Very bad fuel odors and presure in fuel tank

[Helicopter]

Did a quick scan of previous threads but did not find anything exactly matching this problem. So here we are talking about a VR V8 calais.

On a warm to hot day in particular, when stopped with windows down, there is a very strong fuel odor comming from the outside. With windows shut and A/C operating, the smell still gets inside, but when the A/C is set to re-cycle, no fuel smell are evident

Now the fun part, whenever this strong smell is evident, even with the engine idling, there appears an enormous amount of pressure is building up in the tank. When you start uncrewing the fuel cap, it hisses out like a leaking tyre and with the cap removed completely you can clearly see the fuel vapour pouring out the filler neck. Just need a naked flame and presto, a giant blow torch!

The fumes are continous, until the engine is stopped, then no more presure, obviously! Start the engine, wait a few seconds and you can hear the presure begin to build up once again.

I know that excess fuel vapour should not be trapped in the tank and should vent via the carbon canister which in theory re-directs the vapour back into the intake manifold. I have previously checked that the line between the fuel tank and canister is not blocked. I did this by removing the hose off the canister and was able to blow air back into the tank, so figured its OK. I also checked the canister operation according to the instruction manual and it appeared to be also functioning correctly.

Has anyone else been doen this track with so much vapour pressure in their fuel tank?

Could it still be the carbon canister? Replacing this with a new one may be a good starting point perhaps, or any other suggestions?

[Ozzie]

It happens to all cars. The heat makes the fuels gasses build up preasure and unless you have a breather on the cap itl keep doin it.
Best to buy a cap with a breather or if yours has one try unblocking it

[Cursed CX8]

Try a new canister, also try to get replacement fuel lines and fuel pressure regulator. The 304's have a history of being a blow torch

[Helicopter]

It happens to all cars. The heat makes the fuels gasses build up preasure and unless you have a breather on the cap itl keep doin it.
Best to buy a cap with a breather or if yours has one try unblocking it

Thanks for the suggestion, I will check the cap, but didn't they stop fuel vapours from exiting the cap many years ago for pollution reasons?
And isn't that the reason the canisters where introduced so any pressure/vapour build-up can escape via the canister back into either the engine or the atmosphere? Isn't that what its meant to do?

But even if you are correct about the vent in the fuel cap, as I already stated previously, when I blow back through the rubber hose, I can hear the air going into the fuel tank. Furthermore, when I blow into the canister on the same port as the hose from the tank is connected, the air just goes through the canister and out the bottom vent. Its a clear path between the fuel tank and the canister.

I need to read my VR manual again as its driving me nuts!