Fuel tank vacuum lines

[blazerparts]

For busy work I am going to start replacing all my vacuum lines. I have noticed three or four lines at the back below the fuel filler tube that are plugged with bolts. I also see one going to the back from the charcoal canister in the front. I assume this is all part of a smog set up and am wondering if I should just remove all those existing lines or return it to stock condition. What have you guys done with these vacuum lines?

[71RestoRod]

It's a vapor recovery system to trap and burn fuel vapors from the gas tank that would otherwise be released to the atmosphere. If the lines are blocked off, you should make sure that you have a vented gas cap on the tank.

I hooked mine back up. I found that with the in-tank pump that I have installed for the EFI, there was a lot of pressure building in the tank. Hooking up the vapor lines fixed the pressure problem and got rid of the gas smell that had been emanating from the cap.

[DANL]

I think something is not connected correctly as I am getting a strong gas smell in my K5. I have reviewed section 8 of the 72 10-30 series truck service manual but do not have a good handle on it yet. Any updates on your progress would be appreciated.

...Dan

[blazerparts]

Anybody have a detail on how and where these things install?

[71RestoRod]

Originally Posted by blazerparts

Anybody have a detail on how and where these things install?

You have two vent lines that run from the gas tank to the vapor tube, one from each side of the gas tank. This is so that when the fuel sloshes back and forth you always have venting on one side or the other. The third line on the vapor tube, the one in the middle, runs to the charcoal canister.

At the canister you have three lines. They are all marked on the canister. One leads back to the vapor tube. Another line from the canister is spliced into the PCV line to the carburetor. The last line is a vacuum line that should be hooked up to ported vacuum on the carb.

When you run the engine off idle, the ported vacuum opens a valve on the canister and the engine consumes the gas tank vapors via the PCV line. At idle, the vapors vent out through the canister.

Here's a pic from the factory assembly manual of the gas tank and vapor tube lines:

[LUV2XCLR8]

Should this be hooked up on a standard gas motor, like OE type motors

[blazerparts]

Thanks RR.

[71jimmy]

71RestoRod, that's a good explanation. I know what it is, but I could not have explained it that well. And I now really understand how it works.

LUV, I think that's a personal question wether you want it to be "stock" or not. I think most of these system have been dismantled over the years or are just "bad", ie cracked hoses, rusted lines etc. Since our vehicles are smog check exempt, no one will ever check if the system is in place and working. As far as helping with gasoline smell while parked in a garage, I don't think it will help or hinder. Remember, the engine is off in the garage ( at least I hope so) so the system is not ON.

[71RestoRod]

Thanks, 71jimmy. I read once that it was one of the few smog devices that actually improved efficiency and with the EFI setup I kinda got forced into researching it...

As far as the gas smell... hooked up this way it's a closed system. The cap is not vented, so even with the engine turned off you're not apt to get the gas smell around the cap like you would with a vented system. I have no gas smell at all now. Before I hooked up the canister, it stunk up the garage enough that the wife was complaining. (all that work and it smells like gas!!)

[bpmcgee]

71,

Not ALL of our vehicles are exempt. Here in the great state of corrup-- Illinois, if your vehicle was ever subject to emissions tests, it will continue to be forever. The only way to get around that is to register it outside of a major city (although that's changing), or get antique plates for it.

Brian

[Frizzle Fry]

I found this thread with the search function. I just installed a poly tank and I have no vapor recovery system. I also put a new gas cap on (the old one was just ugly). I just got what the parts store gave me per their book, so I don't know if it's vented or not. How can I tell the difference? Is there a check valve?

My concern is that I will crush the tank with my electric pump if it's not vented.

[71RestoRod]

Frizzle,

The tank won't crush. It might be just the opposite. All of the gass caps let air into the tank. The vented caps let air out as well if the pressure builds up in the tank.

I've never looked at a poly tank from the top, but the stock tank (later years anyway) has 5/16" fuel line connections on the left and right hand sides of the tanks.

[VinceY]

Originally Posted by 71RestoRod

You have two vent lines that run from the gas tank to the vapor tube, one from each side of the gas tank. This is so that when the fuel sloshes back and forth you always have venting on one side or the other. The third line on the vapor tube, the one in the middle, runs to the charcoal canister.

At the canister you have three lines. They are all marked on the canister. One leads back to the vapor tube. Another line from the canister is spliced into the PCV line to the carburetor. The last line is a vacuum line that should be hooked up to ported vacuum on the carb.

