at 30 degrees (outside).longer if it is colder, and still the hvac air feels like i've got a heat pump- luke warm. i was hoping to find someone else that may suggest an answer to this vexing problem.

 

I have noticed my heat not getting the car nice and hot like i like it for a little longer than expected. Especialy the foot area. dont know why though.... I dont however have the 20-25 minute problem your seeing.

 

possible a failing heater core.

 

same problem in my car (97 a6 quattro) replaced the heater core and water pump now blows very hot the other a6 seems to have this problem with the heater core if you put the wrong antifreeze in them it eats the heater cores away