non emissions fuel tank breather questions

[Cav Trooper]

Ok, I've been driving my mutt for over a year now and today I took it for a spin and after about 2 miles I smelled gas. Didn't think a lot about it but got home and checked and the fuel tank breather was seaping gas out onto the tank cover plate. Tank is about 3/4 full and I removed the gas cap and got a hissing sound-pressure built up???? How does the breather work? The gas cap is new old stock last year and the vent is in the opened position on the cap. I removed the breather and noticed it has a rubber gasket with a fine mesh screen in the gasket. All of the fittings were tight and this is the first time it's leaked. The gasket does appear to be swollen slightly which I assume is from the ethanol gas. Is the breather vacuum operated? Any insight?
Thanks,
CT

[muttguru]

Tank is about 3/4 full and I removed the gas cap and got a hissing sound-pressure built up???? The gas cap is new old stock last year and the vent is in the opened position on the cap.

CT.... the cap sounds like it is faulty. The vented cap with the vent "OPEN" should vent out the vapor. If it isn't working (and you say it isn't) then try a different cap (a vented cap). A photo of your current cap plus a photo of the top of your fuel tank would be helpful. And make sure you do not over-fill the tank.
Ken

[rickf]

And the vent line to the carburetor should also be open to the atmosphere, albeit in the air cleaner but it should not allow pressure to build.

[Cav Trooper]

Ken, I haven’t figure out how to post pictures on the website. I’ll send you some privately. Rick, the vent line is open to the air cleaner and shouldn’t allow pressure buildup in the tank. I am curious as to how the vent valve works. I removed it and it passes air in either direction, more when applying pressure but will flow vacuum also. The valve doesn’t seem to seat. Vin is looking for a valve for me.

CT

[rickf]

If the vent valve you are talking about is the one that has the line going to the carb I am pretty sure it does not close. This has been an argument that Ken and I have had for years but I am pretty sure that line both vents excess pressure from the tank and also lets in air for driving with the fording valve closed. The only time I could see it closing is in a rollover as in the vent valve in most cars will do. A simple ball and seat setup. upside down the ball lands on the seat. But in normal driving it is open all the time. If you had a full tank of gas and had to ford a large area and that vent did not let in air you would die in middle of the fording. We have watched videos of the guys from the Vietnam Jeep Club swimming around for 20 minutes under water with nothing but the windshield above water. That gas tank was getting air from somewhere and that vent line was the only place available.

[Cav Trooper]

Rick, that sounds reasonable. I just can't figure out why the tank had pressure in it and gas bubbling up around the breather valve screws. I took the valve off and the gasket holes are larger than the screws. Since the screws thread directly into the cover and the threaded holes are not encapsulated, I can see why the gas was able to perc out of them and past the gasket. All of that doesn't indicate why it was doing that though, or why the tank had pressure in it. The tank is about 3/4 full and of course the fuel sloshes around and would wet the screws protruding through the cover. Just another puzzle. I'll keep digging. I reinstalled the valve and used loctite thread sealer on the screws (gas and oil proof stuff) and high tack gasket cement on the gasket. The vent line to the filter canister is open, I removed it and was able to blow air through the lines. So ?? unless the valve was stuck closed. I'll drive it some and keep an eye on it until I can get another valve to replace it. I'll put my old cap back on also, I replaced the old cap only because it did not have the ears on it to open it easier. All of your help is greatly appreciated from an old tinkerer with a new toy.