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Hi all, I have a 1996 XK8 coupe with the normal 4 litre engine, not filled her up for quite a while, had a hiss of air during opening of petrol cap - is this normal ??

Yes, shows you have a good seal from the petrol cap.

Some cars have been known to collapse fuel tanks when their tank breather arrangements have failed. The symptoms associated with these failures are strong inwards hissing every time the car is filled with fuel and unexplained 'twangs' from the fuel tank flexing under strong vacuum. A one-off or occasional hissing is probably nothing to be concerned about. However, it would be worth having the car checked out if it's a strong hissing and occurs regularly. Mike.

Thank you for the replies - not had any noise from the tank but will keep listening.
I am also getting a slight smell of petrol when very close to the flap....