should the petrol tank be pressurised?

should the petrol tank on a 1995 850 t5 be pressurised? ie. should there be a hiss when unscrewing fuel filler cap?

petrol tank pressure 

do you know what would happen if there wasnt a hiss from the petrol cap when unscrewed? as mine doesnt seem to be hissing after a run

as it's cold - there's no expansion - so no hiss!!

hi, i thought it should do neither cos the breather hole in the cap allows air in to replace fuel used and the evap system releases fumes into the inlet manifold. wayne

petrol tank pressure 

interesting. i saw a pipe at the top of the pump, but not sure if its a breather. if it was a breather, would the fumes not go in the car. also, how does the fuel cap hiss when unscrewing? it says in the haynes manual to renew the pump gasket seal at the top of the tank pump rim. if that hole was a breather, wouldnt the petrol pour out when the tank is full? any more comments and veiws on this topic please?

i presume if you have a +pressure in the tank the evap system would not be functioning correctly and if there was a -pressure the filler cap breather is blocked ??? would a vacuum not overwork the fuel pump ? hmmm, im interested now. wayne

On our racing sidecar the tank is not pressurised, the fuel pump sucks fuel out of the tank and therfore if the tank was pressurised (i.e no breather hole) then the tank would cave in under high fuel pressure (suction). Fuel tanks need to be able to breathe in case of this. Also in case of expansion especially in summer when the tank get s hot and some fuel inside the tank turns into gas. If the tank was pressurised it would be quite dangerous. However i believe the tanks do build up a certain amount of pressure but do breathe too, as i have experienced a small hiss on all the cars i have had especially during summer.

It is a natural thing to obtain a bit of pressure in the fuel tank. The hiss should only occur when the tank is filled and driven to low. This is natural and a waste of time worrying about it, unless you own an older jag with the twin filler/saddlebag tank.