From: John De Armond
Subject: Re: Gas tank fills to only 3/4
Date: Tue, 18 Jan 2000 14:23:47 EST
Newsgroups: rec.outdoors.rv-travel

Bryan Camus wrote:
> 
> Hey MW,
> We have a '99 Winnebago Brave with the same problem.  In fact, at one
> station I spent 5 minutes and only was able to put in $2 worth of gas
> because the pump kept shutting off.  We talked to the service manager at the
> local Ford dealership and he pointed out the way the tank is fed from the
> fill tube, i.e. it is almost a horizontal feed.  My best work-around has
> been to use travel plazas to refuel instead of smaller stations.  It could
> just be me but it seems that those hoses are less sensitize to the
> backpressure caused by filling and I'm able to spend a lot more money on
> every refill <G>.


I have the same problem with my Itasca and for the same reason. 
There is a solution.  First, let me explain how a fuel nozzle
works.  If you look in the end of the nozzle, you'll see a block or
a tube or something similar.  If you look on the bottom side of the
nozzle underneath the block, you'll see a small hole.  Inside the
nozzle, there is a restriction, typically a venturi, that generates
a vacuum when the gasoline is flowing.  This vacuum is vented
through the hole and is applied to one side of a diaphragm inside
the nozzle.  The other side of the diaphragm is referenced to the
gas pressure in the nozzle.  When gas is flowing normally, a vacuum
is produced by the gas flow but is vented through the hole.  When
liquid gas reaches the hole, such as when the tank is full, the gas,
being more viscous than air, partially blocks the hole.  The vacuum
increases, the diaphragm moves and the valve is tripped shut.  Very
elegant little design, no?

The problem arises when you have to stick the nozzle through that
little government-mandated plate inside the filler neck AND when gas
puddles in the horizontal filler pipe.  As air bubbles back through
the trapped gas, it brings bubbles and droplets up toward the vent. 
When a drop large enough to block the nozzle vent hole hits the hole
(or dribbles down from above), the nozzle trips even though there is
no actual liquid gas present.  A related problem is that as the air
bubbles back up, it is blocked from exiting the filler by that plate
and so the pressure fluctuates.  This affects the diaphragm also,
making it more likely to trip.

Now the solution. It varies according to whether your vehicle has to
be inspected for emissions.  If it doesn't, the solution is easy -
simply remove the barrier.  Since leaded fuel is effectively
unavailable in the US, the plate serves no useful function (It used
to block the entry of a leaded fuel nozzle) other than to help a
bureaucrat keep his job.  Removing this plate will let the vented
air out more quickly and will not trap gas droplets up around the
nozzle tip.  This is the solution I used.

If your vehicle has to be emission-inspected, the solution is more
complicated since that plate is something they look for.  In
general, one wants to vent the tank as effectively as possible and
make it more difficult for gas droplets to coalesce on the nozzle to
form drips.  The most direct way is to cut the plate out and then
insert one of the replacement plates before each inspection.  They
are available inexpensively from auto parts plants.  If the filler
neck can be easily removed, a neck with the plate intact could be
substituted in for each inspection and then swapped out again.  A
very enterprising individual could tee into the filler pipe and run
a vent it to the outside through a suitable vent or valve.

For either solution, the second part is to insert the nozzle the
least possible amount.  If possible, insert it so that the vent hole
is still on the outside of the plate (if it remains) or near the air
(if not.)

Using these techniques, I can usually fill my tank completely with
only a couple of tries.

John