BUILD THREAD “The Juggernaut”

69hurstolds

Geezer
Supporting Member
Jan 2, 2006
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GATES 31615 is a twist to vent cap.

MOTORAD MGC819 is a push button to vent cap.

I'm sure they have other caps that are continuous vent as well.

Just something to think about.
 
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Rktpwrd

Builder of Cool Shjt
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Feb 2, 2015
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Calgary, Alberta, Canada
You will need a vented cap to make this particular setup work. Cars with their EVAP systems intact use a non-vented cap from the factory. I ALMOST mentioned it the other day, but figured you had that taken care of as you pretty much have three steps ahead already planned out.

It's true, on Oldsmobiles with Oldsmobile V8 engines with the canister, the canister vent line ran from the two or three connections at the tank pickup (that third connection was for fuel pump return line if applicable) and immediately made a left turn at Albequerque and over to the driver side. After all, the tank was vented into the front of the wheel well on our cars with canisters and didn't need a vented cap. The vent pipe ran along driver side until the rear of the engine cradle/crossmember and up along top of the frame on the passenger side. If it was on the "wrong" side, like Chevies.... :) ...j/k... it was left alone on the left side and didn't cross back over since the canister was on the driver side. So if your car used to have a canister, you may remember taking off that line somewhere in the past. If not, it still could be up in there.

GATES 31615 is a twist to vent cap.

MOTORAD MGC819 is a push button to vent cap.

I'm sure they have other caps that are continuous vent as well.

Just something to think about.
Thanks for the responses Mike, it’s appreciated. I came to the same conclusion after doing some extensive research last night, the tanks on all G bodies are vented through the evap canisters. No such thing as a vented cap from the factory on a G body.

Only real issue now is that I could not find a good part number (and anybody with stock that’ll ship to Canuckistan) for a CONSTANT venting cap. All the ones I found were like the ones you posted links to. Push button or twist to vent, neither of which is desirable for my application.

Any help you can provide on this would be awesome, if possible.
 

Supercharged111

Comic Book Super Hero
Oct 25, 2019
3,940
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Colorado Springs, CO
Thanks for the responses Mike, it’s appreciated. I came to the same conclusion after doing some extensive research last night, the tanks on all G bodies are vented through the evap canisters. No such thing as a vented cap from the factory on a G body.

Only real issue now is that I could not find a good part number (and anybody with stock that’ll ship to Canuckistan) for a CONSTANT venting cap. All the ones I found were like the ones you posted links to. Push button or twist to vent, neither of which is desirable for my application.

Any help you can provide on this would be awesome, if possible.

That 2 port fuel sender I put in my Monte claimed to be from a non-emissions 1980 Buick Century IIRC, maybe that got a vented cap.

PXL_20221231_205704443.MP.jpg


But that 2nd line isn't 1/4" now is it? It's a Spectra FG08B and it claims to fit 78-81 Buick Centuries. Maybe I'm onto nothing. Looking at it again on their site the 2nd line has that weird thing on the end of it that I usually see for a charcoal canister. So I guess that's a no to any sort of G with no can.
 
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Texas82GP

Just-a-worm
Apr 3, 2015
7,728
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Spring, Texas
Donovan, unless I'm mistaken, the fuel cap for the diesel cars is what you want. I believe the diesel cap is vented. Dislaimer: I haven't dropped the tank on the GP so it is possible it has a vent in the pickup/sender. It's been a long time since I was under the rear of the car.
 
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Texas82GP

Just-a-worm
Apr 3, 2015
7,728
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Spring, Texas
I think the AC Delco GT172 is what you want. This looks like what is on my car. The specifications say it is a vented cap...
Screenshot_20230124_174031_Samsung Internet.jpg


Screenshot_20230124_174043_Samsung Internet.jpg
 
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Streetbu

Know it all, that doesn't
May 22, 2011
3,446
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Central NY
I know for a fact some G bodies came with vented gas caps. Fuel sloshing out of the cap under hard accel is a very common issue. Obviously most of these cars didn't have issues bone stock, but rather once they have some real hp.
 
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69hurstolds

Geezer
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Jan 2, 2006
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The question is which way does the venting on the cap go? I’ve honestly never delved into the fuel caps other than ensuring I got the right GM part number. Is the vent into the tank? Or out? Or both ways? I think this will take a bit of investigation.

My guess is the factory cap is vented to allow air in so when the evap system is vented through the canister and vapors are pulled into the carb, any negative pressure in the tank is compensated for by the cap venting air into the tank. Same for the fuel pulled from the tank by the fuel pump. But it could be a check valve arrangement where air can only enter the cap but not leave.

Wouldn’t make sense for the cap to vent outward otherwise there's no sense for the charcoal canister.

That GT172 is the factory gas cap for many a G-body. GM p/n 22525300.
 
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melloelky

Comic Book Super Hero
Oct 22, 2017
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mass
it's been years since but i came across a vented cap # from stant,10834.the local napa had it.I'm running the two line sender w/a axle vent from dorman on a short piece of hose on the unused line.i have zero venting/drivebility issues/zero gas smell in my life.just my .02.
 
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69hurstolds

Geezer
Supporting Member
Jan 2, 2006
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Now we're getting somewhere. Fitment for the Stant 10834 is basically the same as the GT172 Delco cap.

The Stant 10834 descriptor states: "Equipped with carefully designed and tested pressure and vacuum relief valves to maintain a specified amount of pressure or vacuum in the tank before relieving" Which I believe may answer the question whether the factory caps are vented.

With that said, there's STILL an issue with how much can be relieved via the cap, either pressure or vacuum when driving around. The vent doesn't really go anywhere the way it's set up now from the top of the tank to the neck of the tank. I don't think that was the intent for the vent port on the pickup. I still believe a separate vent line from the vent port on the top of the tank needs to be located somehwere outside of the tank. This is just me musing here...I would disconnect the vent from the neck. Plug the neck hole and attach an L bracket nearby and yes, I'm going to say it, use a check ball vent with rollover protection on the end of that hose. I want to say someone here did that. They ran the vent to behind the filler neck when you drop the license plate you could see it back there. Optionally, you could mount it way up in the top area in front of the tank, which would alleviate any issue of potential acceleration problems with the vent in the back.

I would have a tendency to believe what Stant says about the caps because, well, they actually made the factory caps.

BTW, the CSM doesn't say squat about the caps or fuel system operation. It does discuss fuel vapor canister and canister purge system, and does mention that the cap is threaded deep so that when you start to remove it, the threads allow any residual pressure to be relieved when opening the cap yet not allowing it to blow out and hit you in the face. I'm simply summarizing.

Found an older, yet not that old, thread on this subject. May come in handy.

 
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