Bad gas in a lawn mower is a common topic that comes up, but how do you determine if your gas is bad? Like most engine problems, your lawn mower will show symptoms if it has bad gas. So, knowing the signs to look out for could save you a headache and repairs, as bad gas will end up doing damage. In this article, I’m going to run through several symptoms that could indicate your lawn mower has bad gas in the tank.
Main Symptoms of Bad Gas in a Lawn Mower? (The Short Answer)
Degraded gasoline no longer has the ability to combust efficiently and progressively gets worse with time. Lawn mowers with bad gas will be hard to start, run poorly, and will typically stall during use.
3 Things You’ll Likely See If the Gas in Your Mower Has Gone Bad
You’re experiencing symptoms of bad gas in a lawn mower if you start noticing problems with how your engine runs. Let’s take a closer look to understand better what you’re experiencing and why it’s happening.
Hard to Start the Lawn Mower
Bad gas gives you a lot of trouble when starting your lawn mower. You may be cranking the engine for some time before noticing a slight splutter. Degraded gas has reduced flammability, making it much more difficult for the spark plug to ignite the engine. If you have attempted to start an engine with a flooded carburetor, you’ll know how this feels.
Poorly Running Engine
If you manage to start your lawn mower with bad gas, you’ll quickly notice that it’s unable to run smoothly. It’s common to find that your lawn mower surges. The engine’s revs will continuously increase and decrease even when just idling. This is due to the irregularity of the gas. It’s also not uncommon to find that your lawn mower only runs with the choke on.
Lawn Mower Stalling
A lawn mower running slowly and stalling is another common symptom of bad gas. The engine stalls once it is no longer able to combust the fuel. As a result, you’ll be back trying to start the engine, which will probably be even harder, if not impossible.
1 Quick Test You Can Use to Confirm You’re Dealing with Bad Gas
Luckily, there is a quick and easy test you can do to determine if you have bad gas in your lawn mower. You might come across other methods on the internet for testing bad gas, but there is only one that I recommend. I’ll explain why below.
Gas Discolouration Test
Once gasoline begins to degrade, it starts to change color. The oxidization process will change gasoline from its original color to brown/amber and will also become cloudy. Comparing a sample of gas from the tank with fresh gas will help you to identify if degradation has occurred. Carrying out this test in a safe and controlled environment is the best and most practical method to determine if you have bad gas in your mower.
Steps to Compare Fuel Sample
First, take a sample of fresh gasoline and pour it into a clear glass. Then use a syringe and remove gasoline from the lawn mower’s gas tank. Follow this by transferring the gasoline from the syringe into a second clear glass. Next, hold both samples towards the light and compare the color and clarity. If you notice a difference in color and clarity, you have successfully diagnosed that there is bad gas in your lawn mower.
Equipment & Materials Required to Compare Fuel
- Plastic Syringe
- 2x Clear Glass Containers
- Fresh Fuel
Gas Smell Test
You have likely heard or read about the smell test for determining if your lawn mower’s gas has gone bad. It’s true that gasoline starts to have a different smell as it degrades. Aged gasoline has a sour odor that is distinctly different from fresh gasoline.
However, I do not recommend this method because intentionally sniffing gasoline is extremely dangerous. Gasoline contains methane and benzene hydrocarbons, both highly poisonous and should never be purposely inhaled. Regardless that some claim this to be a good test, it’s hazardous to your health and should not be tried.
Bad Gas in Your Mower: Here’s Your Action Plan
If you have carried out your gasoline test and discovered that you have bad gas in your lawn mower, you’ll need to remove it. It is best to approach this in two stages. Firstly, removing the gasoline from the fuel tank, and secondly, releasing the gas from the fuel systems. These two actions will reduce the fuel system’s exposure to bad fuel.
Removing Fuel From the Fuel Tank
To fix the issue of bad gas in a lawn mower, you will want to either siphon or pump out the fuel into a separate fuel can.
Steps to Remove Fuel From a Fuel Tank
- Position separate fuel can next to the lawn mower
- Insert either siphon or pump into fuel tank and fuel can
- Start siphoning or pumping process
- Allow fuel to drain from the lawn mower fuel tank completely
- Dispose of bad fuel
Tools Required
- Fuel Can for Waste Fuel
- Pump/Siphon
Removing Fuel From the Fuel System
Once the fuel tank is empty, some fuel will remain within the fuel lines, fuel filter, and fuel pump. Disconnecting the fuel line before the carburetor and cranking the engine will force the remaining fuel out of the fuel system.
Steps to Remove Fuel From a Fuel System
- Remove the fuel line from the carburetor
- Place the fuel line from the carburetor into a small container to catch waste fuel
- Crank the engine until fuel no longer drains from the disconnected fuel line
- Reinstall the fuel line from the carburetor
- Dispose of bad fuel
Tools Required
- Screwdriver
- Small Fuel Container
Frequently Asked Questions About Mowers and Bad Gas
Knowing the symptoms of bad gas in a lawn mower and how to remove it will save you both time and money. Consequently, knowing how long gas lasts in a lawn mower could save you from ever facing the symptoms and associated problems. Getting ahead of bad gas will keep the lawn mower’s engine running smoothly and increase the lifespan of the entire fuel system.
How Long Does Gas Last in a Lawn Mower?
Gasoline will start to degrade after 30 days. Yep, that quickly! Filling your tank between weekly cuts isn’t going to cause a problem, but if you leave your lawn mower for longer than 30 days, it will start to degrade and go bad.
Can Old Gas Cause a Lawn Mower Not to Start?
Certainly, old gas can cause a lawn mower not to start. Old gas isn’t as volatile and, therefore, is less combustible.
Hi. Just read your article about bad gas. The trouble my cub cadet riding lawn mower has been having is that it will run for a while and then eventually it will start sputtering and wants to die. I have changed the fuel filter but still retaining the same problem. The length of time it does the sputtering varies. Sometimes it will run for a long time and other times maybe 15 minutes or so. What does this sound like it might be.
Hi Ronald,
It sounds like your lawn mower is struggling after it has warmed up and that it starts to overheat. Now, this could be because your lawn mower is under excess stress due to things like dull blades, a clogged cutting deck, or cutting long grass. Additionally, you could be dealing with something like clogged cooling fins on the flywheel, low/old oil, an old oil filter, or a dirty air filter. Because you have already tried the fuel filter and fuel, you can count them out.
So, if your lawn mower runs fine to start with, then It’s something that’s changing as time goes on. It’s very likely an overheating issue.
Do you think your lawn mower would keep running if you didn’t engage the blades? For example, start your lawn mower up and leave it running. If your lawn mower can sit on full rev without the blades engaged without an issue, then I would say the stress of cutting causes overheating. If your lawn mower stalls during this test, then you’ll want to take a closer look at issues with the engine, such as the oil, filter, flywheels fins, etc.
Take a look at this article I wrote about overheating to get some ideas: https://lawnmowerguru.com/lawn-mower-overheating/
Thanks for the question, and good luck!
Tom.