Pressure in fuel tank with engine running

[MapisM]

I can't see into the tank because the hose is bent in two dimensions (but not kinked)

Makes me wonder what the builder was smoking, to dream of a filler pipe so short and at the same time so complicated.
Or did you possibly mean three feet rather than inches?

 

[rotrax]

No, you won't, not even if the tank is plastic. Fuel is sucked from the tank by the low pressure lift pump, it isn't capable of collapsing a tank.

I have changed two car tanks, both collapsed due to incorrect filler caps fitted. Aftermarket locking filler caps that completely blocked the inlet, stopping atmospheric pressure into the tank.

It is not the suction inside the tank, but atmospheric pressure outside that collapses the tanks. Allow atmospheric pressure inside the tank, you get a balance. Without it you easilly get a collapse.

Both vehickes - Fiats - were running fine. Both came in for severe rattles from the rear.

The tanks were rattling about as the straps could not retain their new distorted shape...........................

From direct experience.

Car fuel tanks are often plastic now and are moulded with ribs for added strength. Plus fuel systems are totally different to the old carburettor jobs.