Gas Tank Ventilation - Impala Tech
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post #1 of 14 (permalink) Old 06-23-2010, 01:46 PM Thread Starter
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Gas Tank Ventilation

My 65 Impala has a non-vented gas cap, as it should. It has a vent tube at the top front of the tank which uses a tube that extends up through the trunk floor, looping back down and venting outside, under the car.
What I don't understand is that when I remove my gas cap, I get an extreme amount of air sucking back into the tank.

Does anyone know if that vent tube has a flapper valve inside the tank and is possibly only meant for expansion within the tank to relieve pressure? It sure doesn't help the tank draw air in when needed, resulting in what I'm seeing when I remove the gas cap.

I checked my service manuals and it doesn't totally explain the function of the vent tube, but does confirm having a non-vented cap.
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post #2 of 14 (permalink) Old 06-23-2010, 05:08 PM
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Quote:
Originally Posted by palalt View Post
My 65 Impala has a non-vented gas cap, as it should. It has a vent tube at the top front of the tank which uses a tube that extends up through the trunk floor, looping back down and venting outside, under the car.
What I don't understand is that when I remove my gas cap, I get an extreme amount of air sucking back into the tank.

Does anyone know if that vent tube has a flapper valve inside the tank and is possibly only meant for expansion within the tank to relieve pressure? It sure doesn't help the tank draw air in when needed, resulting in what I'm seeing when I remove the gas cap.

I checked my service manuals and it doesn't totally explain the function of the vent tube, but does confirm having a non-vented cap.
That alleged vent tube is in fact a liquid-vapor separator designed to prevent fuel surging up the intake tube and having a water hammer blow your needle off the seat. You will need a vented cap unless you have a charcoal filter under the hood sitting on the drivers side with three tubes connecting the canister to the car and the motor you need a vented cap.

I don't know what year manual you are reading, but you should have a vented cap. You can actually crush your gas tank enough for it to fall out of the car. As the fuel pump will draw enough of a vacuum to crush the tank just like Mr. Wizard’s science experiment on TV when he sealed a heated can and took it off the stove. Air pressure crushed that can as it cooled and the pressure fell inside he can.

Big Dave
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post #3 of 14 (permalink) Old 06-23-2010, 07:06 PM
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Both my 65's have non vent caps on them and they are original. Sometimes in hot weather and a full tank of fuel it acts up as you describe.

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post #4 of 14 (permalink) Old 06-23-2010, 08:31 PM Thread Starter
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Larry, I agree. The factory chassis repair manual says hardtops, sedans and converts have nonvented caps. Station wagons have a vented cap, because they fill from the side, not behind the rear license. If I put a vented cap on, I expect gas to leak out the cap under full tank acceleration times. My vent tube and tank are new and clean so I know it's not clogged. It is 85-90 (and miserable) here in Illinois so maybe it's temperature related. I don't think there is much I can do, other than open the cap often to let air in, if that's what the tank is looking for. I still don't understand why the vent tube doesn't provide full ventilation though, whether expanding or contracting of the fuel is occuring. That's why I think it has an internal flap, to only allow air 'out'.
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post #5 of 14 (permalink) Old 06-23-2010, 08:41 PM
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I think that may be the reason GM changed the design in later years.

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post #6 of 14 (permalink) Old 06-24-2010, 01:40 AM
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My experience with this is I had muddobber wasp nest built about 2" inside the the tube. I disconnected it from gas tank ran a coat hanger through it, flushed out; and blew out with air hose. It worked as it should then. Since then I went with a vented cap and plgged vent fitting at tank. 65's did not come with vented caps
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post #7 of 14 (permalink) Old 06-24-2010, 09:30 PM
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The 65 and 66 (and maybe others) use non-vented caps. If you use a vented cap, every time you step on the gas, it pours out on the street.....

Tom

1965 Impala SS Tahitian Turquoise 427 4 Speed
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post #8 of 14 (permalink) Old 06-25-2010, 12:07 AM
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I never had an issue with gas pouring on the 2 lane blacktop only because I never had enough money to fill her up !
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post #9 of 14 (permalink) Old 02-16-2011, 08:54 PM Thread Starter
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So much for the hot weather theory. My car has been in storage here in Illinois for months (obviously not a warm climate) and today I opened the gas cap and an enormous amount of air sucked into the tank. The car hasn't been started in three months so the engine drawing gas is not causing the vacuum. The tank actually clunked and popped like it was trying to return to its original shape. I think it permanently kinked the tank as one of the straps looks like it has a gap now, not fitting as tight as the other one.

Something is definitely up with the venting on this new tank. I don't want to go with a vented cap and have gas pouring out of the filler tube but I'm confused as to the purpose of the vent tube if it is not allowing the tank to breathe.
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post #10 of 14 (permalink) Old 02-16-2011, 09:37 PM
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So are you saying you have the same problem with 2 different tanks? Defiantly a vent problem.

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post #11 of 14 (permalink) Old 02-16-2011, 11:57 PM Thread Starter
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No, the problem is with the only tank that's been in the car since it was restored. It is a new tank.
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post #12 of 14 (permalink) Old 02-17-2011, 07:46 AM
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So I guess you have checked the vent assembly that goes up through the trunk for being plugged or bent?

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post #13 of 14 (permalink) Old 02-17-2011, 10:56 AM Thread Starter
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Yes, I ran a wire through the tube and blew air through it before I installed it. That's why I originally asked if their might be a valve in the tank part of the tube that only allows pressure to escape and does not provide full breathing, like allowing air to enter the tank under vacuum conditions. I'm half tempted to put light air hose pressure on the short tube connected to the tank, with the gas cap off and have someone feel if air comes out the filler tube.
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post #14 of 14 (permalink) Old 02-17-2011, 11:08 AM
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Well if the problem is not in the vent tube it must be in the tank. I cut a tank in half a couple of weeks ago to take to the schreder. I didnt look at how the vent worked.

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