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Discussion Starter #1
I have what may be OE gas tank cap, reads "anti surge" and "vented." The fit seems quite loose, maybe because the gasket looks pretty thin. It lets air in okay but IDK if.it is working properly going the other way.

Hate to spend $6 on a new chinese one if the old ones are fail proof. But I don't want my gas to evaporate while the car sits in the garage either.

Anyone know enough about how these things work to know if mine is still good?
 

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the tank is already vented by a tube on the fuel sender unit, so don't worry about the evaporation, at least-
 

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Discussion Starter #3
There is a vent in the sender? What happens when the tank is full?
 

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ok so the vent is someplace else. on 63 its a tube that goes up and over the filler neck. get under there and look. i know its not easy. find a cushion to lay on:D
 

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My 64 does not have a vent. The tank would expand when the temperature increased with the cap that was on the tank when I purchased the car. I purchased a new cap, same problem, I modified the valve in the cap to let it vent when the tank built up a little pressure. If you search the forum, there is a post about what cap I used and what I did to correct the vent problem .
 

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Discussion Starter #8
With my cap, I've never heard a pssst of pressure buildup. What is the mechanism for allowing just a few lbs of pressure?
 

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There is two valves in these new gas caps, one lets air in, the other is suppose to vent when the pressure gets to great. The problem I had was the cap didn't vent and the pressure built up in the tank. So much that the cap would hit the fuel door and open it a little. I shortened the spring on the pressure valve so it let the valve open with very little pressure. Works great now. This is what I finally used.

 

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There is two valves in these new gas caps, one lets air in, the other is suppose to vent when the pressure gets to great. The problem I had was the cap didn't vent and the pressure built up in the tank. So much that the cap would hit the fuel door and open it a little. I shortened the spring on the pressure valve so it let the valve open with very little pressure. Works great now. This is what I finally used.

I also had that issue on my 66 Gal. Shorting the spring didn't even help, I had to totally remove the spring.
 

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Discussion Starter #11
Maybe my PO shortened the spring too much and now I get no pressure in there, or over the years the spring got too weak.

A third possibility is I don't give the pressure a chance to build up because I keep the car garaged. I'm taking it out golfing tomorrow. It will be sitting in the sun for a couple hours with a 1/3 full tank. should be a good chance to check the cap, do you think?

if there's no pssst, buy a new cap.
 

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Discussion Starter #14
Makes sense to allow a few lbs of pressure before venting. If you leave tank open to the air your gas evaporates.
 

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if the tank gets pressurized, it can open the valves in the fuel pump and possibly overcome the needle valve in the carb- flooding
 

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Back in high school I worked at a gas station. When full service was still a thing. Add gas, wash windshield, check oil, etc. I removed a lot of gas caps. If there was pressure we would try to sell a new gas cap.

Up until the early 70s cars just vented fumes from the tank to the atmosphere. Either through the cap or through a vent line.

I agree with extech.

- John
 

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Discussion Starter #17 (Edited)
Is the flooding a realistic possibility? As I appreciate it the caps are designed to vent after reaching x psi.
 
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My vented cap I bought for my 66 had a lot of pressure when I opened it up and I felt it was too much. I took the cap internals apart and removed the spring and diaphragm so it could vent both ways. No more problem now. Also no fuel smell in the garage, so what vapor vents is minimal.
 

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Is the flooding a realistic possibility? As I appreciate it the caps are designed to vent after reaching x psi.
unsure. but - a fuel pump puts out 3-5 psi, anything over that overwhelms the needle/seat in the carb.
at what psi do these caps vent?
 

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Discussion Starter #20
Hard number to find, I don't know why.

Closest I can figure, the air pressure inside a party balloon is one psi above atmosphere. The maximum a person can blow is 2psi.
So if you hear Psssst coming out past the gas cap similar to that you would hear from a balloon then I guess 1 psi would be the number.
 
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