So I pulled this gem off the back of a 1970 a few years back Here is the thing, I am ditching the vent tube & charcoal canister when I do the mustang gas tank swap and planned on running a vented gas cap. I am looking at this gas cap and note it does not have a little plastic relief valve in the center as I have read that some gas caps have (can be drilled out to get pressure relief in both directions). However, if you look at the third pic, you can see there is a gasket that is off center. It almost looks to me like it is done that way on purpose so there won't be a complete seal and thusly the cap will "vent". (This sounds a little goofy as I write it but I'm doin' it anyway) Did any of the older models come from the factory without a vent tube on the tank and has one of these gas caps that ends up providing venting? Help me out here?
your theory sounds good, but I would think if you have any slosh in the tank it would leak gas at the cap seal... not the same but...some tanks have a valve in the top that is...one way. when air/gas tries to go out, it closes the valve. don't know how hard it would be to drill a hole and install that valve...my 75-77 tank has this valve. I installed a short hose and small filter to it. ...Frank...:Handshake
1970 had a drop in tank like a mustang, Did they have a vent tube ? if its from a 70 and they didn't have a vent then I would say the cap is already vented... Someone will chime in with an answer...
I know the tank in my 70 wasn't vented. Seems like I remember pulling the cap off on hot days and it would release built up pressure. If you really wanted to vent the tank, you should be able to tap into the filler and form a line and run it through the floor (like when you vent a fuel cell). Maybe you could have a bung welded to the filler (as high as possible so gas doesn't get in it when filling) and attach a vent. Some bent the vent in circles but others say that may hold gas and not let it vent properly. I simply ran my vent up as high as I could and over then down through the floor and I attached a small filter so it wouldn't suck up road dirt. The down side is that the way it's vented, you can smell a little bit of gas fumes when the car is parked in the garage. I've been thinking about installing a pitcock in the vent so I can close it off when the car is stored. I now have a vented fuel cell so the next thing on my list is to modify one of my gas caps where it can be bolted on solid. No need to come off anyway and nobody can steal it.
