vented gas cap for '56

[mark '87 930]

Hello everyone, for those of you who have converted your stock, behind the seat tank with an EFI setup... how did you vent the tank?
I am thinking of going EFI AND will retain the stick tank. I have a couple of options for return lines (will likely go with the FItech system with their FCC or the one from Rob), either through the sender or via the filler tube.

My question is, what do you guys do about a vented gas cap? With the return line, there will undoubtedly be pressure buildup of sorts, i don't want the cab smelling of fuel (for obvious reasons), so what do you guys do? I haven't been able to find one for the stock setup. It's not like it's a great seal with that cap anyhow, but thought i'd ask.

[1project2many]

IIRC the stock cap vented at about 2 psi. It needed to so the fuel pressure in the tank wouldn't overcome the float in the carb. I'm still using the stock cap but it's taken a few tries to get the tank to seal. I'm mostly happy but I still think I'm going to connect a hose at the filler neck vent and run it out to a carbon cannister.

[mark '87 930]

Interesting. So are you saying that 2PSi is too high? and that vapors are still making their way into the cabin?

[1project2many]

2 psi was too high. I had made a custom sending unit / pump hanger combination. This necessitated cutting a new hole in the top of the tank. Paper gaskets were never good enough to seal the tank once the fuel began to warm up. Along with using a power steering cooler to remove heat from the returning fuel, I ended up buying a sheet of neoprene from Grainger and making a good gasket for the new sender.

Sometimes after a fill-up on a warm day I will smell some fuel in the cab. And now that the tank builds pressure and vents out the cap I'm seeing some fuel staining around the fuel filler neck. So I'd like to install a charcoal cannister and a purge valve to handle fuel vapors properly.

[mark '87 930]

so you are saying that you have a fuel pump inside the tank?
My intention was to use that Fitech FCC. I don't this unit returns any fuel or high pressure vapours. I may try it with the cap that is there and If I need to run to a canister, I can always do that later.

Do you have any pictures of your current setup?

[1project2many]

Yes, I have a pump inside the tank. It was my first ever EFI retrofit and I missed a couple of important details. The first is that carburetor tanks often don't have baffles because the carburetor has a fuel well to supply fuel if the pump temporarily fails to supply fuel. The second is that fuel returning from the tank is warmed up and vaporizes easier. My second retrofit, in a '67 Chevelle, adapted a marine system which employed the mechanical pump on the block supplying fuel to a reservoir with a high pressure pump. This system worked well and required a minimum amount of adaptation.

The FITech system is similar to the marine system and should not require a high level of concern for vapor buildup in the tank. Be sure to place your high pressure supply pump where it will not become overheated, away from exhaust manifolds and air coming through the radiator, and consider a cooler on the return line. The shortcoming of a short system is that may have no way to shed excess heat.

I might have some photos of my tank and sender. I built it in '92 so the pictures are real photos and will need to be scanned. I did have the sending unit out in 2012 or 2013 and I *think* I might have taken some digital shots. Maybe I can locate those as well as they will show the baffle I added to reduce pump starvation.

You may want to remove the stock sender and replace the 5/16" steel line with 3/8" line to ensure plenty of fuel available to your engine.