Which trucks had vent lines on the gas tank?

[Vintage Windmills]

Seems like none of the trucks around here have it. (based on a few 67's to 71's I have sitting here) but I was reading about these on the board.

Also, at least one replacement tank LMC sells has the vent ports. Thoughts?

[whoryou1979]

1972 had vent
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[yuccales]

My '70 has vented tank & charcoal canister. Even in 1970, General Motors was doing its part to keep us Californians breathing clean air.

[71K20chevy]

I brought this topic up a while back, and someone informed me that 3/4 tons did not have vent lines.

[yuccales]

Originally Posted by 71K20chevy

I brought this topic up a while back, and someone informed me that 3/4 tons did not have vent lines.

Your right there, my C-20 does not have a vented tank.

[Daddy'O]

Interesting - My 72 GMC 2500 4x4 does not have vent lines.

[72 K20]

None of my 3/4 ton trucks have vent lines. 3-'72's and a '71.

[70cst]

I am signed on

[weever82]

My 69 has a non vented original tank. I was told the 67-69 were non vent and 70-72 were vented.

Weev-
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[fireman064]

I'm interested in knowing how to tell or how to fix my awesome fountain of gas spray I get every time I open my fuel cap... my flat black paint doesn't like the gas spray that well
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[Vintage Windmills]

Originally Posted by fireman064

I'm interested in knowing how to tell or how to fix my awesome fountain of gas spray I get every time I open my fuel cap... my flat black paint doesn't like the gas spray that well
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Buy a vented cap assuming you don't have vent lines. Your system should not build pressure.

[Littletony]

It looks like I ordered the wrong tank if non of the 3/4 tons had vent lines. So can you run a vented tank with the vent line plugged? Would this hurt anything?

[Vintage Windmills]

Originally Posted by weever82

My 69 has a non vented original tank. I was told the 67-69 were non vent and 70-72 were vented.

Weev-
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Perhaps they changed in mid 1970 since I have a very original C/10 1970 parts truck with no vent lines.

Originally Posted by Littletony

It looks like I ordered the wrong tank if non of the 3/4 tons had vent lines. So can you run a vented tank with the vent line plugged? Would this hurt anything?

Tony, Capping the lines should not be a problem but its up to you.

Jud

[yuccales]

[QUOTE=Vintage Windmills;5705837]Perhaps they changed in mid 1970 since I have a very original C/10 1970 parts truck with no vent lines.

I'm thinking it must be a state by state emission thing for the C-10's ? My SPID on the '70 shows "option NA9, Evap Emission Cntrl". Maybe all C-10's made at the Fremont plant in 1970 had a non-vented tank. I find it laughable that California put emission stuff on the C-10 yet nothing on the C-20, but both share the same engine. Now that i think about, that's not the only thing laughable that my state has done!

[Littletony]

I say cap the vent line Mr. Jud, just let me know if I need to send another fuel cap or not.

[da-burb]

Of the things I have read over the years half ton trucks were considered to be light duty and had similar emissions devices as the cars. 3/4 ton and heavier trucks were considered heavy duty work trucks and were exempt from emissions add-ons like vents, TSC(transmission spark control devices, air injector pumps, and charcoal canisters.

[Sir Psycho C10]

So did all C20's and above have vented gas caps? I have a 72 C10 built at the Fremont plant and the emission system is long gone. The vents are wide open under the truck and need to be cleaned up. If I cap off the vents and get a vented gas cap I should be good to go....just like the C20's and above were back in the day?

I certainly don't need pressure issues.

Thanks all,

[71K20chevy]

Yes, you will need some form of ventilation if the emission crap is all gone.

[Sir Psycho C10]

Thanks 71K20 -

Do the caps have the ability to vent each direction to keep the pressure in the tank stable?

[1972SUPERSB]

My 72 has the lines and they go up the passenger side pillar. I'm assuming that's what they are for the vent
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[69GWC]

Originally Posted by Sir Psycho C10

Thanks 71K20 -

Do the caps have the ability to vent each direction to keep the pressure in the tank stable?

Yes the cab will vent both ways.
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[71K20chevy]

Which brings me to a question, in general, what psi do they begin to vent? Sometimes on my vented cap there's some pressure (sometimes vacuum while driving or in the winter months), not a lot, but it's there.

[Sir Psycho C10]

71K20,

I just found this bit of info about the PSI range. Now I need to find the proper vented fuel cap for my 72 truck...since my canister is gone. Hope this helps....

The fuel cap contains an integral pressure and vacuum relief valve. The vacuum valve acts to allow air into the fuel tank to replace the fuel as it is used, while preventing vapors from escaping the tank through the atmosphere. The vacuum relief valve opens after a vacuum of -0.5 psi. The pressure valve acts as a backup pressure relief valve in the event the normal venting system is overcome by excessive generation of internal pressure or restriction of the normal venting system. The pressure relief range is 1.6-2.1 psi over atmospheric pressure. Fill cap damage or contamination that stops the pressure vacuum valve from working may result in deformation of the fuel tank.

[71K20chevy]

Thanks for the info, answers my question!

[Z10]

So, can someone clarify for me, what gas cap to use on a 72? The truck is as it came from the factory. Vented or non-vented?