I believe auto shutoff fuel nozzles work by a slight suction created by a
Venturi that the fuel jets through inside the valve assembly. On the lower
surface of the fuel nozzle near the tip there is a small port connected to
a tube that runs inside the nozzle up to the valve assembly. Air is sucked
in through that port to relieve the vacuum until fuel or foam reaches it.
Then the unrelieved vacuum moves a diaphragm inside the nozzle and trips
the fuel cutoff.

It's a mystery to me why two different nozzles would fail to cut off, but
it may be because the nozzle was not inserted deeply enough into the filler
opening. It's unlikely but possible that both nozzles were defective. I
think you need a larger sample than two fill-ups to determine if there is a
real problem.

Nick
'82 Westy, '88 Jetta, VW camper owner since '67
Mendocino CA

At 02:58 PM 5/3/00 -0700, Stuart MacMillan wrote:
>Fuel nozzles cleverly detect the sudden rush of air up the filler pipe
>that happens as gas gets up to the top of the tank and shut themselves
>off.  Really slow filling can fool this system, as can having the nozzle
>too far out of the filler neck, but I don't think an air leak in the
>tank would.  My '84 does not pressurize the tank, I don't think any
>Vanagon does.
>
>The only way to fix a leak would be to drop the tank and check the hoses
>and tank condition.