Fuel Tank Venting, where to?

[Bigbeardbiii]

My '73 only has the rear tank and I was told to plug all vacuum lines. Here's a link to pictures of my filler neck to the tank & sending unit/gauge. I have no idea if it's connected to the charcoal canister. I know I have one but I thought when plugging all the vacuum lines except for the one's to the tranny that the canister became useless.
http://www.supermotors.net/mygarage/content.php?id=27354&sID=85758

 

[DirtDonk]

the charcoal canister on our trucks does not use engine vacuum directly from the manifold, but instead gets a very light taste of it from the air filter housing.
Modern vehicles with computers can control a CAN-P (canister purge) solenoid valve to let full engine vacuum evacuate the canister, but we don't have that option.

Your canister did still become useless however, just by plugging the hose lines in the tank. Even if you did still have the tubes in the canister to the engine, there's nowhere for the vapors to escape the tank.
And since your fuel cap is only vented "inward" to allow the fuel to flow easily, it can't vent pressures or vapors out through it like an early cap would. An early cap won't fit your filler neck, but many people drill a small hole in the later caps to allow better fuel flow.
However, it obviously has the not-so-nice side effect of letting fuel vapors vent into the atmosphere. Makes for very stinky garages. As does leaky lines, seals, tanks and even newer vent caps like you have on the side of your tank.
In fact, if those are more than a few months old, they might be where some of your fuel smell is coming from.

You can test the fill part of all this theory by uncapping the two vent lines and running hose to a high spot on the truck temporarily. Next time you fill it see if that changes the dynamic. Simple cheap and easy.

Good luck.

Paul

 

[Crush]

My 77 has the factiry vent system. Top of tank to the rollover valve on the passenger side then up to the charcoAl canister. From there i ran a piece of heater hose from the too of the canister to the air cleaner. This seems to be working for me. No gas smell in garage and i can fill the tank without having to hold nozzle upside down like i did on my 76 that had nothing for vents on it but a hoze

 

[Bigbeardbiii]

the charcoal canister on our trucks does not use engine vacuum directly from the manifold, but instead gets a very light taste of it from the air filter housing.
Modern vehicles with computers can control a CAN-P (canister purge) solenoid valve to let full engine vacuum evacuate the canister, but we don't have that option.

Your canister did still become useless however, just by plugging the hose lines in the tank. Even if you did still have the tubes in the canister to the engine, there's nowhere for the vapors to escape the tank.
And since your fuel cap is only vented "inward" to allow the fuel to flow easily, it can't vent pressures or vapors out through it like an early cap would. An early cap won't fit your filler neck, but many people drill a small hole in the later caps to allow better fuel flow.
However, it obviously has the not-so-nice side effect of letting fuel vapors vent into the atmosphere. Makes for very stinky garages. As does leaky lines, seals, tanks and even newer vent caps like you have on the side of your tank.
In fact, if those are more than a few months old, they might be where some of your fuel smell is coming from.

You can test the fill part of all this theory by uncapping the two vent lines and running hose to a high spot on the truck temporarily. Next time you fill it see if that changes the dynamic. Simple cheap and easy.

Good luck.

Paul

Paul,
You spoke about my fuel cap but I don't remember uploading a picture of it. Did you see it in my batch upload link I sent or where? I'm lost now. Are you speaking of my gas cap on the outside LH fender of the truck?
Also, when you say to connect a hose to the capped off vent ports coming from the tank and place them in a high spot are you saying as long as they are higher than the top of the tank?
Thanks,
Dean

 

[DirtDonk]

Maybe we should start another thread for your tank setup BB. It looks like it's going to be an ongoing discussion since there's so much to do and verify with your rig.
In the meantime though...

Yep, the gas cap on the side of the fender. They are all "vented" but early ('70 and older) ones differ from later (mid-'70 and later evap) compatible caps, in that they are not supposed to allow vapors to escape. Only to allow outside air to flow inward to take up the space left by the gasoline that the engine uses.
Early caps can't be fitted to a later filler neck, so you can't vent pressure and vapors outward (from heat expansion) into the atmosphere.
I'm not the cap expert I should be however, so there are probably things caps can do that I'm not aware of.

With your later cap and the closed off vent tubes you can get a pressure buildup inside the tank from normal heat that can push vapors out, or even push gas out the filler neck on to the side of the body!
So capping off any vent lines is just not a good standard practice.

So my suggestion to uncap the two vent lines and run hoses from them to a high point was to avoid this potential issue. You would want them higher than the top of the tank for the obvious reason of keeping gas from flowing out. You'd also want to either bend them over in an almost 180 bend, or cap them with some kind of filter or thingy to try to keep at least some of the fumes from escaping and smelling up the joint. Also keeps other junk and dust and stuff out of the tank.

As I mentioned, I would start another thread and post the link to the pictures, and add more pics of your charcoal canister and it's hoses (if any) so we can discuss how best to proceed with yours so you get full function while filling up and not all the associated stink.

Paul

 

[reynard101]

So my suggestion to uncap the two vent lines and run hoses from them to a high point was to avoid this potential issue. You would want them higher than the top of the tank for the obvious reason of keeping gas from flowing out. You'd also want to either bend them over in an almost 180 bend, or cap them with some kind of filter or thingy to try to keep at least some of the fumes from escaping and smelling up the joint. Also keeps other junk and dust and stuff out of the tank.

I have this, still pukes a little. And still builds up pressure at times. I might need to re-route and maybe using large fuel line to have more volume.

 

[Bigbeardbiii]

Paul,
I did start a thread on the issue and I think you have responded on it too but here's the link: http://classicbroncos.com/forums/showthread.php?t=266588

I'm about to go outside & take pics of the lines coming out of the evap. tank & the line running from the charcoal canister to where they are cut as well & will post the link to them on the aforementioned thread.
Thanks,
Dean