Couldn't that condition be caused by a worn out throttle plate, leaking air
around it?
If the van is very old or has lots of miles, it might be worth taking the
throttle housing off
to examine.  Don't know however if one can open it up easily or do any
reconstructive work.
But isolating the problem would help.  I too would start by checking leaks
in hoses and
air plenum.

> 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.