Since your mechanical fan, and your electrical fans are working, I would suspect the cap itself. The spring could be worn out, but its probably the rubber seal that is getting old.
They are not expensive.

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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