Audi A3 pressure build up in petrol tank

Dear All,
I hope someone might be able to advise me - I have founf various posts relating to the Evap Emmissions systems on these systems but I don't
think these help me - they refer to flooded carbon canisters or faulty purge valves but it seems to me that the blockage is closer to the tank itself...
To describe the symptoms: Every so often there appears to be a build up of pressure in the tank which causes petrol to push out via the fuel filler cap. If I remember to occasionally open the fuel filler to release the pressure I just get a gas release and I can void the fuel flooding out. But if I leave the fuel filler undone with the engine running fuel floods out.
So it seems that something is pumping pressure into the tank (that is why fuel floods out with the filler cap undone).
If I undo the breather pipe (where it goes into the carbon canister in the engine bay) it doesn't seem to make any differentce, the pressure is not released from the petrol tank. So I do not think that it can be the carbon canister or the purge valve.
If I release the pressure in the tank and then pump air into the tank via the breather tube (again, the one in the engine bay that goes into the carbon canister) then I can build up the pressure manually. i.e. it looks like the breather pipe allows air into the tank but not back out again -as though there is a valve that has been put on the wrong way. Is there such a valve on the tank? How do I get to it?
I am at a loss - any helpful suggestions gratefully received - note: I am aware of the carbon canister and the purge valve, the pressure does not appear to be geeting that far
Tx Brightwell
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On Jun 1, 5:24 pm, "brightwell snipped-for-privacy@yahoo.co.uk"

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.
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Fuel vapour is definitely being pushed out of the tank (past the sealed cap) - if I leave it to build up further fuel itself pushes out past the cap. If I open the cap quickly I get sprayed with petrol! - I have to open it gradually to let the pressure diminish.
The noise is definitely gas/fuel coming out rather than air being sucked in
The symptoms are worse if the tank is near to empty and particularly in the hot weather
I'll see if I have one of those spill pipes and it sounds like it is worth looking at the fuel pump if I can get to it.
Thanks for the suggestions
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