The Stovebolt Site Forums The Shop Area General Truck Talk Vented Gas Caps

What is the function of a vented gas cap on our vintage trucks?

There has been allot of information/misinformation, both on this forum and on the internet on the function of a vented gas cap on our vintage trucks. I’m not talking about a charcoal canister vent or the vent along the gas filler neck that helps vent the air when filling the tank, but just the cap itself.

It has always been my understanding, that a vented gas cap is vented in one direction, “in” to allow air makeup as the fuel pump empty’s the tank. In addition, the cap seals sloshing fuel and vapors from exiting the tank. This theory is supported by some vented caps that give a warning printed on their cap “Fuel Spray Many Cause Injury, Release Slowly”.

If you do a internet search you will get answers from so called experts that are all over the place. From my theory, to venting equally in both directions, to venting “in“ for air replacement and venting “out” for pressure release after a few PSI, like a pressure release valve.

Many of these posts originate from smelling gas vapors inside the cab or outside by the gas cap or a failed gas cap causing a vacuum in the tank, starving the engine from gas. Some ill advice is the drill a hole in the cap so it will vent in both directions which to me, skirts the real issue.

So getting back to my question, what is the function of a vented gas cap and then specifically how it applies go our beloved trucks.

The replacement gas caps which I have purchased in recent years allows pressure to build up in the tank. I do not like that idea. It would seem to lend itself to cause fume leakage into the cab at the sending unit gasket or the hoses on the filler neck.
I assume the design is supposed to be environmentally kind. And maybe it would be for everyone who is not inside the cab...
I drill a 1/8th inch hole in the plastic on the underside of the cap. No more pressure build up, yet no fuel spilling out either. I avoid filling the tank up 100% also.

That has always been my understanding also. That is why running a non vented cap will cause a vacuum to form in the tank while driving and eventually stall out the engine, due to lack of fuel.

Or flatten your tank in & bend sending unit / pick up tube.

I believe the "non vented" caps in question here are usually non vented for pressure but do have a valve to relieve vacuum.

No vented gas cap I've ever owned/used worked that way. In fact, I think a well-vented cap must allow flow in both directions...at least if it is to work properly. But that flow doesn't need to be much. The fuel use of an inline 6 isn't that high. The original cap of the AD was vented. I have one in the attic somewhere, but in Dallas a locking cap is the smart way to go. As for fuel vapors in the cab, my experience has been this has nothing to do with vented/non-vented gas caps although a cap which does not allow pressure to escape to the outside could exacerbate an existing problem. Fumes generally come from one of three (or maybe more) places...a poorly sealed fuel gauge sender unit will do it. Poorly sealed or deteriorated rubber hoses connecting the fill/vent tubes will do it (less often than the sender gasket, though in my experience). And I've seen in a couple of AD tanks a spot at the top (about 3/8 inch to 1/2 inch long) where the pinch weld simply didn't work. It is very hard to see this unless you know just what to look for, but with gasoline it doesn't take much to stink up a cab. As for sucking the tank in and distorting it, I haven't ever seen that happen. My experience has been the engine will stop running first.

Ok, folks...it isn't so hot yet this morning, so I got up into the attic and found the original AD gas cap. The one which GM put on my 1952 when it was new. Please see below 2 images and we can see how this was designed. Image 1 shows the vent provision in the AD gas cap. If I suck and blow on this, air moves nicely in both directions. Feels about like the amount of air I could displace blowing / sucking through a tube of maybe 1/8" to 3/16" ID...not any more than that, however. And if you look at image 2, you will see there's a space between the cap and the brass piece which (along with the rubber ring which fell out when I took this out of the envelope it was resting in) forms the seal for the filler neck. Air escapes in/out from beneath this brass piece. Neat design and hopefully this question is solved.

And out of idle curiosity, I removed the locking gas cap I'm using now to see just how they vented it. If you look below, you'll see a rubber ring seal and if you look carefully you can see a metal piece (ring, actually) inside and below that seal. As it turns out, vapors can go past that ring to the outside in either direction (they seep out on the back side of the rubber ring where a gap exists between that ring and the body of the cap, but the actual filler neck will be sealed against wind, rain, snow, whatever. So, they sort of used the same scheme as GM, but in this case there is no bi-directional valve like the one GM used.

And I should add my wife is fairly horrified to see me sucking and blowing on gas caps here so it may be a while before I get a kiss. But you guys don't care about that, do you?

My old '65, Mean Green, had the side auxiliary tank that was definitely not vented. It also didn't have a sending unit. My gas gauge for that tank was a 3 foot piece of hose I kept behind the seat. Cruising the backroads or around town it never built up enough vacuum to suck-in the sides of the tank. But going on long trips I'd top off both tanks and start the trip with the side tank. And when it was empty, if one was paying attention or didn't have the music too loud, one would hear the tank go "dong". If one heard that and reached down to switch the valve on the floor, all was well, but if one didn't hear it, the engine would sputter and then one knew the tank was empty. I miss having the extra tank, and it will be swapped in as truck gets its makeover.

Seeing these photos and reading the responses, I need to check out the cap on my current truck.

Early '80 G.M pickups had a big air filter housing w/ sections with charcoal between wire mesh built into the housing, filter went around it. Idea was to catch vapors when engine shut off. We would bet the drivers that would suck a tank all the way dry before switching tanks. Backfire & set the fume laden charcoal on fire.

Thanks everyone for participating in this discussion. I now have (and I hope everyone else does also), a much better understanding of the different caps that are/were installed on our trucks.

There is also a cap design that came along after the tree huggers got a case of the vapors (pun intended) about venting a gas tank to the open atmosphere. It's called a "pressure/vacuum" cap. It allows a certain amount of pressure to build up in a tank and force the vapors to a charcoal canister, and it also has a vacuum-sensitive flap to allow makeup air to enter the tank as fuel is used. The problem with that type of cap on an older system is that it can cause enough pressure rise in an unvented tank to overcome the needle valve seal in the carburetor. Under high tank pressure caused by extremely hot weather or a clogged charcoal canister, the pressure/vacuum cap has a pressure override feature to allow venting to the atmosphere. Obviously, a pressure/vacuum cap should not be used on unvented tanks like stovebolts have.
Jerry

Yet, many (if not all) vendors sell them to us...
Hence, my 1/8" hole drilled through the plastic guts of said valve.
Carl

FWIW my 53 210 has an unvented cap however the system is vented. There's a small notch on the filler tube rim that allows the passage of air. The notch is in the 12 o'clock position in the pic below.

That is something you wouldn’t really notice. I have to check my truck when I get a chance!