Could it be that at idle you are not producing as much vacuum as when driving, and that this drop in vacuum, is causing the recirc flaps to switch to the outside air, instead of cooling off cabin air?

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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D