That is the way it is supposed to be.
vapor's cannot get out to the atmosphere with the cap on.
Gas tank pressure
- Thread starter Carmine
- Start date
vapor's cannot get out to the atmosphere with the cap on.
I know on a 65 impala that I have the cap is not supposed to be vented as it has a port with a tube that comes out of the tank kinda by the sending unit but anyways I put a new tank and cap and I was missing the vent so I plugged it so it would dump gas all over and the first time I ran it for an extended time I stopped to fill it up and when I twisted the cap it had a lot and I mean a lot of suction in there and the tank made a pop sound like it was sucking in. I know the difference in tanks as I have 2 63's also and don't think they would do that but a 65 66 tank is big and more flat so it's more prone to this.
It could have been all in my head as my wife says I all kinds of crazy up there!
Not sure on 65's ,but i think that tube is for when filling the tank it allow air pressure to release from tank as you are filling because it is laying flat, it will puke back up while filling if the tube was not there.
Oh that makes since ,also can't believe I never thought of that and I have owned 65's since 1986.
That is the way it is supposed to be.
I guess the EPA didn't think of all those vapors coming out when you take the cap off!
The cap will be vented to the inside. It will draw in but not out.That is the way mine is. You are trying to reinvent the wheel Carmine!!!!!
Then I am totally confused again. Why would it want to draw in?? Why wouldn't there be a simple passage way in the cap to allow pressure to escape?? Or maybe air in to equalize the pressure from the fuel sloshing around and the draw of the fuel pump?? It doesn't make sense to this novice as to how a large pressure build up can be a good thing. To what purpose does that serve?? Just seems to me that the pressure should be released as it occurs. My pressure is definitely outward. Regardless, I certainly have a long time to think about this, Carmine.
Just reread thread #18. Guess I understand it a little better, but not much.
I miss wrote this explanation ....THE OLD CARS ARE JUST THAT WAY,,,, they are not like the new car's .....
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Carmine, take your cap ,you should be able to put it to your mouth (if you want to )
and suck on inside of cap and draw air in if it is vented ,BUT you will not be able to blow outward's. So the tank will draw air in as gas level goes down, but no vapor's can escape . So there are expanding vapor's that cannot escape the tank. Do you understand now???
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Carmine, take your cap ,you should be able to put it to your mouth (if you want to )
I do, but I'm just not comfortable with that kind of pressure in the tank. If that's how it works and the cap/tank are designed for this, then so be it. Thank you, Carmine.
The check valve in the fuel pump should hold the pressure from getting to the carb.
The pressure in the tank will be above the fuel level so it will have some liquid pressure in the line going forward, but as James said above the fuel pump will not allow it to go any further.
You get a change of pressure with a change of temperature, heat expansion, cold contraction. Most of the serious pressure in my opinion only happens if you have a non vented gas cap and you empty the gas tank. Regular heat and cold, not that much in comparison. But then I don't live where it gets above 100F, we do get it in the 90's though.
If you left the gas tank cap off, over a few months you would lose a lot of the tank of gas.
Carl 1962
Well Known Member
As some have said this is pretty normal for these old Chevy's even with the vented cap which vents for negative pressure. My 62 (with the proper vented cap) can build up a really high positive pressure if the tank is low and it's a hot day. This is because there's a lot of air volume in an empty gas tank. You usually only notice the high positive pressure when you take the gas cap off at a gas station when you need to fill up again. This is usually when the tank is nearly empty so there's a lot of air volume, and the hotter the day the higher the air pressure in the tank. It has never affected the performance of my car, I've never had any leaks from it and my original gas tank has never split any seams or anything drastic so I don't worry about it any more. If you're really worried about it, the only way I can think of to keep the positive air pressure down is keep the tank near full so there's not much air in there to build up pressure.
I see I'm not the only one with this issue. The above is kind of identical to mine. Actually, the only time I take the cap off is when I fill it. I usually let it get to about 1/4 tank or a little less. That's when I experience a lot of pressure. Since it seems to be the norm, time for me to leave it alone and move on. I have other things (oil consumption) that need my attention, Carmine.