Dual Tank Venting

[learyw]

I have a 78 K20 with a 350 and dual fuel tanks. My fuel senders have three outlets. The fuel line, return line, and vent line. Both vents were plugged when I bought the truck. Clearly, this causes problems when the tanks heat up creating a build-up of vapor. It shoots out when I open the gas cap. Whenever I am at the junkyard I look at other trucks hoping to find a vapor canister or get an idea for how to vent mine. A few thoughts:
1. Every truck I look at that is similar to mine is different. Different dual tank fuel selector, a different way of running lines. Clearly, people change this system a lot.
2. What is the best way to vent mine? LMC sells a vent line for 2wd (part 32-3654). Not sure if that works on my 4wd. It doesn't look like mine every had a line. I would be surprised if someone went through the effort to remove it. Maybe. I could run that to a canister and then to my carb. I would also need to buy an aftermarket canister. Several hundred dollars in parts for all that...
3. What is the cheapest/easiest way to safely run mine? Plug the vent lines and buy a venting gas cap? I read a few people ran the vent line directly into the carb.
4. I have looked at a lot of vacuum line diagrams. There is even a diagram for my year and engine that shows no vent lines at all. How did they vent? Is this why some only have two outlets on the sending unit?

I have read every post I can find, but Im kind of lost on this one. Thanks! -Will

[DieselSJ]

I have a 87 with dual tanks.

The vents do not go through the selector. The vent from each tank goes to a T on the right frame rail by the tank, and from there it goes to the canister.

[rgunlock]

I have no idea whether it was an original setup, but my '78 K15 with dual tanks just had a piece of short hose with one of those caps that keeps out water, etc. connected to the senders. I replaced with the same setup when I rebuilt the truck and it seems to be venting fine. I don't have a canister and never had a return line on mine.

[sweetk30]

Originally Posted by DieselSJ

I have a 87 with dual tanks.

The vents do not go through the selector. The vent from each tank goes to a T on the right frame rail by the tank, and from there it goes to the canister.

x2 on a 87 i parted out .

[hatzie]

Get a 1970-1973 Corvette charcoal can. It's still available and easy to hook up.
The cans for many early 70's rigs are the same but not available new. Worth a shot if you don't want to spend much.

The Vent and purge hoses should be SAEJ30R7.

Put a Tee in the vent lines and run them up to the charcoal can Tank or Fuel Tank connection.
Put a Tee in the PCV hose and run the PURGE or PCV connection to this Tee.
Run a vacuum line from the vacuum valve aneroid on the charcoal can to ported vacuum on the carburetor. This one may or may not be marked. It's the connection to the top of the hat on the vacuum valve.
The carburetor bowl is directly vented inside the air cleaner ring so that connection is usually capped off.

[Dead Parrot]

My 75 with dual tanks has the 2 port senders, and no canister and no return lines. The vents are a short hose with a plastic covered vent like you use to vent the differential.

Mine was HD emission from the factory. No cats, only had PCV and THERMAC for emission controls.

There was a lot of variation in square emission setups due to rapidly changing rules, weight rating on truck, state of sale, phase of moon, etc. That is why you are finding so many variations.

[learyw]

Thanks, Hatzie. I think Im going to make that my long-term fix plan. Any problems running rubber line instead of a metal line that distance? Condensation or anything?
I think for now those one-way breathers might be a good idea. Parrot, do you know if it came factory that way?

[hatzie]

I'd run steel line over the tank to engine bay distance because it's easier to hang it. Rubber fuel vapor line will work but it will need to be protected. Rubber line will probably be more expensive than steel as well.
I've bought 25ft coils of marine steel (Cu/Ni/Fe) brake tubing online somewhere between $0.50/ft & $1/ft.

You can run an axle vent on a Tee if you aren't parking it inside. My 76 K20 didn't originally have a charcoal can but it does now.

[Dead Parrot]

Yes, the vent caps were factory. Mine is a HD emission model. No cats, air pumps or other emission stuff.

If you are going to redo your fuel plumbing, probably better to install a canister. The gas fumes from the little plastic vents can get a bit strong on hot summer days. But as a fix for un-vented gas tanks, the plastic vents should to the trick. I also snagged a 90's era canister as it looked fairly easy to hook up. May do Hatzie's Covette canister suggestion instead if I ever make the change and they are still available at that time.

I found factory lines at the local Pull-a-Part for a canister setup. Easy to get as previous pickers had taken the bed and cab. Waiting until I get around to doing some long needed body repair and have the bed off.

[tucsonjwt]

My 83 C20 had no evap can from the factory. It did have the short hoses with differential caps. I bought 25 feet of gas line hose and ran it to the back of the truck - terminated the hoses with the differential vents above the spare tire. You can smell fuel ant the back of the truck but not inside the cab.

[hatzie]

Originally Posted by tucsonjwt

My 83 C20 had no evap can from the factory. It did have the short hoses with differential caps. I bought 25 feet of gas line hose and ran it to the back of the truck - terminated the hoses with the differential vents above the spare tire. You can smell fuel ant the back of the truck but not inside the cab.

I kinda figure there's no performance hit for installing an EVAP can.
The fuel smell is fuel... I paid for that fuel that I smell dissipating into the atmosphere without an evap can. I might's well burn it in the engine if I can.

The stink from the un-captured fuel vapors makes the barn smell like an old salvage yard too.

[tucsonjwt]

Losing some gas through evaporation is true, but installing an evap system also costs money. A clogged evap system also affects performance. My BB drinks so much gas I don't think I would notice some evaporated gas.