I have been fighting this problem to no avail for YEARS in my 96 and 97 A6's (2.8 12V). Water pumps, bleed, thermostats, heater cores, flapper doors, servo motors, bleed, bleed, bleed, etc...

After reading this thread, I too feel it is a flow problem but I have found no blockage in any of the steel tubes in the cooling system. Perhaps it is a design problem with some aftermarket water pump impellers?

I am going to put one of these up at the heater core connections and see what happens:

http://www.carpartsdiscount.com/auto...2630&3671=1002

If it works, I'll try to figure out some sort of switch to turn it on only at low RPM.

These two cars are the winter rigs and I am tired of driving in second gear to keep the windshield clear in slow traffic during snow. I can handle freezing, but like to see out the windshield My wife on the other hand, she is tired of being cold at red lights.

I hope this works, will let you all know when the weather gets below freezing.

 

I installed a dedicated water pump just for the heater and it solved the problem for me. You can purchase a complete kit from Amazon

 

Can you post a picture of how you mounted/connected that? The area seems sort of tight.

Also, do you run the pump 100% of the time the car is running or shut it off somehow at high RPM's?

 

I installed mine by the thermostat. It is a very tight fit, I had to remove a few items just to access the space. The pump runs 100% of the time that the car is up to temp and the outside temperature is under 60 degrees. I already burned up one transmission by driving in 2nd gear on cold days and all winter. This is the best alternative I could find.

 

Guys, having heat at speed or with RPM but not at idle is indicative of air in the heater core. Check the B5 A4 forum - in the last week or two there have been several posts about heat-related issues and there's a good amount of info in the threads to help you out.