How long does gas last?
May 18, 2005 6:03 PM   Subscribe

How long can I expect unused gasoline in a car's gas tank to last before it breaks down into something that the engine can't burn?
posted by nmiell to Travel & Transportation (11 answers total)
I think it really depends on a lot of factors that influence the condensation of gasoline. I've heard anywhere from a month to half a year. If you're worried buy some stabalizer which will prolong it for up to a year.
posted by geoff. at 6:11 PM on May 18, 2005

I've just mentioned this to a petroleum engineer and this is his reply:

Gasoline does not "break down" in a human lifetime, however:

1. You may notice issues if you have a tank full of gas purchased in winter and you start your car up in the summer, or vice versa, as oil companies modify their formulations for a consant viscocity at seasonal temperatures.

2. The most volatile parts of the gasoline may evaporate over a period of months, increasing the "gumminess" of the tank contents, but this isn't likely to cause issues you'd notice unless you fed your car a steady diet of old gasoline.

3. If the tank is not full, you may get water condensing in the tank at night and getting trapped under the gasoline. Since the fuel line draws from the bottom of the tank, this may result in water in your fuel line, which will present a problem.

His recommendation was if the car has been through a lot of dewy mornings, you should add one of those ethanol gas-line additives to the tank, as it will allow the water to form a solution with the gas in your tank and you'll be able to get the water out of your tank, after which the gas, no matter how old, will run your engine acceptably well until you can fill up with some fresh dinosaur soup.

Other problems (like gasket shrinkage, battery charge, or fluid-levels dropping due to small leaks) are probably going to be bigger concerns.
posted by Crosius at 6:38 PM on May 18, 2005

anecdotally: we have a back-up car and have often left it parked in the driveway for up to nine months at a time with no ill effects other than a dead battery.
posted by jessamyn at 6:46 PM on May 18, 2005

I had an old car that I bought and gave to a panel beater, mentioning to him at the time that I was in no hurry.

This was a mistake.

Three years later, I got it back from him, unfinished (it had been sitting in his factory the whole time). It still had the quarter tank of fuel I had left in it at the time, and once I got the car over the initial starting problems, it ran fine, without the need for additional fuel.
posted by tomble at 6:58 PM on May 18, 2005

All this is true. There is one factor, though, that most people don't know about: There is actually an organism that will begin to grow in gasoline that may gum things up. There is an additive that prevents this organism. But it takes years, not months, before anything bad happens.
posted by Doohickie at 7:10 PM on May 18, 2005

We have ignored warnings of gumming up and repeated recommendations to use additives, and have left cars for up to 2 years with no noticeable problems due to the fuel degrading.

The only things we do are to leave them with as little gas as possible so that we can dilute what is left with new gas, disconnect the battery completely, and overinflate the tires.
posted by Geo at 7:56 PM on May 18, 2005

My dad has a boat that's been sitting for 25 years or so. The gas in the tank doesn't even smell like gas anymore. It smells like varnish. There is no engine the boat so I can't say if the gas would even run an engine or not.
posted by 6550 at 8:25 PM on May 18, 2005

I helped a friend rebuild a couple of Vespa's, in Jr. High (mostly I sat on a broken stool and drank Dr. Pepper). The scooters were '74 Sprint Veloces, and had been parked with gas in the tank for twenty odd years. All that was left was some kinda gelatin crap (my money is on napalm). So I'd say, twenty years-- too long.
posted by cosmonaught at 5:56 AM on May 19, 2005

Aging gas is something most people won't see causing a notable issue anymore since everything is fuel injected... unless you're talking boats, generators, motorcycles and lawnmowers. So for the purposes of this question the answer is : you're gonna have 97% of everything else on the car go to hell before the gas causes you an issue.

For your carborated engines there's Sta-bil which you can put in the tank to minimize the problems when you pull that donorcycle out in the spring but the easiest method and the one I used to use for the small outboard on my sailboat was to cut off the supply of fuel to the engine when you're done using it.

For the outboard I would just pull the intake line which was connected on a quick-connect. In about 30-60 seconds it would run out and stall. Once I adopted this system the 3-5 months between runs which used to be a CONSTANT problem which required pulling and soaking the carb stopped completely.
posted by phearlez at 9:45 AM on May 19, 2005

My dad has a boat that's been sitting for 25 years or so. The gas in the tank doesn't even smell like gas anymore. It smells like varnish. There is no engine the boat so I can't say if the gas would even run an engine or not.

Was it plain gasoline to begin with? A lot of boats take a 50/50 gasoline mixture, and that's kind of odd smelling to begin with.
posted by Kellydamnit at 2:14 PM on May 19, 2005

Carbs are the real problem point. The burnable bits of gas will evaporate out of the fuel bowl(s) in the carb leaving only additives that resemble a nice coating of varnish. This will cause all sorts of problems with floats and metering jets. If your car has FI and an operative evap canister your fuel system is basicly sealed and you can get away with using gas that is several years old. Having said that I always try to leave as little gas in a stored car as possible so that I can put as much fresh gas in as possible when I first start it up.
posted by Mitheral at 12:51 PM on May 25, 2005

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