What is the difference between a vented and non-vented gas cap?

Can you turn a vented gas cap to a non-vented gas cap?

Does it make any difference if you use a vented gas cap to a non-vented gas cap?

This is what I know. Newer cars use non-vented caps because of the EFI system. The EFI system has high pressure in the fuel line and so use a return line back to the tank. So the tank is apart of the pressurized system and can not have a vented cap. That being said there is a release valve in the cap if the tank has to much pressure in it. This is for safety.
Older cars dident have to worry about returning excess fuel back to the tank. And so the tank was not pressurized and dident need a non-vented cap. The vented caps do have a seal around them to keep the vapors in the tank for the most part.
I know you cant use a vented cap on a EFI system for it would trip a fault code. Im not sure on the other way around.
I dont think one cap could be changed into the other. Why would you want to? I think the tanks were different with the different caps so just changing caps might not work for ya. I assume you are wanting to switch from Carb. to EFI.

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~Nic

I run a TBI system with a stock 65 yearold vented cap.....no problemo.

You need a nonvented cap if you are using EVAP emission controls,

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Yeah....Im the one who destroyed this rare, vintage automobile.....

The non vented caps started around mid 70 on A bodies when they went to the EVAP system.

You will have a lot more issues if you use a non vented cap on a vented system than if you use a vented cap on an EVAP system.

A vented cap on a late model car will set a code.

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Official Cookie and Mater Tormentor.

I have a 62' Signet and saw a vented fuel cap on Ebay that will fit the Signet.

Hence my original question.

Thanks

That would be correct.

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Official Cookie and Mater Tormentor.

Would there be a problem if I fitted a non-vented gas cap to my 62' Signet?

A 62 will have a vent line attached to the filler neck.

So a non-vented cap will work with a 62

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Vented caps are not used on our cars. All Chrysler Corp cars through '70 ('69 in California) used non-vented fuel caps. All vehicles '70-up in California and '71-up elsewhere use Pressure-Vaccum caps. See here.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I have now read the article, thanks.

But can I still use a vented fuel cap on my 62' Signet?

I dont see why not.

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~Nic

If you want to aggravate the tendency of these cars to vomit gasoline all over the quarter panel, then yes, you can use a vented cap.

Why do you want this?

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

X2 ????? Venting is done w/ the small tube in the filler neck up high, it
is to allow air to enter the tank as fuel is removed w/o developing vacuum,
otherwise fuel flow ceases in short order. Many carbureted cars have fuel
returns, ever see three lines on a fuel pump or filter? All tanks build press.
FI or not, it is evaporative emissions that is being managed by canister
equipped cars that eliminated the free venting systems, not FI. Unless
you have a fuel venting problem, and it's poss. spiders-etc. have plugged
the vent-line, there is no reason to put one on your vehicle.

The previous owner of my '64 put a non-vented cap on the car, and as a vacuum was created in the tank, it sucked the filler neck and the quarter panel inward.... W.T.F. ????????

This was due to the filler neck vent (which is supposed to vent the tank) being tied to some mystery nipple on an incorrect fuel tank that was installed on the car.

http://www.slantsix.org/forum/viewtopic.php?t=59460

So Dan is right...... assuming that the correct tank is installed on the vehicle. No need for a vented cap. Guess I need a new cap for my new tank.

Lol!! The power of atmospheric pressure should not be underestimated! It
could crush that fuel tank if you developed enuff vacuum. We have a parts
guy who kept putting the small bung back in one of the 55 gal. drums of oil
we pump out of. One day another tech was pumping away and BOOM!! It
imploded, looked like a three lobe paddle wheel!!
Back to your tank, the mystery nipple may be the orig tank vent for that
tank, and should've just been capped/plugged. And your vent line from the
neck to the frame/atmosphere like it was, you'd be fine.