Can I Use Stale/Old Gasoline in My Lawn Mower?

by Mike Palmer

You shouldn’t. This is a question that’s asked by many lawn mower owners who, after taking their mowers out from winter hibernation, face the quandary of whether to use the gas left in the fuel tank or siphon it out for new gas.

Does the Gas Go Bad If You Just Leave It in the Fuel Tank?

The gas definitely degrades over time. Gasoline is a refined product containing highly volatile components. These volatile components can evaporate over time degrading the fuel’s volatility which in turn affects how efficient it burns in your lawn mower’s engine. If the fuel is not burning efficiently, your engine will not run properly. Your mower may still run, but it may run roughly. You’re definitely not doing your mower any favors by using old, stale gas. I like to tell people that using stale gasoline is like feeding yourself nothing but junk food. Your arteries will get clogged up over time. Sure, you can function, but you have less energy and it takes more effort to perform physical tasks. Walking up a long flight of stairs becomes a challenge. Get the picture?

But Doesn’t the Fuel Cap Prevent Evaporation of the Gas?

No. The fuel cap does not provide a complete seal. The cap has a tiny vent hole which allows air to get into the fuel tank. In fact, the more serious issue you have to face is oxidation caused by hydrocarbons in the gas reacting to oxygen. The result is buildup of deposits in the fuel system clogging up gas lines, filters, and even orifices in the carburetor and fuel injector. That’s why it’s recommended that at the end of every mowing season you run the gasoline dry before putting the lawn mower in storage for the off season.

How Do I Siphon Out the Old Gas?

Use an Oil Vacuum/Fluid Evacuator to vacuum out the old gasoline. Replace with fresh gasoline and you will extend the life of your lawn mower because its engine will be running smoother and more efficiently.


Do you remember the last time you changed the oil in your lawn mower? If not, then it’s definitely time. Regular oil changes should be part of your must-do list for lawn mower maintenance. You can check out the easy and detailed instructions at: How to Change the Oil in Your Lawn Mower.

You might also like these posts:

How to Give Your Lawn Mower a Tune-Up

How to Change the Spark Plug on a Lawn Mower

How Do I Sharpen or Replace the Blade on a Lawn Mower?



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