If the engine still idles with the idle screw all the way in you have an
air (vacuum) leak somewhere.  Keep looking.  One trick that can work if
you are careful is the propane gas test.  Open a propane torch slightly
and hold it near hoses, connections, intake manifold, etc.  Be careful
not to let gas collect anywhere, be quick.  If the engine speed
increases at some point then you are near the leak.

As far as interchangability, the 2.1 throttle body has a larger engine
end opening and needs the 2.1 intake plenum too to work on the 1.9, and
the 2.1 AFM is different and not interchangeable.  I don't know anything
about the diesel.

Douglas Miller wrote:
>
> Thanks folks for the advice already coming through,
>     As I sat eating dinner, I thought of one thing that I left out that might
> be another clue to the puzzle. Then Zoltan also asked about it. When I took
> the van to a retired Volkswagen friend, he adjusted a few things. One being
> the idle speed adjusting screw.Through the archives, someone had suggested
> turning the idle screw in to cut the idle surging out/down. When I tried that
> earlier today, I found that the idle screw was already screwed in all the
> way. I can only back it out.
>     Could the idle speed adjusting screw/throttle valve housing be my
> problem?  Is a 1.9 throttle housing interchangeable with a diesel or a 2.1L ?
>  Is the AFM on a 1.9L interchangeable with a diesel or a 2.1L?
>     Thanks once again,
>     Doug