Fuel tank venting configuration on pre-1970 cars with no evap system?

Just curious, was the only venting pre-1970 the small tube presumably from the airspace at the top of the tank to the little hole near where you put the nozzle in? Or did the gas tanks also have a vent at the top?

I recall they either used vented gas caps or there was a vent in the filler pipe.

And I remember several times where the owner replaced a vented gas cap with a non-vented cap, only to end up having the tank later implode on them.


That space has to be vented somehow, both to allow air in and to allow fumes out. It seems like it would be a bad idea to rely entirely on the gas cap for that function.

Around the transition to modern evap system using canisters, the gas pumps changed to have a pleated rubber gasket, presumably to capture fumes that get pushed out of the tank during refueling. I guess the way it worked, those fumes would move up the vent tube and exit through the small hole in the nozzle area, and from there they moved into the pump somehow. Maybe the pump had a big canister?

Relying on just the gas cap worked fine on millions of cars for years.

You had to remember to not fill up your car to the top, and then park it in the hot sun. If you did, when you got back to your car, there would be a big puddle of gas under your car due to the expansion.
I don’t remember anyone being too concerned about it, other than it being messy.

I took a gas cap apart one time, looked to be from a modern car with sealed fuel system airspace. It had two spring-loaded valves, one to allow air in, and the other to allow fumes out. I recall the fuel-overflow problem w/60’s cars, but didn’t know exactly how it got out. Was the exit path via the valve in the cap?

I still see that from time to time, and it’s usually due to ignorance and sloppiness on the mechanic’s part, imo

We carry both vented and unvented caps at work

And they look IDENTICAL . . . the only visible difference is the part number on the box

So the mechanic has to take the time to determine which is the correct cap for the application

And some guys don’t do that . . .

Interesting. What’s the def’n of a “vented gas cap”? Does that mean it is vented in both directions, into and out of the tank? & in each direction, is it vented all the time (e.g. via a small hole), or only when the pressure reaches a set value (e.g. via a spring loaded valve)?

My 1976 is vented from the cap…

I’m guessing a 76 model year would still have a simple evap canister system. My similar vintage truck has one. The canister has an vent port to the outside air, but I think that’s used only on refueling, the fumes are scrubbed from the vapors pushed into the canister during the refill, and the cleaned-of-vapors air exits from the canister vent port. The canister has a one way vent port in other words. The gas caps I’ve taken apart have air-inlet valves (presumably so air can enter the tank there to replace fuel used when driving) & also have fume-exit valves, which I presume is only used when the canister clogs.

Not for a very long time, it was all disconnected back in the 90’s… lol
But yeah, OE was very basic, mine is also pre lean burn system…

Someone stole my 3 gal gast tank for the little boat. Had to buy a new one. The cap would allow air in,but not out. Tank would build up so much pressure sitting in the sun gas would spew out of the carb. Nice thought,but big fail environmentally.

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Good point. Saying a gas cap is vented is pretty ambiguous. Could be vented in, vented out, or both. And either direction vents could be a spring loaded valve that only vents above a certain pressure differential. Or just a small hole to the atmosphere. The two automobile gas caps I’ve taken apart were vented both directions, both using spring loaded valves. On more recent/complex evap system designs, the evap system could do the venting itself, so the cap could be 100% sealed, unvented. Just exactly how the tank venting is done is a difficult thing to ferret out for some reason. Maybe manufacturers consider the specifics a trade secret.

The fuel line should be switched off or the fuel line coupler disconnected from the tank when the engine is off. There are probably directions with the fuel tank or boat motor explaining proper use.

No directions, was a surprise to me. Just loosened the gas cap and called it a day.

Thinking back to the 50’s and 60’s this was just how it was done. The tank was vented and the carbs, which were often mounted above the exhaust manifolds to allow for faster warm up, had pipes to direct any overflow under the engine. Generally worked OK until the carb floats got stuck and the overflow lines failed dropping gas on the hot manifolds.

Yeah, terrible environmentally but back then the gas was leaded, fuel metering was primative and “lean burn” was considered a problem (vapor lock) instead of a virtue so the regular loss of a few ounces of fuel wasn’t even considered.
So much for “The Good Old Days” :wink:

I think I see why someone stole your old tank. :wink:

These new portable tanks are supposed to eliminate the vapor release issue but in my experience, I spill more gas now than ever, all of the pent up vapors are vigorously released when I open the valve to dispense gas and it takes 10x longer to dispense gas, all the while releasing gas vapors.

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That is just taking it to far, I would drill a hole in it (new portable gas tank) so it works right (breathe)…

Next they will have a Vapor Canister with Solar Powered Purge Valves on every new portable gas tanks you buy…

Heck the last one I bought was a 1 gallon for my Weedeater and it took me a while to figure out how to get it to pour out, I still hate it and I have had it for years… but to cheap not to use it… lol…