How Long Can You Leave Gas In A Mower? Do You Have To Drain It?

There are various reasons why you might want to leave gas in your lawnmower. Perhaps you’ve just finished mowing your lawn and don’t want to empty the fuel tank or you’re storing your mower for a short period while away on vacation. Whatever the reason, it’s essential to know how long you can safely leave petrol in the tank without it degrading.

Leaving gas in your lawnmower between regular mowing during the growing season is absolutely fine as you can safely store it for around a month. Any longer and it would be best to either drain the tank or add fuel stabilizer to prolong the life of the gasoline.

In the following sections, I will look at the factors affecting gasoline shelf life, why old gas should be drained, and the benefits of using fuel stabilizers to prolong the life when it’s stored in your lawnmower.

Leaving Gas In Your Lawnmower During The Growing Season

Generally, it’s advised not to store gas in your lawn mower for more than 30 days. 

That means you can safely leave leftover fuel in the tank when you’re using it every couple of weeks or so to mow the grass without issue. There’s no need to drain it or try and use up the remaining fuel before putting it away.

Beyond that time frame the fuel will begin to degrade or ‘go off’ and could lead to engine damage or uneven running. The specific time frame depends on the factors mentioned below.

Factors Affecting Shelf Life

Type Of Fuel: The type of fuel used in your lawnmower, whether regular or ethanol-free, can affect how long it stays fresh. Ethanol-free fuel generally has a longer shelf life.

Storage conditions: The conditions in which the lawnmower is kept can impact how long you can store it without going off. Make sure to keep the machine cool and out of direct sunlight for the best results. 

Fuel stabilizers and additives: The use of fuel stabilizers and additives can significantly prolong the shelf life of gas in your lawnmower and prevent varnishing or gumming up.

Can You Leave Gas In Your Mower Over The Winter?

While it is possible to leave gas in your lawnmower while it’s stored over the winter, it’s important to follow the manufacturer’s recommendations as their advice often differs.

Some recommend it as a full tank prevents moisture and corrosion building up in the fuel system, others advise that you drain your mower completely after running it with stabilized gas to coat the insides of the carburetor, fuel lines, etc.

If you do decide to winterize your mower with fuel in the tank, make sure that the mower is stored safely in a separate shed or garage well away from your house and any possible source of ignition. 

You will also need to add a fuel stabilizer to prevent problems caused by the gas deteriorating.

If you decide to drain the leftover gas into a can, make sure you store it safely. If you aren’t sure how to do this, you should check out this article

Additionally, there are a number of factors you need to consider when storing your mower with a full tank of gas. I wrote about those in more detail here

When Should I Drain Gas From My Lawnmower?

If you’ve left your mower for a while with gas still in the tank, it’s important to check if the fuel has gone bad before using it to avoid performance problems.

Old gasoline forms a varnish that can coat the insides of the fuel system and gum up the filter and carburetor. This could lead to expensive repairs to restore the mower to clean running.

Signs That Gas Has Gone Bad

Smell: Old gas has a distinct, sour smell that is different from the scent of fresh gas.

Color and consistency: As gas ages, it may darken in color and become more viscous (meaning it takes on a thick or sticky consistency).

Engine performance: If your mower has difficulty starting or has reduced performance, it could be due to old gas.

If you notice any of these issues, or if the mower has been stored for more than 90 days without fuel stabilizers being added, then it’s a good idea to drain the fuel tank and fill it with fresh gas.

How To Drain And Dispose Of Old Gas

Before draining old gas, ensure the mower is turned off with the spark plug disconnected, and that you’re wearing protective gloves and eyewear.

Carefully drain the old gas into a suitable container or gas can, using one of the following methods:

  • Siphon it off using a hose 
  • Disconnecting the fuel line at the carburetor
  • Using a large pipette baster
  • Manually tip the mower up and drain the fuel into a can with a funnel (although, turning a mower on its side can be rather tricky – you can read more about this here)

Dispose of the old gas at a designated hazardous waste facility or recycling center. Never pour it down drains or onto the ground as this could lead to explosions or damage to the environment.

The Benefits of Using Fuel Stabilizers When Leaving Fuel in a Lawnmower

Fuel stabilizers are additives that help preserve the quality and shelf life of gas when it is stored for an extended period, especially in the fuel tank of your mower.

There are various types available on the market, each designed to cater to specific types of fuel and storage durations. Some popular brands include STA-BIL, Sea Foam, and Lucas Oil.

They have many benefits and are well worth the investment, even if you don’t plan on storing gas in your lawnmower for a long time:

Preventing Engine Damage And Performance Issues

The primary use of fuel stabilizers is to prevent the formation of gum and varnish in your mower’s fuel system, which can lead to engine damage and reduced performance.

They also help to reduce oxidation, helping stop fuel degradation that can result in engine issues, such as hard starting or stalling.

Saving Time And Money

By using a fuel stabilizer, you can reduce the frequency of needing to replace your gas, saving you time, effort, and dollars.

As these additives can help prevent engine damage caused by old or degraded gas, they also reduce the likelihood of expensive repairs and maintenance costs.

How Do Fuel Stabilizers Work?

Preserving fuel quality: Fuel stabilizers work by inhibiting the chemical reactions that cause gas to break down over time. This helps maintain the fuel’s quality and ensures it remains suitable for use.

Extending storage time: With the use of fuel stabilizers, you can extend the recommended storage time for gas in your lawnmower, sometimes up to 12 months or more, depending on the stabilizer used.

Final Thoughts

Overall, leaving gas in your mower during the times you use it most often is no problem. Over longer periods of storage, such as during the winter, it’s best to follow what the manufacturer recommends for your particular mower.

Generally, untreated gas should not be left in a mower for more than 30 days to keep it running reliably for years to come.

Peter Toth

Hi! I'm Peter, the owner of BackyardGadget. Working around the house has always been a big part of my life. I've created this site to share my experience, and to help people choose the right tools for the job. Thank you for stopping by!

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