Fuel Tips: What Happens to Gasoline as It Ages?

By Staff Writer
Jul 17, 2021 | Fuel Tips | Posted in Features | From the Nov. 2020 issue | Never miss an article

Sponsored Content Presented by Sunoco Race Fuels.

Like many things in life, gasoline isn’t forever. As gas sits around, parts of it evaporate and can escape the fuel container—and we say “parts of it” because gasoline is a fairly complex substance. 

Gasoline is made up of aliphatic and aromatic hydrocarbons—let’s call them open chains and closed rings. The light ends, technically known as short-chain aliphatic hydrocarbons, evaporate first, leaving behind the long-chain aliphatic and aromatic hydrocarbons. You don’t need to own a lab coat to understand the results: A fuel that’s light on short-chain hydrocarbons is harder to ignite because it needs a higher temperature to vaporize enough fuel to support combustion, explains Zachary J. Santner, technical specialist at Sunoco Race Fuels. 

Scientists have developed antioxidants to protect gasoline from undergoing the process of oxidation. Oxidation can lower the octane rating and increase the amount of gum in the fuel. However, these antioxidants are consumed as they react with oxygen. Regular-grade gasoline is designed to be used quickly and doesn’t have the high-antioxidant package found in high-octane gas. On the other hand, Santner notes, race fuel’s expensive antioxidant package gives it the longest shelf life in a sealed container—up to three years in some cases.

As gasoline evaporates, it also leaves behind gum, a technical term for the varnish residue that can gradually build up inside a fuel system. “There is a spec for how much dissolved stuff can be in fuels,” Santner continues. The standard test for pump gasoline allows 5 mg of gum per 100mL of gasoline, he adds. Meanwhile, Sunoco’s race fuels, for example, contain a tenth of that. 

Another issue: The moisture inside the air that we breathe is attracted to the ethanol found in most pump gasoline. Water does not burn nearly as well as gasoline. 

Why are the aging issues related to fuel not a more common topic? For one, most gasoline is consumed rather quickly: People tend to fuel up and then drive around. Those of us who let cars sit can mitigate problems by storing fuel in a tightly sealed metal container and choosing the right type of gasoline—either an ethanol-free or a highly refined, high-octane product.

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Carsandbikes Reader
1/5/21 3:11 p.m.

I thought that I was taking fairly good care of my latest lawnmower.  Last year, at the end of the season, I ran as much of the gas out of the tank that I could before shoving it into the shed.

This year it sat for nearly 2 months and before shoving it in the shed I tried to start it.  That was enough for any of the flammable ingredients to evaporate. 

damarble New Reader
1/5/21 10:36 p.m.

I just fixed a car that's been sitting for 2.5 years, it started up on the old gas and ran fine. Very surprised. 

rslifkin UberDork
1/6/21 6:59 a.m.

Modern cars have the fuel systems sealed up pretty tightly, so gas lasts a lot longer than it does in a more open environment (like most small engine tanks).  That said, with stabilizer added right at purchase, I've never had any equipment fail to run perfectly on year-old gas. 

frenchyd PowerDork
1/6/21 11:18 a.m.

In reply to rslifkin :

I've got 5 gallons of VP12 that's probably 25 years old. Sealed can never opened. But what to do with it? Is it hazardous waste? Can I blend a little ( quart) at a time and burn it in my daily driver? Suggestions please. 

Vajingo Reader
1/6/21 12:18 p.m.

In reply to frenchyd :

Bonfire from hell

rslifkin UberDork
1/6/21 12:25 p.m.
frenchyd said:

In reply to rslifkin :

I've got 5 gallons of VP12 that's probably 25 years old. Sealed can never opened. But what to do with it? Is it hazardous waste? Can I blend a little ( quart) at a time and burn it in my daily driver? Suggestions please. 

Open it up.  If it still smells and looks like it's supposed to, I'd blend small amounts into gas to use.  However, if it's leaded, don't burn it in anything that has O2 sensors or cats, as you'll destroy them pretty quickly. 

Vigo (Forum Supporter)
Vigo (Forum Supporter) MegaDork
1/7/21 1:37 a.m.

Since ive picked up, parted and scrapped out sooo many long-dormant cars I've learned a lot about old fuel. I think one of the most formative moments was when i had a scrapper come pick up a car and he pulled the gas tank right then and there and dumped the whole stinky thing into his tow truck's fuel tank. 

Mostly what I've learned is that almost all 'bad' gas is 'blendable' with good gas and the only trick is not going too far! I've still honestly had more problems with water accumulation in fuel than with 'non water-y' fuel causing issues. 

Main disclaimer: I never run any old fuel in any boosted engine ever and whenever i start messing with one of my dormant turbo cars again i immediately pump out the old gas and refill with new. 

What's kinda funny is that this whole thing about figuring out what you can get away with dumping old gas into running cars was sort of by necessity because it seemed like the least harmful way to dispose of it! 

I thought it turned into parts cleaner and fire starter. 


AAZCD (Forum Supporter)
AAZCD (Forum Supporter) Dork
7/17/21 10:43 a.m.

In reply to Vigo (Forum Supporter) :

I recently drained about ten gallons from an F-250 that had been sitting for almost five years and another five gallons from an Audi TT that had been dormant for at least a year. Both had a very orange color from varnish and some rust. I won't put it in a car, but after filtering it, it burns just fine in my riding mower mixed with a little of the fresh stuff. I'm mowing neighbor's yards now just to dispose of the stuff. ...I need an empty can for the 924.

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