On Jun 1, 5:24 pm, "brightwell_...@yahoo.co.uk"
<brightwell_...@yahoo.co.uk> wrote:

A6 1996 petrol. In the Hynes manual there is a diagram of sorts.  At
the filler pipe, where the nozzle is put in, there is an overflow
pipe, it seems to be there to collect spilled petrol at the filler and
take it down to the tank.  In the filler pipe there is an insert which
seems to be a restriction of some sort.  Rather like an S bend or Z on
its side.
The electric pump is in the fuel tank, the fuel is pumped to the
injectors and excess returned to the tank.  On the pump is a breather
pipe which probably goes to the evaporative control ( which is the
thing with the carbon filter in) fuel vapour is, when the engine is
warm, periodically allowed into the air intake to the engine. ( the
vapour is sucked into the air intake).
It is unlikely that the tank is pressurized via the evaporative
control.  However, an inlet valve that is not seated properly could
cause some back pressure, the evaporative control would need to be
open to pressurize the tank.
If the evaporative control valve was open all the time then the fuel
tank could have a vacum.

When the filler cap is removed air could be sucked in.  The sound is
rather like air coming out.  Phfumb.
Mine sometimes makes the phfumb noise.

You could check this using a piece of tissue and see which way the
tissue is moved.

Does the fuel gauge work properly?  If you are putting in too much
fuel there may not be enough room left for expansion.  Do you live in
a hot climate, is the car left in a warm or hot place?

Generally as the fuel is used up from the tank and the evap control
opened periodically there could be a vacum in the fuel tank.  It is
unlikely that the tank becomes pressurised.

Can you see if the fuel comes up from the big filler pipe or up from
the small spill pipe?
There could be a partial blockage in iether of the two pipes which
allows petrol through but holds some back and when the vacum is
released the retained fuel comes out.  Rebounds.

Many years ago I had a J4 van which worked for various periods of time
from a few minutes to weeks I never knew for how long the engine could
keep going.  After several months of changing things the van once
again stopped the only thing I had not looked at was the fuel line
from the tank to the lift pump.  In this line was a union, when I
opened the union no fuel came out.  When I poked a small screw driver
inside fuel came out.  Eventually I was able to see that there was a
piece of shell just the right size to act as a butterfly valve where
the pipe was nipped in by the union olive.