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Old 01-29-2007, 10:13 AM
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Gas Tank Venting - Help


I'm redoing the fuel lines from the rear mounted gas tank to the engine mounted fuel pump on my 72 chevy and I have a question.

I cut out and removed all the emission and vapor canister lines that connected to the fuel tank because I didn't want them in the way and they were probably trashed and rusted through anyway.

My question is - Is the fuel tank supposed to be vented to atmosphere somehow? (probably through some emission device right?) If the fuel filler cap is air tight, which it is, and the fuel line will be sealed up as long as the floats in the carb are at level, where does the pressure go inside the tank when the temperatures rise and the air expands. Is there supposed to be a pressure relief strategy or what?

I will begin the fuel line portion of my upgrades next weekend and want to plan ahead. I need your help.

Please explain.


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Old 01-29-2007, 10:23 AM
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My '72 El Camino has a very convoluted system. The previous owner(s) chopped it up for some sort of racing application, but I was able to restore most of it. I don't know if your '72 is the same, but the charcoal cannister is/was part of the venting system. I would suggest at least a vented cap, provided gas doesn't slosh out when full if the filler neck is in the rear.
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Old 01-29-2007, 12:08 PM
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Instead of a vented cap allowing vapors into the atmosphere, the tank had a line to a charcoal canister which collected those vapors. Upon startup, a solenoid opened allowing the engine to suck in the vapors from the canister. The solenoid venting system allows proper pressure balance inside the tank.

Old cars, especially those with the filler neck behind the license plate, had a line running higher than the tank under the body for venting rather than a vented cap which would have allowed fuel spillage during rapid acceleration.

You should consider using the original tank vent line to an elevated vent filter. You can use a common inline fuel filter on the end of the vent line to prevent injestion of dirt, etc. into the fuel tank. Point the open end down for water protection.

On a hot day you will smell gas vapors around your car.

Last edited by xntrik; 01-29-2007 at 12:24 PM.
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Old 01-29-2007, 08:25 PM
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I vote for getting another carbon canister and hiding it under the truck somewhere (not like you don't have the room under there). Other then the obvious fuel economy/emissions reasons the carbon canister is the vent for the gas tank as you discovered. If you are going to do without it be sure to install a roll-over check valve in the vent line. Otherwise if you ever (God forbid) get the thing on it's lid you have a much higher chance of going up in flames. 70's trucks had so little safety equipment as it was...don't go ditching the few that it did actually have Because I am cheap...a carbon canister will most likely be less money then a roll-over valve.

Roll-over (or tip-over as Summit calls them) valves are made for racing and are therefore will need adapters which can be $$$ in themselves.

Last edited by Triaged; 01-29-2007 at 08:31 PM.
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