Can I Add Fuel Stabilizer to Old Gas?

Have you ever found yourself at the end of a summer or winter season and you didn’t use very much of that 5-gallon gasoline can in your lawnmower or generator?  Or maybe you haven’t run your car in 6 months.

If you haven’t added a fuel stabilizer, you’re probably wondering if you should store the gas for a later date, use it immediately, or if it will even work at all.  Is it too late to add fuel stabilizer or is the gas already ruined?

Can I add fuel stabilizer to old gas?

Adding fuel stabilizer to old gas will not harm or decrease the current efficacy of your old gasoline in any manner but it will not restore your gasoline to its original state.  Fuel stabilizers are designed to be used as a preventative measure against evaporation, oxidizing, and the loss of volatile gasoline vapors.

I’m going to cover why your gas goes bad in the first place, why fuel stabilizers won’t restore your gas, and what you should do with your old gasoline.


First, Why Does Gas Go Bad?

There are a few factors that come into play with why gas goes bad, but in general exposure to oxygen is the primary culprit.  The time it takes for your gas to start becoming “old” is about 1 month for ethanol-free, or 3 months for regular. This is also assuming that you are storing your gas in a vented container and that no stabilizer has been added.  

Containers made prior to 2009 were primarily vented containers and didn’t create an air-tight seal.  These were easy to pour, but many people left the small vent behind the handle pulled out. This caused the gasoline to oxidize.  Even if the vent was closed, these containers were not air-tight by any means.

When oxygen is allowed to come in contact with gasoline, the volatile “light ends” that are found in the gasoline will evaporate out into the atmosphere.  These “light ends” are what your engines are designed to run on. Remember, your engines want to run on the vapors of the gasoline, not the liquid.  

Over time, generally 1 month for ethanol-free, or 3 months for regular, these “light ends” will become in short supply and you’ll start to see performance issues on pickier engines.  As time goes on (6 months for ethanol-free or 1 year for regular) you will likely see serious performance issues in most engines if this gasoline isn’t diluted with fresh gasoline.  

At this point, the lighter ends are no longer present and you are left with the heavier ends on the volatility spectrum which will cause your engines to not run as efficiently.  The oxygen in the air will also cause additives in the gasoline to start to turn into varnish and gum up your various engine’s fuel lines, carburetors, and pumps. 

Gasoline stored in self-sealing containers made after 2009 has much more leeway as far as its potency.  Assuming that you created a proper seal after you added this gasoline to the container, stabilizers would be redundant since the gasoline isn’t evaporating the “light ends” or oxidizing. 

I know it’s anecdotal, but I tried an experiment where I filled a 2-gallon gas container with regular and didn’t treat it with any stabilizers.  I verified that I had an air-tight seal by watching it expand and contract with the temperature swings of the seasons and didn’t open it for 2 years. 

As a test, I added it to my lawnmower at the beginning of the season when the fuel tank was empty after being winterized.  The engine ran flawlessly.  


Why Will Fuel Stabilizers NOT Restore Your Old Gasoline?

Fuel stabilizers are to be used as a preventative measure – not to reverse the damage done by failing to use them. 

As we mentioned above, your fuel goes bad due to oxidation.  Once the volatile “light ends” evaporate out of the mixture that is your gasoline, there won’t be any new ones.  The best you can do is dilute some old gas into some new gas to bring up the “light end” ratio a little bit but it will never run as well as when it was fresh.

Keep in mind that there are between 150 and 1,000 chemicals, detergents, antioxidants, and other various additives in your gasoline and they are meant to work together in the designated proportions for optimal engine performance.  If the most volatile compounds are allowed to evaporate, you won’t have the proper mixture for combustion in your engine. 

Theoretically, your engines want to run on only the vapors of gasoline. In reality, most run on a fine mist of liquid gas and vapors. Liquid gas ignites and gasoline vapors combust. 

We want to have as many of the volatile “light ends” as possible to create the most efficient combustion of the gasoline as possible within the combustion chamber. 

The way to keep these volatile “light ends” in the gas is to keep the gas from oxidizing.  This can be done by either storing the gasoline in an air-tight container (those created after 2009), or adding a fuel stabilizer.  

I believe doing both is best.


What do Fuel Stabilizers Do?

The purpose of fuel stabilizers is to inhibit the evaporation, oxidation, and loss of volatile vapors of your gasoline.  This is accomplished with proprietary blends.

Some products like Stabil create a layer on top of the stored gasoline that prevents air from touching the gas and also prevents evaporation. 

Some products like SeaFoam blend with your gasoline to achieve the same effect (and more, in SeaFoam’s case).  

I prefer to stay away from Stabil at this point.  Don’t get me wrong, the product works and I used it for years.  However, it’s best to use this if your gas container will remain undisturbed.  When you disturb it the layer that was formed on top of the gas will lose its integrity and efficacy. 

I prefer SeaFoam so that I can move the gasoline container, or even pour a portion of it and not worry about having to add new stabilizer to create a new protective layer on top.


Why Use a Fuel Stabilizer If My Gasoline Container is Air-Tight?

In theory, your gasoline should be fine in an air-tight container.  

However, I still recommend using a fuel stabilizer before you put it away for storage.

This is because you might have an improper seal caused by a torn gasket, or a gasket that has shifted when tightening the collar around the spout and the container itself. 

If you store your untreated gasoline and this occurs, you’re going to be left with gasoline that is going bad in 1 to 3 months. That won’t be good when you reach for that gasoline in a year to use in your generator during a power outage. 

An easy way to tell if you achieved a good seal is to see if your gasoline container expands in the summer and shrinks in the winter. 

If you’re storing the container at its max-fill line with gasoline, the expansion and contraction will not be as noticeable since the room for vapor is limited and the vapors are primarily responsible for the bloating and shrinking of the container.

Again, not being able to really notice if your container is expanding or shrinking with temperature changes due to storing full containers is another reason to add fuel stabilizers after purchase.

If your plastic gas container is concerning you with its bloating or shrinking, I have an article here that you can check out to see if it’s normal or not.


What do I do with my Old Gasoline?

If your gasoline has been untreated and not stored in an air-tight container, the factors that you need to consider are the time the gas has been going bad and the temperature it has been exposed to. 

Gasoline that is exposed to the air in a hot environment will be thicker, gummier, less volatile, and will leave varnish in your engine and fuel lines. 

Time plays a role too.  The longer it has been exposed to air while untreated, the worse it will be to use.  

I personally have no problem blending old gasoline with new gasoline in a 1:4 ratio if the gas is between three months to a year and using it in any of my engines.  

If the gasoline is over a year but under two years, I would blend it maybe 1:6 or 1:8 with new gasoline and use only in my non-essential engines such as my lawnmower.  

It’s not worth it to me to use up a few gallons of old gas that cost only a few dollars and cause damage to my fuel lines, fuel pump, or more.  

Beyond two years and you will not see me using it in any of my engines.

Whatever your cut-off time is with your gas, you can always take it to your nearest center that will dispose of gasoline and other chemicals. 

Robert Van Nuck

Robert lives in central Michigan and enjoys running, woodworking, and fixing up small engines.

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