Petrol Filler Cap

[Ammy]

When I release the screwed filler cap on the petrol tank, I always get a "hissing" sound.  Not sure whether it's air coming out or going in. Comments please on whether  this is normal.   B.T.W  I have an aluminium tank. Ammy

[MattNoVAT]

Its likely that it's sucking air in when you unscrew the cap, as fuel is consumed it creates a slight vacuum inside the tank.
The hissing is it equalising.

I have seen some cars with a small hole drilled in the cap to stop vacuum building up.  Or you can check your fuel tank vent pipe is not blocked or kinked, I managed to compress my vent pipe and after driving 100 miles the vacuum drew the fuel back into the tank and the car didn't fancy starting for a while.

[droptop]

THAT'S why mine is difficult to start after a long journey, and I have the hissing!
Thanks Matt.
Problem solved without even having to ask

[Ammy]

Rather than drill the filler cap,  I decided to drill a small hole in the breather hose to the filler neck and insert a piece of "washer pipe" which then follows the overflow pipe through the bottom of the rear wing.  Same result.