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Approximately how long does it take for unleaded gasoline in the tank and fuel lines to become unusable in a car that is not being used? What causes this, and what can be done to prevent or remedy the issue?

UPDATE: The car that prompted this question ('96 Lincoln Mark VIII, parked for about 6-9 months without preparation) has started successfully. The engine really seems like it doesn't like the gas, but it runs.

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I'm not sure if it is related, but I've had gas in a gerry can for ~6 months that will still start a fire just fine :) – alex Mar 8 '11 at 4:49
@alex I'm sure you can start a fire with the stuff that is old enough to plug up a fuel system, too. – Mark Johnson Apr 12 '12 at 4:54
Just a thought, when you store something with a motor ( rv, boat, car ect.), if your able to drain the tank and use it for your car or truck. Leave the tank dry for long storage. Don.t forget to empty the carburetor, if it has one. – user11026 Jun 27 '15 at 3:13
up vote 22 down vote accepted

I left my Mazda Protege unused in my garage for over a year, and the fuel filter was clogged when I went to start it again. It started fine, but would not rev and was basically undriveable. I'm sure it wasn't great for the fuel injectors, too. Also, I once bought a motorcycle with 10 year old gas in the tank, and the bike wouldn't run at all. I could hear a cylinder occasionally fire, but the gas was totally unusable. I poured it into a metal bucket and lit it on fire. The carbs were very badly gummed up and clogged, too, and had to be torn apart and soaked in carb cleaner.

If you use STA-BIL you can store gas for over a year without issue. Just make sure you run the engine for a few minutes to circulate the STA-BIL through the car. Once gas goes slightly bad, you can still use it if you put a little (1 gallon) in an (almost) full tank of fresh gas. The stuff I pulled out of the motorcycle was much darker than normal, and smelled so bad that I didn't even consider using it in an engine.

If you're planning to store a vehicle for a very long time, it makes sense to spend the time to totally drain the fuel system. It's cheaper than replacing the fuel system, later on, due to dried up, gummy gas deposits.

Here is some info I found on the STA-BIL website:

Q: How long will STA-BIL Fuel Stabilizer keep fuel fresh?

A: For 12 months when mixed into fresh gasoline. Doubling the dosage will keep fuel fresh for up to 2 years.

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So, what about a quadruple dose - will that get me four years? If I just fill my tank with straight STA-BIL, will it be good forever? ;-) – Iszi Oct 24 '13 at 13:28
Marine stabil plugged up our carbs and fuel tanks.. bad stuff. – user9771 Mar 15 '15 at 18:15

It really depends on the storage conditions, but The Straight Dope suggests a storage life of 6 months to a couple of years.

The biggest issue to worry about is that the gas degrades and leaves deposits or gums, particularly in sensitive areas like injectors or jets. Another issue is that if accumulation of water in the tank. If the tank is kept full, there is less space inside it to collect condensation, which can help decrease this effect, but it also depends on the humidity where the gas is stored.

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The octane rating is a key factor, too. Premium lasts longer than regular. – Mark Johnson Apr 12 '12 at 4:55

Stabil is not recommended in ethanol blended fuels as it causes a chemical reaction corroding brass in carburetors and speeds up the process at which ethanol mix gas breaks down fuel lines in vehicles made 2006 or older. All auto gasoline in the US contains a 10% ethanol mix as of 2007.

A version of Stabil was released that was supposedly ethanol friendly but it in turn sped up the process of corrosion and breakdown in older vehicles. So, the best solution is to just drain your gas before you leave the vehicle parked for an extended period. Just about every car has a drain plug in the gas tank. Simply take the plug out and drain it. Of course, make sure to capture it in a container appropriate for gasoline storage so you can either use it in something else or have it disposed of properly.

Once the tank is drained turn your key to the key on/engine off position for 5 seconds then turn it off and repeat about 3 times. This will clear the fuel pump and most of the fuel line of any fuel.

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for lawnmowers and power equipment gas can go bad in as few as 30 days. always drain your tank in winter and or add a stabalizer which will prolong gas life up to 12 months.

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Why are lawnmowers and power equipment so special that fuel deteriorates so quickly? – theUg Mar 15 '13 at 13:04
@theUg My first guess would be due to the smaller tank the chemical reactions happen faster, proportionally speaking, than they would in a larger tank. Also, smaller engines probably have less tolerance for bad gas in general. Would be nice to see more detail added here to explain. – Iszi Oct 24 '13 at 13:21
I cannot generalize, but what you say is not true with our lawnmower. It has sat in the shack September-May for each of the last 12 years, and it starts with no issues every May. – Martin Argerami Jun 22 at 5:45

You can use Mix - n - Go after you get the car out off storage or begin to drive it again. When you put it into storage the ethanol starts separating with in weeks. Lucas sells a new blend for storing cars. Make sure for any storage blend that not only do you put the stuff in but that you run the car so that it gets into every single line and injector / carburetor.

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