Petrol tank pressure

[timshap]

Following a track day at Elvington last Sat whilst un doing the fuel cap there was a hell of a pressure in the tank. Seems as if I had not opened the cap the tank would have ballooned....and then....
Any ideas please
I have had this problem before but not so severe as it was effecting the idle rate and or fuel supply. once pressure had dissapated all was fine.

[shpub]

Some pressure is normal. Hell of a pressure usually means that you can't undo the cap without difficulty and can be caused by a blocked overflow (pipe or purge cannister itself) from the tank to the purge cannister in the front of the car. If it did it on a track day last Saturday when it was hot, the pressure is caused by fuel vapourising in the tank.

If it does it occassionally, don't worry.

Steve
www.tvrbooks.co.uk