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Old 08-06-2019, 06:37 AM   #1
History
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Default Gas tank cap venting?

I assume the gas tank cap is designed to vent from internal pressure and as the gas heats up it expands,,,,,,,,,. I also assume it's designed to let air in to equalize pressure as the gas gets cold and condenses? How could I check my cap before I put gasoline in as my tank is bone dry at the moment? Should I be able to hear air escaping if I pressurized the tank slightly?

I'd like to not have to wonder later.
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Old 08-06-2019, 07:07 AM   #2
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Default Re: Gas tank cap venting?

The gas cap is not high tech there is a small hole that vents pressure. Look at your cap. If there is a small hole in the inner cap(caps are made in two pieces) and the hole is not blocked, your good.
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Old 08-06-2019, 07:09 AM   #3
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Default Re: Gas tank cap venting?

warm soapy water around the cap and look for bubbles when slightly pressurizing the tank?. (on some gas caps I haven't been able to see the hole - hidden under a plate?)
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Old 08-06-2019, 08:44 AM   #4
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Default Re: Gas tank cap venting?

Blow compressed air through the vent hole in the cap.
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Old 08-06-2019, 09:05 AM   #5
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Default Re: Gas tank cap venting?

Shake the cap!
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Old 08-06-2019, 09:15 AM   #6
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Default Re: Gas tank cap venting?

Quote:
Originally Posted by History View Post
I assume the gas tank cap is designed to vent from internal pressure and as the gas heats up it expands,,,,,,,,,. I also assume it's designed to let air in to equalize pressure as the gas gets cold and condenses? How could I check my cap before I put gasoline in as my tank is bone dry at the moment? Should I be able to hear air escaping if I pressurized the tank slightly?

I'd like to not have to wonder later.
The cap also has to be vented so air can enter to replace the gas used as you drive, or the carb would soon run dry!
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Old 08-06-2019, 09:53 AM   #7
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Default Re: Gas tank cap venting?

What year is your A?
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Old 08-06-2019, 10:29 AM   #8
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Default Re: Gas tank cap venting?

Had to lay the cap upside down and soak the vent hole with carb cleaner. Then I used compressed air to assure an open vent passage way.
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Old 08-06-2019, 11:20 AM   #9
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Default Re: Gas tank cap venting?

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What year is your A?
^^^^

The most important first question to ask! The '28-29 caps are pretty reliable; the '30-31 reproductions are a real pain, not venting.

You should never hear pressure escaping when you take a cap off.

It is also easy to accidentally put an unvented radiator cap on the gas tank. Not good.
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Old 08-06-2019, 12:14 PM   #10
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Default Re: Gas tank cap venting?

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Originally Posted by 40 Deluxe View Post
The cap also has to be vented so air can enter to replace the gas used as you drive, or the carb would soon run dry!
I'd bet that's the (main) reason it's vented.

My car is a 30. I put a new gasket on the cap as the old one was almost gone. I believe the gasket is leather?? Cap is as mentioned a two piece part. I can't see a hole anywhere and the part under the cap is a pressed part. I'll look at it closer and see what I discover. I just thought I'd ask as it might help me and others

Last edited by History; 08-06-2019 at 07:10 PM.
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Old 08-06-2019, 12:36 PM   #11
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Default Re: Gas tank cap venting?

30-31 reproduction gas caps are prone to rotating on the single rivet in the center that will block off the vent hole. Snyders sells and Eaton style cap that incorporates two rivets to prevent rotating. See the attached.

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Old 08-06-2019, 12:37 PM   #12
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Default Re: Gas tank cap venting?

try again
Attached Files
File Type: pdf reproduction gas cap.pdf (639.9 KB, 62 views)
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Old 08-06-2019, 12:58 PM   #13
Purdy Swoft
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Default Re: Gas tank cap venting?

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Old 08-06-2019, 01:59 PM   #14
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Default Re: Gas tank cap venting?

Light bulb moment!
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Old 08-06-2019, 04:46 PM   #15
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Default Re: Gas tank cap venting?

Years ago, Tom Endy fixed my repro gas cap per his attached article.....works great!
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