Hi Debi,

I'm surprised that no one else has mentioned this...

'84 right?  Probably has a microswitch or two where the throttle cable
attaches to the throttle body.  Make sure that the wires are hooked up.
A faulty "full throttle" switch (the one that activates when the
throttle is wide open.) will do just as you have described.  Sudden!
drop of mileage.  Ask me how I know.  BTW, if the switch has failed,
find a boneyard one.  New they're about $50.  Used... about a buck.

Jim Arnott
List Admin
WetWesties
Spare Vanagon
Union, OR (mountains too)