pressure in fuel tank???

[dozzum]

Whenever i open the fuel tank there is a very audible release of vacume in the tank? I thought that the breather must be clogged but hadnt got round to looking at it. Just seen on the for sale ad for the black Scala that it is supposed to do this???
Is this true and if so why?

[stu.gtx]

As far as I'm aware the breather hoses should prevent the tank from getting pressurised.

[GT Matt]

When removing the filler cap, there may be a very small "hiss" of pressure (expanding vapour) but fuel tanks should not be pressurised.

The vent is essential in allowing air back into the tank as the fuel level drops. Technically the tank is always "full". It's just what it is full of is the question (100% fuel - 50%fuel:50%air - 100% air (empty)).

If air is not allowed back into the tank to replace the used fuel, a depression is created in the tank. This then allows the atmospheric pressure of the air outside the tank to be higher, which in turn acts on the tank and starts to crush it or implode.

If you ar getting excessive pressure when removing your filler cap, you really need to check the vent pipe out (but it's easier said than done i'm afraid!). Look for it having been crushed somewhere (normal case with breathers) or blocked by something.

[p in phcas]

Same here and has done it for as long as I can remember. I haven't quite worked out if it's a suck or a blow though