ok, i'm stumped. or having severe brain pharts. :(

a friend of mine has a 90 multivan automatic.
IF he fills the gas tank, the bus refuses to accelerate from a dead
stop until like 5 or 10 seconds after he mashes the gas. and if he
shuts off the bus and waits, the gas tank makes a thunking noise a
short while later.

my diagnosis was a failed gas cap ... too much vacuum on the tank.

so we tried running without the gas cap on, and with a screwdriver
(big handle) in the gas filler neck to keep the little flapper thingie
open.

no luck. same thing.

so my next guess is the charcoal cannister and the vent hoses from the
tank.

but here's the Gotcha: it does NOT act up (no stalling and no
thunking)
once the fuel gauge gets below about 2/3rds tank (between 1/2 and
3/4).
takes off from stops just fine. shut it off and no noises from the
tank.
WITH the gas cap in place.

anyway, i'm stumped as to why the charcoal cannister, if that is the
problem,
would act that way on a full tank but not on a half tank.

anybody got any ideas??

unca joel