New gas tank sucked in. "Vented" cap? Hmmm... help!

[Big Bad Dad]

I put a new stainless tank in my 64 Fury. I noticed back in the summer, when it was hot and I pulled the gas cap off, I got a pressure release. Today, it has been cold. I pulled the cap off, and heard the tank pop out like a tin can and got a big suck inward. I am hoping the top of the tank isn't creased! Anyway, I also put on a new gas cap when doing the tank. It is a Gates from RA. However, the cap is actually a Motorad, and is supposed to have some type of special 'approved" vent system. I checked it, and it does not seem to have any venting at all. Looking online, I see every supplier seems to have this same cap under different brand names and labels. They all have a part number ending in 46. Looks like Motorad is the only supplier?? I am afraid this cap is either defective, or else I will need to modify it. For now, I just put the old original cap back on. But it is a loose fit with a hardened old gasket.
Anybody else ever run into this?.. What did you do to solve the problem?

 

[69a100]

For now, I just put the old original cap back on. But it is a loose fit with a hardened old gasket.

Can you wrestle a new gasket on the old cap and just go with that? Did your online check include seeing what NAPA has? Good Luck

 

[diesel_lv]

I've noticed a lot of the vented caps are vented for pressure release and not for vacuum release. They should be vented both ways, but lately it's hard to find. I drilled a cap w an 1/8" hole once. Never had fuel splash out and it worked.

 

[Dan64]

I had the very same thing happen to me on my first test drive of my 64' Polara. What I did was to get a fuel cell roll over vent from summit and I installed it into the fill neck. It actually works better than a vented cap (I also put a non-vented cap on) because if you have a full tank and hit the gas it won't dump fuel all over the place. The vent has a built in shut off in case of a roll over and that is activated when the gas moves toward the rear of the tank and slams the valve closed.

 

[Dan64]

I mounted it just forward of the filler tube mount bracket, sorry not a great picture. The valve installs like a bulk head fitting with the smaller nut part inside the filler tube. I can’t remember exactly but I think I cut off the AN fitting threads to give myself more more and their not used anyway

 

[Big Bad Dad]

I found this old thread about the same problem over at Jalopy Journal. Looks like the same issue happens on other old cars. I think I will perform surgery on this new cap just like the guy there did.

Technical - Vented fuel cap issues

 

[Hemirunner]

I fought this same issue with my stainless tank causing fuel pressure issues even with the factory vents. My application is a little extreme as I’m fuel injected with a mechanical cable drive pump but I deleted the factory vents, welded in two 10AM bungs and ran them up into the trunk in a loop and back through the floor to a rollover valve. No more issues.

 

[Big Bad Dad]

I found this article to be interesting. It is for Corvairs, but the same principals should apply to our cars. And the cap pictured that was dismantled is the same style "Motorad" that is giving me a problem.

Clark's C90 Locking Gas Caps Are Not Vented

 

[Robert Schaad]

I put a new stainless tank in my 64 Fury. I noticed back in the summer, when it was hot and I pulled the gas cap off, I got a pressure release. Today, it has been cold. I pulled the cap off, and heard the tank pop out like a tin can and got a big suck inward. I am hoping the top of the tank isn't creased! Anyway, I also put on a new gas cap when doing the tank. It is a Gates from RA. However, the cap is actually a Motorad, and is supposed to have some type of special 'approved" vent system. I checked it, and it does not seem to have any venting at all. Looking online, I see every supplier seems to have this same cap under different brand names and labels. They all have a part number ending in 46. Looks like Motorad is the only supplier?? I am afraid this cap is either defective, or else I will need to modify it. For now, I just put the old original cap back on. But it is a loose fit with a hardened old gasket.
Anybody else ever run into this?.. What did you do to solve the problem?

