No air pressure for fuel tank

[Richo2]

Hi guys

I want to know how commodore tanks keep their air pressure. I've got an HJ but I'm using the original charcoal canister which doesn't hold the pressure whatsoever. Is their a pressure relif valve on commodores (maybe it's just the fuel cap?) and if so what is the max air pressure? 15 PSI?

thanks

Andrew

 

[hako]

the fuel cap is the primary pressure device - remember it keeps pressure in when the tank expands but allows a slight vacuum when it contracts. Dunno what the pressure is but must below 2psi. 15psi would blow most tanks. Your HJ has a carby so you dont want pressure in the fuel lines otherwise the carby will flood when the engine is not running and then....BOOM. Remember the Commodore has fuel injection where fuel line pressure is about 35 psi but that is from the pump in the tank to the engine...not the internal pressure of the tank. If tanks held 15psi, every time you opened the tank there would be a great gush of petrol vapour waiting to be ignited...probably would blow the cap off just opening it. A HJ should have just the slightest hint of pressure in the tank when you remove the cap....maybe a slight hssss. I'd be happier with none. There is no advantage in having a pressurised tank unless you want to make a big bang.

 

[Richo2]

Thanks Hako, you answered my question - I have adapted the EFI from a VS including the fuel filler pipe and plastic fasia/lid but I am still using the original charcoal canister. I haven't checked out the commodore canisters yet but I imagine that they only allow fuel gas to enter them over say 2 PSI, otherwise the system would have no pressure at all.

What stops fuel leaking into the charcoal canister when the tank is full and you brake hard?

 

[hako]

I dont think there is a relief valve in the purge charcoal canister - it should be sealed with a pipe ending up back at the tank (I may be wrong). Strictly speaking, the prime reason for a "sealed" tank is pollution - it stop the fuel evaporating out of the tank so you dont get that fuel smell when near the filler. Fuel should not flow to the charcoal canister under heavy braking because (amongst other things) the hose is mounted on the top of the tank where there is only fuel vapour not liquid fuel. The fuel tank does not have to be pressurised!!!! Good Luck