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6/25/13 5:33pm - Original Message: 'Vented vs Non Vented Gas Cap'
manget1 Lifetime Member
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Indianapolis, IN - USA

Silver 1974 Convertible - L82 4 speed Low miles (was only 8,900 when I bought it in March 2006) - Now 42K miles. Original owner bought it as an investment for son's college expenses. 2008 Jetstream Blue Convertible - DAILY DRIVER!!

Joined: 9/3/2006
Posts: 2389
In my recent travels, I was driving the car in very hot and very dry conditions, in some higher than normal altitudes. When I stopped to fill up, I got a big rush of air as I opened the gas fill cap. Being a 74, the car has a non-vented gas cap. Besides the big pressure release, the gas in the tank actually appeared to be BOILING! I did a quick google search which told me it was just doing a "heat exchange," and the info I found said that it was no big deal and to just let the vented gas cap do its work. That's great for the person with the vented cap! But what about for me who doesn't have a vented cap? As I've been on this trip, the car has had troubles either when stopping for lunch (not wanting to start right away) or towards the end of the day (not wanting to stay running at idle). I'll be doing more driving over the next few days in similar conditions and just wondering... Is there something I can do short term to improve things? Is there anything long term I should do? Should I investigate a vented cap? Should I release the pressure often? Sorry for the "run-on" "no paragraph" text, but this is how things come out on the iPad...

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6/25/13 8:33pm - Reply: 'Re: Vented vs Non Vented Gas Cap'
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Zachary, LA - USA

1974 L-48 Auto. Just your basic Vette. GM crate motor w/vortec heads and intake. Crane cam. Scorpion rockers, Edelbrock carb. Lots of parts still in boxes... :(

Joined: 1/27/2013
Posts: 201
I think your tank should be vented through the charcoal canister, in the engine bay, under the windshield washer reservoir.  There is a vent line that runs from the tank along the frame rail to the engine bay, and that line can get plugged with rust and mung.  Disconnect it on both ends (tank and canister) and blow compressed air through it to make sure it's clear. 

The idea was that vapors from the fuel tank would be stored in the charcoal canister, then at warm engine temps a vacuum line would suck the fuel vapors out of the canister and burn them in the engine.  This (presumably) kept the fumes from stinking up your garage, like old cars used to do, as fuel evaporated slowly in the tank.  Not that we've ever known an old Corvette that stunk a little bit.  :)

|UPDATED|6/25/2013 5:33:07 PM (AZT)|/UPDATED|


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6/25/13 10:15pm - Reply: 'Re: Vented vs Non Vented Gas Cap'
Adams' Apple Lifetime Member
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Duncanville, TX - USA

#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

Joined: 11/8/2003
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Ed Zachary..the tank is vented thru the charcoal canister. Chances are the metal lines to/from the tank are stopped up, creating the pressure inside the tank. There is a relief valve built into the top of the tank, and it will pop if the internal pressure gets too great. Bad news is, once it pops, it's popped...no fixing it.
For now, I would suggest leaving the fuel cap a little loose. Don't do it with the tank full tho, or fuel will splash out. Wait until you used a few gallons, and then loosen the cap.
The metal lines may need to be replaced, if you can't get them blown clean. Also, the valve on the canister might be messed up. I would first take the hoses loose on both ends of the metal lines, and blow thru them, and see how that works.

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