Cooling System Wierdness

[David Giannandrea]

It could be a valve or the vacuum that operates the valve. Audi's air
control valves are designed to fail in the defrost position. You can test
the vacuum theory by inducing vacuum with a throttle off test. Make sure you
have enough revs so the engine is sucking against the closed throttle. Going
downhill in a lower gear works. If this 'cures' the problem, it's a vacuum
leak.

David G.