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gas tank pressure?
#1
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David Grubbs
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Most of the snow finally melted today, so I took the 39 120 out for a test drive, having messed with the timing. I usually time it by ear, advancing it until the engine hesitates a moment before starting, then turning it back a bit. Drove it for about 20 minutes and it ran nicely after the one stuck valve finally opened up and it quit back firing. Drove it to the gas station and removed the gas cap and heard a "pop" as the tank released pressure. The cap is a new one. Do I need to switch back to the vented version? My dumb assumption is that if the cap wasn't breathing, the tank would have a slight vacuum, not an overprice. Ideas?

Posted on: 2014/11/28 19:10
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Re: gas tank pressure?
#2
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HH56
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If it's a Packard tank with just the single feed line to engine it will need the vented cap. If there was pressure, wonder if the gas is so volatile or tank close enough to exhaust the heat or sloshing around caused gas to expand & trapped vapors were freed.

Posted on: 2014/11/28 19:26
Howard
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Re: gas tank pressure?
#3
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Owen_Dyneto
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Dave - are you absolutely certain it was a release of pressure you heard and not the sound of air rushing into the tank? Without a vented cap you'll create a vacuum in the tank, I've seen cases where it's enough to actually begin to collapse the tank. It's pretty hard to conceive of a practical situation where the tank would pressurize as you've described.

Funny story. I had a friend with a 57 Plymouth. He filled the tank and lost his gas tank cover so he installed a non-vented cap, not knowing the difference. Drove a couple of hundred miles and refilled - it only took a few gallons so he thought he was getting 50 or so miles per gallon. Drove about 50 miles and ran out of gas. The tank had collapsed top down nearly to the bottom and also sprung a couple of leaks at the seams.

Posted on: 2014/11/29 0:10
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