It is probably the throttle position switch.  This is located beneath your
throttle valve and controls when the computer goes into idle mode.  The
easiest way to check it is to leave the van shut off, move the throttle valve
to full throttle position.  You should hear a small click.  Then slowly move
it back to idle position.  You should hear the same click again as it reaches
idle.  If not then your switch needs to be adjusted so that it clicks when it
is at idle position.  If it is clicking then you should test it via an ohm
meter for continuity when closed (idle position).  Adjust the switch per
Bentley manual.  If you need a new switch I have these for $60 and I also
have the cams for the switches (normally what wears out) for $6.

Thanks,
Ken Wilford
John 3:16
www.vanagain.com
Phone: (856)-765-1583
Fax: (856)-327-2242