How long is Gas Good in a Lawnmower?


You have been using your lawnmower to trim your lawn or yard throughout the mowing season, and now you want to winterize it. Or you have another reason why you do not use your mower for some time. But there is some leftover fuel in your gas tank, and you don’t know if you should drain it or let it stay there so that it could be used again once the next season starts. Gasoline is one of the most common fuels used in lawnmowers. It is easy to find and fill with the real advantage that gas-powered mowers still provide longer cutting time than the electric ones. But gasoline sitting in the mower’s fuel tank becomes harmful after a certain period, and that’s why you should not store your mower with gasoline sitting in its fuel tank.

How long is Gas Good in a Lawnmower?

The amount of time that gasoline can last before going bad in your fuel tank depends on certain factors, such as the type of gas and the contents present in it.

  • Pure gasoline can stay ok in the tank for as long as 6 months.
  • Gasoline with ethanol goes bad earlier because of ethanol’s oxidizing tendency. Ethanol blended gasoline can last in the tank for up to 3 months.
  • Stabilizers can be added to gasoline to extend its shelf life. Stabilizers prevent the gas from oxidizing or evaporating. Gasoline with stabilizers can last for as long as 1 to 3 years, depending on the stabilizer’s type, quality, and strength.
  • It is important that gasoline should be stored in tightly shut, sealed containers. Usually, you can find information in the manual or on the tank itself on how to store gas and how much gas to fill for your mower’s fuel tank.
How long is Gas Good in a Lawnmower? 1
(CC BY 2.0) by Robert Couse-Baker

Now that you know that gasoline cannot be stored in the fuel tank forever, you should also go through the details of why and how it goes bad after a certain period of time and the adverse effects bad gasoline has.

Why and How Does Gasoline Go Bad?

Gasoline can go bad because of several chemical phenomena, e.g., evaporation or oxidation. The chances of these phenomena happening and the speed with which they proceed depend on gasoline contents. The details of how and why gas goes bad because of these phenomena are given below.

● Evaporation:

The proper combustion of gasoline in the combustion chamber of your mower’s engine depends on its chemical and physical properties. The correct proportions of its contents and weights etc., play a vital role in the combustion process. Even a slight change in these properties can adversely affect the combustion process.

The evaporation of a pure substance and a mixture are different in that the mixture’s contents evaporate at different times and rates depending on their own nature. For instance, if there were a mixture of two liquids A and B, and A was more volatile, it would evaporate first. The same things happen with gasoline that has other chemicals blended in it. The more volatile components evaporate first. The bad thing is that the lightest components of the mixture evaporate first, which makes gasoline heavier. As said earlier, a change in gas properties affects combustion. The heavier gasoline does not combust properly. This means that the engine will still run but not properly.

● Oxidation:

Oxidation is the chemical process that occurs when chemicals react and combine with oxygen to produce new compounds called oxides. For example, rusting is a form of oxidation that takes place in the presence of moisture. Oxidation of gasoline is an even more serious issue than evaporation.

Oxidation of fuel takes place as hydrocarbons present in the fuel react with oxygen to produce new compounds. This results in the production of a semi-solid called gum, which can create deposits all over the fuel system and clog filters and fuel lines. Gum makes the affected components malfunction, causing the mower not to run smoothly or not even start at all.

Gum can also affect the carburetor very badly. Gummed-up carburetors won’t work unless they are fixed, and fixing them is quite expensive.

● Ethanol:

Gasoline these days takes much less time to expire than it did some years ago. This is because of the introduction of ethanol in the fuel.

The Environment Protection Agency implemented the use of ethanol in the fuel in an attempt to make the fuel more environmentally friendly. Ethanol is a type of alcohol that is derived from corn. Nowadays, ethanol takes up to ten percent of the 87 octane gasoline. The addition of up to ten percent ethanol made the fueling environment friendly and cost-effective but created a problem for small gasoline engines. Remember that the gasoline engine of your lawnmower is also a “small” engine.

The problem with ethanol is that it draws moisture from the air as it sits in the tank in the garage or the shed. This moisture waters down the gasoline. An engine with this watered gasoline in its combustion chamber will not start, stutter, or die during the operation.

The problem with small engines such as those in lawnmowers is that they are used only for a short time after several days. After use, leftover gas keeps sitting in the tank until the next use. This causes exponential growth in the chances of your gasoline reaching its expiration date. Once the gasoline expires, it causes all sorts of problems for the components of your mower’s fuel system. Fuel lines, carburetors, and seals deteriorate. A varnish buildup caused by expired gasoline can cause small fuel pores to clog. Clogging of the fuel ports will make it impossible or at least almost impossible to start the engine.

Gasoline with ethanol can last for a maximum period of 3 months. However, we advise you to consume it before that period. Using fresh fuel is very important for the health of your machines.

● Water:

Water can buildup in the tank that you are using for storing gas. Water buildup occurs due to condensation because of temperature changes over the period of storage. Water buildup degrades the gas and makes the proper combustion of gas impossible.

Reusing Old Gas:

Although reusing old fuel is not a good practice, it can be done in some cases, but the gas must be treated first to make it less harmful. Old gas can be reconditioned by passing it through nylon and cotton cloth and adding fresh gasoline to it in a certain ratio. This can make the old gasoline reusable, but reusing old gasoline is not good practice, and we don’t recommend it.

Another important thing to remember as far as reconditioning and reusing old gas is that it can be done only for engines with carburetors and not fuel injection systems. Furthermore, this process requires special care and can’t be done by everyone. Seek the help of a mechanic if you wish to reuse old gas by reconditioning it.

Using a Fuel Stabilizer:

If you intend to store fuel in a tank for more than 30 days, you should mix it with a fuel stabilizer. Using the right fuel stabilizer and mixing it with the fuel in the correct ratio will allow your fuel to sit in a tank without the dangers of pitting and gumming for more extended periods than if no stabilizer is used.

The ratio of fuel stabilizer to gasoline depends on the stabilizer brand. For correct info in this regard, you should contact the stabilizer’s manufacture that you intend to use. An important thing to remember is that the fuel stabilizer ratio depends on the stabilizer brand and not on the mower’s model or brand. For example, some stabilizer brands recommend an ounce of stabilizer for a gallon of gasoline, while others recommend an ounce for 2.5 gallons of gasoline.

Another important thing to note is that the fuel stabilizer works only with fresh fuel. You cannot add a stabilizer to old gasoline and expect it to refresh.

Fixing a Mower Affected by Bad Gas:

If your engine isn’t starting right or is causing trouble because you are using old fuel, stop at once and siphon out the old gas. Any built-up residue in the engine needs to be removed too.

If your engine runs for some seconds and then dies even after you have added fresh fuel, it means that the carburetor has been clogged by the thick varnish-like substance that builds up after volatile components in the fuel have evaporated. Use a carburetor cleaner to clean the carburetor and use fresh fuel with stabilizer added to it.

For more details on how to service your mower affected by bad gas, consult the manufacturer or a professional mechanic.

Final Remarks:

Gasoline can be stored only for a certain time before it goes bad. Gasoline goes bad because of additives that evaporate or oxidize, degrading the fuel. Our advice is to always use fresh fuel, preferably with a stabilizer added to it in the correct ratio. When storing your mower for the winter, drain and dispose of the fuel instead of storing it with fuel. 

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