Date:         Wed, 13 Jun 2007 12:02:12 -0500
Reply-To:     Matt Roberds <mattroberds@COX.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Matt Roberds <mattroberds@COX.NET>
Subject:      Re: gas tank question - full tank woes
Comments: cc: joel walker <jwalker17@EARTHLINK.NET>
In-Reply-To:  <20070613010346.AJW1280.fed1rmmtai103.cox.net@fed1rmimpi01.cox.net>
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed

> From:    joel walker <jwalker17@EARTHLINK.NET>
> Date:    Tue, 12 Jun 2007 16:34:59 -0500
>
> 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.

Here's my guess.

In theory, the normal vent connection to the tank should be at the
highest point of the tank.  There might be another connection (to the
charcoal canister, or whatever - something that eventually leads to
the outside) that's at not-quite-the-highest-point.  If the normal vent
is plugged up, such that the other connection is acting as the vent,
you could get the behavior you describe.  With a full tank, the other
connection is submerged, but after you burn a little fuel, it connects
with the air space in the tank and can act as a vent again.

Outside of that, I've had the fuel tanks out of various (non-VW, fuel
injected, mid-80s to mid-90s) cars, and a common theme is little check
valves or filters or other gadgets in the charcoal canister lines -
some of them there to make sure liquid fuel doesn't get sucked into the
canister; some of them there (I think) to make sure that liquid fuel
doesn't come out the vent lines in the event of a rollover.  If one of
these things is acting up, it might cause the trouble you're having -
you might replace a suspect gadget with a straight piece of hose, or
even leave it off altogether, for a short test drive to see if that
improves the situation.

Matt Roberds