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#1 |
Registered User
Join Date: Dec 2015
Location: Westminster, CO
Posts: 26
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Dual Tank Venting
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
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1978 K20 Camper Special, stock 350, TH400, np203, 265/75R16, GMC 10.5" rear, Dana 44 front 3.73, Q-jet. |
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#2 |
Registered User
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Location: Phoenix, AZ
Posts: 574
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Re: Dual Tank Venting
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.
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87 R30 CC Dually, TBI 454, NV4500, zero rust barn find 87 Jeep Grand Wagoneer, 6.5 turbodiesel, 700R4 |
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#3 |
Senior Member
![]() Join Date: Jul 2009
Location: Tomball, Texas
Posts: 1,592
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Re: Dual Tank Venting
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.
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Rick 78 GMC K15 SWB, 350/NV4500/NP205/4.10s Project Hazel 71 GMC C25 350/TH400 - Project Angie 59 Chevy SWB Stepside (next in line? Not sure now ![]() 2001 GMC Sierra K2500 |
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#4 |
Registered User
Join Date: May 2012
Location: horseheads,ny rust belt
Posts: 2,794
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Re: Dual Tank Venting
x2 on a 87 i parted out .
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77-k30lb BIG truck build . 87-k30lb budget beater build . 85-k30lb the plow machine build . 85-c10sb summer fun toy . ----------------------------- HOLLEY SNIPER efi = worst case of p.i.t.a i ever had . EDELBROCK pro flo 4 = best deal going so far . love my setup works great. |
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#5 |
Moderator
![]() Join Date: Oct 2008
Location: Wentworth, NH
Posts: 4,951
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Re: Dual Tank Venting
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.
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1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...
And please let us know if and how your repairs were successful. Last edited by hatzie; 12-17-2018 at 04:26 PM. |
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#6 |
Registered User
Join Date: Aug 2012
Location: Oklahoma City, OK
Posts: 2,543
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Re: Dual Tank Venting
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. |
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#7 |
Registered User
Join Date: Dec 2015
Location: Westminster, CO
Posts: 26
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Re: Dual Tank Venting
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?
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1978 K20 Camper Special, stock 350, TH400, np203, 265/75R16, GMC 10.5" rear, Dana 44 front 3.73, Q-jet. |
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#8 |
Moderator
![]() Join Date: Oct 2008
Location: Wentworth, NH
Posts: 4,951
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Re: Dual Tank Venting
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.
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1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...
And please let us know if and how your repairs were successful. |
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#9 |
Registered User
Join Date: Aug 2012
Location: Oklahoma City, OK
Posts: 2,543
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Re: Dual Tank Venting
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. |
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#10 |
Registered User
Join Date: Jul 2007
Location: Tucson, AZ
Posts: 3,188
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Re: Dual Tank Venting
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.
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#11 | |
Moderator
![]() Join Date: Oct 2008
Location: Wentworth, NH
Posts: 4,951
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Re: Dual Tank Venting
Quote:
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.
__________________
1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...
And please let us know if and how your repairs were successful. |
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#12 |
Registered User
Join Date: Jul 2007
Location: Tucson, AZ
Posts: 3,188
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Re: Dual Tank Venting
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.
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