Gas cap confusion

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RustOnWheels

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For the ‘65 Valiant station and Barracuda I am in need for gas caps. As the Valiant lives on the street I would like a locking version on that one.

Now here is my confusion: I see the gas cap of damraider and I see the Stant locking gas cap, both of which are NON vented. Now, when I drove back the Valiant home, with its original gas cap on, at one time after refueling I tightened it a little too much. After a while the car started running horribly (way too lean) and the fuel tank started leaking, all of which stopped when loosening the gas cap.

So how does a non vented gas cap work on a ‘65 Plymouth a-body? How does the tank vent with a non vented cap? I did notice that the Valiant has this cork-like gasket around the gas filler tube opening, which is compressed, half broken and missing on a few places, which would almost suggest that air could pass it but somehow doesn’t.

After refueling I now check if the gas cap is loose enough that air can pass as turning until it feels tight gets me into trouble.

Is there something I can change so I can safely use the non vented gas caps as offered by people, for both the Barracuda as the Valiant? Should there be a vent somewhere else at the fuel tank (which would surprise me as the filler necks both are inside the car body)? I am quite confused…
 
Slant6dan sold an awesome gas cap a few years ago I have on my 66’ with 0issues.
It’s not locking. But the best cap I’ve ever owned.

I’m not sure on the locking style. Before the rebuild mine would puke gas out toward the top, hot days, just sitting there. I installed a vent / roll over valve at the top of the tank and followed the info on this site - using the search feature at the top of the page is your best friend.
Plenty of people here have already laid out the styles of caps and uses you can search and understand what you have and what you need to do. I don’t want to quote their work. Good luck!
Syleng1
 
I have noticed after hunting an oem cap for several Fords ( yes whip my *** with wet noodle) that best way to find one is measure the inside and outside diameter of filler neck.\\\

If the tank is not vented ( like said) it will have a tube from it or in case of later B and E bodies, 4 hoses coming out of the darn tank)you need a vented cap and if vented just go with a non vented cap. But like 70 up A bodies each year can have a somwhat different cap.
 
Go to MyMopar. Download the 65 service manual, it's free. The tanks are on page 14-75 -- 14-81

Do some research. So far as I know, NONE of the A bodies (and any other Mopar from ?? 60--?? onwards uses a vented cap. (A body waggin's could be exception) I could be mistaken about some of the older models. The newer evap controlled ones are not vented in the operational sense, either. They are "pressure vacuum" (pressure relief) caps and they are a mess from about 70 onwards
 
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67Dart273 said:
Go to MyMopar. Download the 65 service manual, it's free. The tanks are on page 14-75 -- 14-81

Do some research. So far as I know, NONE of the A bodies (and any other Mopar from ?? 60--?? onwards uses a vented cap. (A body waggin's could be exception) I could be mistaken about some of the older models. The newer evap controlled ones are not vented in the operational sense, either. They are "pressure vacuum" (pressure relief) caps and they are a mess from about 70 onwards
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A body wagons 63-66 use the same tank, sending unit and cap as 2 and 4 door cars only the fill tube is different, but it works the same with the vent tube in the fill tube and the vent tube extends thru the floor.
 
Again, as I said earlier make sure the vent tube is clear and if you want a locking cap Ebay is about your only option. Maybe someone will chime in with the correct part number. Don't trust Ebay's it fits.
 
Thanks everybody, I have the 1965 workshop manual laying around, why I haven’t checked that beforehand I can’t tell you - I’m using it extensively to find some faults in the electrical department so it’s open on those pages. I think it’s because of my confusion (and ADHD fed one-track mind).

The leakages and the surging started on my road trip back home, two weeks ago. I called the previous owner and searched online and found multiple threads on this forum (and Slant Six forum) about the fuel cap being the issue. I could remember that on the last refueling session I turned the cap all the way shut so that’s why it made sense, and loosening it up really helped fixing the problem. The previous owner never had that issue by the way. Anyway, that made me think that the original cap must have been vented normally due to a gap and porous gasket or something.

It now makes much more sense that there must be a clogged vent somewhere else as the tank was leaking badly at the time of leaning out / overheating. The leak seem to come from somewhere at the fill tube side.

I was afraid the tank was leaking because of the vacuum creating leaking seals or even ruptured seams, but after loosely fitting the gas cap the leakage was over too.

So buying an old non vented gas cap will suffice. It will hurt my wallet (there’s one on eBay and shipping costs to the EU are madness) but it’s cheaper than a full tank. And I’m thinking of buying 2 of damraider’s (again via eBay, if his username there is sjd234 - I’d like to support the actual maker of course) so I have one for the Barracuda and one for the Valiant at car shows (locking cap for the daily commutes and parking outside).
 
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Hello, I can make you a better deal then Ebay. Shoot me a conversation.

Thanks,

Steve
 
Be sure the tank is not leaking at the sending unit and mayb the drip runs back toward the filler tube?? If it has been replacd and they usd the new lock ring it fits too loose and will leak at that gasket.
 
The leaks have gone after loosening the gas cap so I guess it’s not that gasket, but it’s good to keep in mind, thanks!
 
image.jpg


Okay, I’m still too dumb to get it I guess… I see that vent tube run from the tank to the top of the filler tube, which makes me think it is still a closed loop if the cap is not vented. Where does it actually vent out to the air to prevent the tank from sucking vacuum?
 
RustOnWheels said:
View attachment 1716121975

Okay, I’m still too dumb to get it I guess… I see that vent tube run from the tank to the top of the filler tube, which makes me think it is still a closed loop if the cap is not vented. Where does it actually vent out to the air to prevent the tank from sucking vacuum?
Click to expand...
Guys, correct me if I'm wrong but the early vent tubes are trying to prevent splash back during filling. Ruety, If I were you I would drill a vent hole in your cap. May be under the bar and on the end or underneath where it can't be seen. It wouldn't leak gas out unless you filled the tube up to the top.
 
RustOnWheels said:
View attachment 1716121975

Okay, I’m still too dumb to get it I guess… I see that vent tube run from the tank to the top of the filler tube, which makes me think it is still a closed loop if the cap is not vented. Where does it actually vent out to the air to prevent the tank from sucking vacuum?
Click to expand...
It goes thru the floor plate for the filler tube any liquid fuel would just drip on the ground. It is looped upward to keep the tube clear of any liquid fuel.
 
I think it goes into the frame rail where it either evaporates or makes it out a drain hole.


Alan
 
Thanks guys, very enlightening, now it all makes much more sense! I’ll check it out this weekend if I have the time.
 
I have driven many miles since this topic and I have had zero problems with leakage ever since. I got two gas caps by damraider; one on the Barracuda which looks absolutely killer in the aluminum look on the silver body, and the other on the Valiant. I kept that one stock (aluminum) for now as well.

A photo of the gas cap today, after a test run I did and cleaning the new tires - hence this picture.

IMG_5873.jpeg
 
RustOnWheels said:
I have driven many miles since this topic and I have had zero problems with leakage ever since. I got two gas caps by damraider; one on the Barracuda which looks absolutely killer in the aluminum look on the silver body, and the other on the Valiant. I kept that one stock (aluminum) for now as well.

A photo of the gas cap today, after a test run I did and cleaning the new tires - hence this picture.
Click to expand...
Thanks again for the support !!!
 
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