Effects of Old Gas On Your Lawn Mower

D.P. Taylor
Written by D.P. Taylor
Updated March 15, 2022
A woman mowing her lawn in the backyard
Photo: Cultura RM Exclusive/Stephen Lux / Image Source / Getty Images


  • Signs of a mower with old gas include difficulty starting, rough idling, and strange noises.

  • Gasoline has a shelf life of just 3 to 6 months.

  • Over time, bad gas can gum up your mower's engine.

  • Siphon out the old fuel from the tank to get rid of bad gas.

Ah, the spring. Now that the weather has warmed and the grass is growing, it's time to bring out the lawn mower and give your yard the first haircut of the year. Unfortunately, no sooner do you start your mower than you run into problems.

After having a hard time getting the mower to start, now it's idling badly and stalling out all over the place. You also hear strange noises. Is there something wrong with your mower?

Possibly, but there's also a chance that you've left gasoline in your mower that has gone bad. Here's what you need to know about old gas, and what to do about it.

Does Gasoline Have a Shelf Life?

Gasoline has a shelf life of three to six months, which may seem surprising considering it’s derived from oil that’s remained in the ground for millions of years. But that’s due to how gasoline is made: refineries turn heavy crude oil into lighter gasoline better suited for engines using a process that involves heating the oil in a furnace until it vaporizes. Over time, oxidation and evaporation take their toll on gasoline’s desirable properties, making it no good for engines anymore.

Considering the fact that most homeowners put their lawn mowers into storage for a few months once the weather turns cold, it's not surprising they often find issues with bad gas when it comes time to put those mowers back out to pasture.

How Can Bad Gas Affect My Lawn Mower?

Pouring gas in a lawn mower
Photo: gabort71 / iStock / Getty Images Plus / Getty Images

Your lawn mower may still run on old gasoline, but you may notice it's not running as well. In fact, the oxidation of gasoline over time can result in clogged gas lines and filters, and it can wreak havoc on your fuel system. As a result, running your mower on bad gas may work for a little while, but you risk doing permanent damage to your mower and may need to replace it early.

How Do I Get Rid of the Old Gas?

If your mower is running poorly or you just suspect the gasoline in it is too old, you will need to drain the old fuel and clean out the engine so that there isn't any damaging residue left behind.

This may result in some dirty, laborious work. It's recommended that you consult with the owner's manual to learn how to get access to the fuel tank so you can siphon out the old gasoline with a siphon hose.

If you still have problems after that, you may need to use a carburetor cleaner to flush out any lingering deposits.

How Do I Prevent This From Happening Again?

The solution to preventing bad gas from spoiling your mower is simple: at the end of the mowing season just before you put away your mower into storage, run off the rest of the gasoline or siphon it. This will prevent a hard cleaning job in the spring, as well as prolong the life of your mower.

And don't forget to perform regular maintenance on your mower, like keeping air filters changed. Tune up your lawn mower regularly. Doing the little things now can prevent you from having to open your checkbook for a brand new mower years ahead of schedule, and considering the cost of a new mower can be thousands of dollars, that’s a good thing.