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Old 03-18-2008, 05:52 PM   #1 (permalink)
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Gasoline loss through evaporation?

I currently have an open vent hose running directly from my gas tank to the air cleaner. My concern is that when the car is just sitting, I could be losing gasoline through evaporation. I've been plugging up the end of the hose when I have the car sitting (I'll soon be getting a valve for a more permanent solution). But does anybody think I could lose a significant amount of gas while it is open (some days I forget to plug the hose)? I know that gasoline is volatile, evaporating at low temperatures...but I don't know if the losses would be enough to actually be noticable or if it would be significant.

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Old 03-18-2008, 07:43 PM   #2 (permalink)
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I don't have any numbers to quantify the losses but I do know they can be quite significant and definitely measurable. If you can smell the gas you are losing fuel. Really, the fumes actually burn much better than the liquid too.
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Old 03-18-2008, 08:29 PM   #3 (permalink)
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I can't give any numbers either, but if you want a baseline: gasoline evaporates faster than water.

The rate will probably depend on temperature, humidity, amount of air in the gas tank, surface area of gasoline exposed (i.e. tank geometry)...

I suppose plugging the line may work, but you'll have to deal with increased pressure. If your tank doesn't generate pressure when the line is plugged, that means it is being vented somewhere else.

Modern cars use activated charcoal canisters to trap gasoline as hot air is expelled. I would look around a junkyard and retrofit one of those canisters. I can't imagine it costing more than $10 in parts. There are schematics online showing different types of setups.

Lastly, preventing gas from evaporating helps lower pollution a bit by decreasing HC's and VOC's. I like my skies blue and lungs healthy...

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Old 03-18-2008, 11:53 PM   #4 (permalink)
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hmmm. My ZJ never has built up pressure in it's fuel system... And That probably explains why it drops so much in the fuel gauge when I turn it on in the morning...

I noticed my friends geo always builds pressure up and it releases when you open the gas tank. hmmm... noticing a trend here?
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Old 03-19-2008, 07:37 AM   #5 (permalink)
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I once read somewhere that a farmer could lose as much as 1% of his above-ground storage tank's capacity on a hot day.

Rather than plumb that vent through the air filter, shouldn't it go to the evaporative canister, the one that catches vapors from the crankcase? Volatile vapors that get cooked out of the crankcase oil get caught there and burned; seems to me that if that's the job it's for, that's a perfectly reasonable place for the fuel tank vent to go, too.

If it isn't plumbed for that already, you may need a larger evap can to deal with the greater volume.
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Old 03-19-2008, 11:15 AM   #6 (permalink)
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Most (newer, but not new) cars have a Charcoal canister for this purpose, they vent the fumes to intake when the car is running. There is a Temperature controlled VSV on the Supra that allows venting only when the engine is abouve about ~160deg coolant temp. May be an option for you, I'm sure I have a spare if you would like to try

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