Taking the hiss

[5.0LTR]

On the occasional refuel these cars seem to need and on opening the fuel cap there is sometimes lots of air hissing out, less gas in tank seems to = more vapour. Is this normal? 99 Griff 500. Ta Bob.

[BliarOut]

Blocked breather? I would have thought a pressurised tank could lead to fuel starvation.

Actually, I know as I spent two weeks tracking down an intermittent fault on my old Triumph 650. Turned out to be wax in the breather and every time I stopped to investigate the vacuum had time to disperse

[5.0LTR]

Sorry perhaps I should have mentioned, the car runs fine, its just this pressure in the tank that concerned me.

[GJR]

Bob,

This used to happen with mine but never so bad that I thought it might be a problem, in fact I just assumed that it was normal. But, I'm sure that the hiss was due to a vacuum in the fuel tank causing air to rush in, rather than increased pressure in the tank due to gases.

I don't know much about fuel tanks and therefore don't know whether the pressure in the tank should normally be at normal air pressure or +/- air pressure?? Maybe someone can answer that for us?

There is a breather tube but is that un-restricted or is it attached to a pressure valve? It seems logical though that as fuel is used (assuming air is not replacing the fuel) a vacuum will be created.

???

G

[Griff4.0]

I had the same problem on mine only the hissing was the tank taking in air as a vacuum gradually formed as fuel was used. The formation of the vacuum didn't affect the smooth running of the car so at the time I wasn't too concerned.

I had to remove the fuel tank recently as part of a body off restoration and the cause of the "hissing" was immediately apparent. The roll-over valve, which seems do double up as a breather, was corroded almost beyond recognition. Replacement of the part has got rid of the "hissing" plus the odeur de petrol - a rather unpleasant fragrance care of Sh(an)ell!

[Argent]

The Bible says that +tive or -tive pressure is normal in a 500.

A.

[5.0LTR]

Seems like its normal and nothing to worry about, then. Thanks all.