I put a new fuel tank on my 1972 Plymouth satellite. The factory cap did not work since it would not tighten down. I got one from NAPA that seemed to work. Every time I stopped in about 10 minutes the tank would pop. Alot of pressure. Later I found that a 71 Plymouth satellite fuel cap was what I needed. It seems the between 71 and 72 the filler neck changed. No more pops. Not sure if that helps but the right cap helped me.

 

[BigFlo]

I mounted it just forward of the filler tube mount bracket, sorry not a great picture. The valve installs like a bulk head fitting with the smaller nut part inside the filler tube. I can’t remember exactly but I think I cut off the AN fitting threads to give myself more more and their not used anyway

View attachment 1370832

View attachment 1370833

Dan64 - how did you mount this into the filler neck? I thought all ‘63-‘65 filler necks have that metal ‘baffle’ welded inside of the filler neck - I believe to let air escape while filling the tank. Curious as to how you were able to work a nut into that spot because there is no access to it.

 

[Dan64]

Dan64 - how did you mount this into the filler neck? I thought all ‘63-‘65 filler necks have that metal ‘baffle’ welded inside of the filler neck - I believe to let air escape while filling the tank. Curious as to how you were able to work a nut into that spot because there is no access to it.

I remember it was a little bit of a PITA but i can't remember exactly how i did it. I'm gonna have to look at it to see if it jogs my memory, but yes, it did have a baffle in there.

 

[Dan64]

Not sure how well it will show up in the picture but I used a rubber o ring on the outside of the fitting. Baffle is still there, can’t remember if I modified it or not

 

[Dan64]

@BigFlo

 

[BigFlo]

Not sure how well it will show up in the picture but I used a rubber o ring on the outside of the fitting. Baffle is still there, can’t remember if I modified it or not

Thanks for the follow up!
Got another question. As far as I know, these bulkhead mount vents are supposed to be mounted internal to the tank with the AN fitting on the outside. That should mean the valve is venting when in that position and closes when the car flips over.
You’ve positioned the valve upside down so it should be closed in that position, correct? I’m not sure how that vents, especially when gas is leaving the tank - it would be closed and not allow air to enter thru the valve. And for expansion in the tank it, the pressure would need to open the valve. Am I missing something?

 

[Dan64]

Mine is venting in the position that it is installed in, it shuts off when gas rushes up the filler tube and slams it shut temporarily,it has to be installed in this manner or it wouldn't work as intened . If you look at the picture i posted in post 5 it's actualy different version than the one i used on my car ( post 13). I just grabbed a pic off of the internet as a example.

 

[Dan64]

Okay, I had to go back and look all this stuff up. The vent I used was a Tanks inc. REMOTE rollover vent which makes a difference . Check out #2 on the install and pictures. It looks like I cut off the barbed portion not a "AN" portion, it hard trying to remember all this little stuff on a build, sorry if that confused anyone.

 

[BigFlo]

Excellent- that makes sense. As I’m just going thru this, it’s all new to me. I really appreciate you going back into your memory banks and sending this out. I think I may give this a try.

 

[toolmanmike]

I put a new stainless tank in my 64 Fury. I noticed back in the summer, when it was hot and I pulled the gas cap off, I got a pressure release. Today, it has been cold. I pulled the cap off, and heard the tank pop out like a tin can and got a big suck inward. I am hoping the top of the tank isn't creased! Anyway, I also put on a new gas cap when doing the tank. It is a Gates from RA. However, the cap is actually a Motorad, and is supposed to have some type of special 'approved" vent system. I checked it, and it does not seem to have any venting at all. Looking online, I see every supplier seems to have this same cap under different brand names and labels. They all have a part number ending in 46. Looks like Motorad is the only supplier?? I am afraid this cap is either defective, or else I will need to modify it. For now, I just put the old original cap back on. But it is a loose fit with a hardened old gasket.
Anybody else ever run into this?.. What did you do to solve the problem?

All tanks need a vent or a vented cap. The check valve above looks like a slick deal. The later model cars (late 60's and up) had vent tubes and later canisters.