I don't know if this could be your problem, but it's fairly easy to check.
A few years back I had an '84 Vanagon Westy, and I took it to Everybus in
N. Carolina. Trip down was fine, van ran smooth and gas mileage was OK. On
the return trip, van still ran smooth but gas mileage dropped WAY down, I'm
talking 5-6 MPG. I couldn't pass a gas station without filling up. The two
buses behind me in our caravan called me on the CB radio to tell me they
couldn't take the fumes coming from my van. When I got it home, I found I
had a massive vacuum leak, the port coming off the air box was
disconnected. A check in Bentley showed this port was supposed to be
connected to the fuel pressure regulator, but mine was tee'd off the vacuum
port to the dizzy. I reran the vacuum lines per Bentley and all was well
again. My guess is that at some point someone had done some engine work and
couldn't remember how to reconnect the lines and just faked it. The port
from the air box was probably plugged with something that fell off during
the trip home. That's just a guess, but the van had run fine up to that
point, and it ran fine after connecting the vacuum lines the correct way. I
had bought this Westy on Ebay, and I had thought it was in pretty good
condition-until I got my latest '84 Westy that I bought back in September,
which is truly unmolested. I realize now just how badly that first '84 I
had had been butchered.   Jeff

Jeff Stewart
fonman4277@earthlink.net
http://autos.groups.yahoo.com/group/vwcampersmidatlantic/