When you run the engine off idle, the ported vacuum opens a valve on the canister and the engine consumes the gas tank vapors via the PCV line. At idle, the vapors vent out through the canister.

Here's a pic from the factory assembly manual of the gas tank and vapor tube lines:

Do you have the same type of diagram for canister in front? All of my system is complete in the rear as per your diagram, but in the engine campartment I am missing the canister and the hose is just open ended under the battey.

[71RestoRod]

I never could find a picture of the canister in the Factory Assembly Manual but each of the ports on the canister are labeled.

The canister is attached with a bracket that bolts to the fender using the two bolts on the triangular brace that goes between the fender and the radiator support.

[Frizzle Fry]

Originally Posted by 71RestoRod

Frizzle,

The tank won't crush. It might be just the opposite. All of the gass caps let air into the tank. The vented caps let air out as well if the pressure builds up in the tank.

I've never looked at a poly tank from the top, but the stock tank (later years anyway) has 5/16" fuel line connections on the left and right hand sides of the tanks.

I have the two extra fittings (one on each side) plus an unused line on the sending unit (return?). Someone was saying a non-vented system could put strain on the pump I was at least worried I could starve the carb if the fuel was not replaced with air--dropping the pressure. I guess the vaporization of the fuel more than prevents that, or the caps are designed to allow air in as you said.

So should I run a vented cap, or a fuel filter open to atm on one of the spare ports as someone suggested?

Do the parts stores carry the canisters?

[JonFrankMc]

Rather than starting a new thread I'm going to try and bump this one. I have a 71 step side that I have a under-the-bed tank in. The tank is not vented and when I take the cap off significant pressure is released. I also have a significant gas smell in the garage.

I don't have any of the original canisters. What would y'all do? Add a filter open the the atm to one of the small ports (like frizzle fry's pic)?

For arguments sake, my tank is similar to the pic above. a 5/8" filler neck vent and then 2 smaller ports...

Thanks!

[Bigb401]

Originally Posted by bpmcgee

71,

Not ALL of our vehicles are exempt. Here in the great state of corrup-- Illinois, if your vehicle was ever subject to emissions tests, it will continue to be forever. The only way to get around that is to register it outside of a major city (although that's changing), or get antique plates for it.

Brian

Wow that sucks! I didn't know about that. Another reason I'm glad to live in Wisconsin! Lol I feel for ya bro.
Posted via Mobile Device

[BBrendal]

Originally Posted by Bigb401

Wow that sucks! I didn't know about that. Another reason I'm glad to live in Wisconsin! Lol I feel for ya bro.
Posted via Mobile Device

That post was from 2006. Since then in Illinois the standards have changed. 1996 and newer cars are now tested (OBD2) and under 1996 are exempt.

[Bigb401]

Oh my bad, guess I need to pay attention better. Lol
Posted via Mobile Device

[infinite-foamies]

Trying to revive this deadthread. I am rebuilding my vapor tube, but now wonder if I even need it since I went diesel. My Suburban also didnt come with the canister under the hood. What could I run the center line from the vapor tube to?

[pound41]

I have 2 blazers when I replaced the tank on my original blazer I replaced the hoses etc. The newest blazer I have The PO replaced the tank and plugged off the other lines. Trying to get gas in that blazer is a pita it belches gas everywhere

[Green Monster]

Originally Posted by 71RestoRod

You have two vent lines that run from the gas tank to the vapor tube, one from each side of the gas tank. This is so that when the fuel sloshes back and forth you always have venting on one side or the other. The third line on the vapor tube, the one in the middle, runs to the charcoal canister.

At the canister you have three lines. They are all marked on the canister. One leads back to the vapor tube. Another line from the canister is spliced into the PCV line to the carburetor. The last line is a vacuum line that should be hooked up to ported vacuum on the carb.

When you run the engine off idle, the ported vacuum opens a valve on the canister and the engine consumes the gas tank vapors via the PCV line. At idle, the vapors vent out through the canister.

Here's a pic from the factory assembly manual of the gas tank and vapor tube lines:

This is wonderful information.


My blazer has stunk up the garage with gas fumes for as long as I've owned it. I thought that's just how they are. Apparently this is not normal.

Upon investigation this morning I found two severed lines on the gas vapor canister mounted near the passenger side headlight and battery tray (dual battery setup). My Blazer has a poly tank and Edelbrock carb installed by prior owners that seem to work properly, other than the gas smell when parked in the garage.

I will need to run new replacement hoses and I don't see the markings on the vapor canister that indicate which line should go where. See attached photo.

Can any of our helpful members indicate what lines should go where when coming out of the vapor canister?