Bad Gas in Your Lawn Mower: How To Detect and Fix It

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If you have a lawn you need to take care of or just help others trim their grass, then you know how important it is to care for your lawn mower. If you leave gas in the tank for too long, it can go bad.

You can detect bad gas in your lawn mower by appearance and smell. The smell is the easiest way to detect bad gas. If it smells sour, the gas has gone bad and you need to replace it. If the gas looks darker and thicker then normal, you have to replace it as well.

The rest of this article will discuss more about bad gas and the signs you should look for that your lawn mower needs some fresh fuel.

Why Does Gas Go Bad?

Gas goes bad when left unused for a long period, causing it to expire. You should avoid leaving gas to sit anywhere for too long. Over time, hydrocarbons in the gas will evaporate, which leaves behind a thicker gas that can clog parts of your lawn mower if not removed.

This can happen all year round, but gas evaporates at a higher rate due to heat. So, as the weather starts to get warm, you’ll notice the gas in your lawn mower going bad at a much faster rate. However, it can still happen over the winter months. Therefore, you must watch for signs that your lawn mower gas may have gone bad. 

How Long Does It Take To Go Bad?

It takes between three and six months for it to go bad. Where the timeline falls in that time frame depends on where you store the gas and how much oxygen, heat, and humidity is present. You should always keep gas stored in an airtight container in a cool, dry area. 

Many factors determine how quickly gas can go bad, as the evaporation process can take a different amount of time depending on heat, humidity, and oxygen. 

If you just leave gas in your lawn mower tank without any additives, it’ll begin to evaporate in about a month. Now, this doesn’t mean that you’ll have a sludgy mess of gas in your tank in one month of not using your lawn mower.

It takes a while for the hydrocarbons to completely evaporate, but after about a month, you can expect the process to begin. 

What Can Happen If You Don’t Detect It?

If you don’t detect bad gas in your lawn mower,  it can cause damage to the engine. The severity of the symptoms you see in a lawn mower with bad gas depends on how old the gas is and how long you leave it in the tank without removing it. 

The first sign that your lawn mower has bad gas is having trouble starting it. However, this is not the only sign that something is wrong. If you notice the engine sputtering or hesitating, this can be another clear sign that something is wrong with the gas. 

However, most of these issues with bad gas are that they can happen for many different reasons. Therefore, it’s important to diagnose the real issue before attempting to use the mower. If you use the mower despite the bad gas, it can develop into bigger problems. 

Bad gas causes the engine to run improperly. With continued use, you can ruin the engine completely. Alternatively, bad gas will leave condensation in your engine, leading to rust. So, bad gas will affect the engine’s lubrication, which helps it run smoothly.

Generally speaking, leaving fuel in your tank for an extended period can cause engine issues and prevent your mower from operating correctly.

Will the Lawn Mower Just Not Start?

If the gas in your lawn mower is old enough that it’s just sludge, your lawn mower won’t start. However, even if your gas is bad, your lawn mower can still start. Therefore, you need to watch for other signs that the gas is bad besides the engine not starting. 

Despite starting, your mower’s engine may struggle to run due to bad gas. This means that you may experience your mower’s hesitating and overall rough running. It may cut off unexpectedly, run louder than normal, or just run roughly. Therefore, keep your eyes open for each of these symptoms to help detect bad gas and save your engine.

How To Detect and Fix Bad Gas in a Lawn Mower

You can detect bad gas in a lawn mower by smelling it or looking at its consistency. If the gas smells sour or looks sludgy, it’s most likely bad. It’s best to remove bad gas from a lawn mower, and you can do this by siphoning it out with a pump or manually removing it from the tank.

Detecting Bad Gas in a Lawn Mower

Smell the Gas

The most obvious way to detect bad gas is by smell. Gas has a strong smell that most of us are familiar with, and you should immediately be able to tell if it has expired or gone bad. Once the hydrocarbons evaporate, and after leaving a sludgy mess, the gas will smell sour. 

If you know the smell of gas, you should be able to immediately detect the strange smell. It will have a sour, musty smell, and it may differ depending on how long the gas has been sitting there. 

However, it’s probably bad if it doesn’t smell how it smelled when you put it in. 

Additionally, it’s important to note that If you don’t notice a smell immediately, that doesn’t necessarily mean you’re in the clear. I don’t recommend attempting to smell the gas too close or for too long because heavy exposure to the gas can cause health issues

Therefore, be careful how much you smell the gas in your lawn mower. If you can’t smell the difference without much effort, then consider another testing option. 

Look at the Gas

Alternatively, you can test the gas in your lawn mower by its appearance. 

Once the gas in your lawn mower begins to lose hydrocarbons, it will also change color. The light brown gas you put in the lawn mower will now be much darker, and it begins to lose clarity. 

To test this, you’ll need to remove some gas from the tank and put it in a clear container to observe it. If you’re unsure what the gas should look like compared to how it looks now, get a sample of fresh gas to compare. 

This is the easiest way to determine the gas quality in your mower without smelling it too much. 

Test the two samples side-by-side to determine whether or not you need to replace the gas in your lawn mower. Remember, the longer you leave gas in the mower, the worst consistency it will be. 

If you pull out a sample and the texture is slimy or sludgy, then you need to get that gas out of the tank to prevent damage to the mower. 

Fixing Bad Gas in a Lawn Mower

Before you try to fix the issue of bad gas in a lawn mower, it’s important to note that you shouldn’t attempt to start the mower more than you may already have. If the gas is bad, then attempting to start the mower can cause the bad gas to flow into the carburetor, which can create a sludgy mess that’ll take a while to clean up. 

The only true way to fix gas that has already gone bad is to remove it from the mower by emptying the gas tank and replacing it with fresh gas. There are a few different ways to do this, depending on your experience level and the tools you have available. 

However, you should start each possible method by allowing the engine to completely cool if you recently started or attempted to start your lawn mower. This will prevent injury and allow you to work more safely. 

If you want more information on how long the engine will stay hot and the importance of letting it cool, check out my article, “How Hot Does a Lawn Mower Engine Get?” 

Use a Siphon Pump To Remove the Gas

You can remove the gas with the use of a siphon pump, but the exact method you will use for a siphon pump depends on the type that you have. Automatic siphon pumps are the easiest to work with as they just require insertion and the flip of a switch. 

However, manual siphon pumps are still easy to use but require more effort to get them going. 

Manually Remove Gas From the Tank

You can still remove the gas if you don’t have a siphon pump. Manual methods require that you use gravity to pull the gas from the tank. 

Follow these steps to manually remove gas from the tank:

  1. Place the container on the ground, and use a hose to pull the gas from the tank. 
  2. When you place the hose inside of the tank, gently blow into the hose and listen for bubbles. If you hear them, then you have reached the gas. 
  3. Once you have the hose in the gas tank and the other end in the container, you will need another hose, though this one can be shorter. Take the shorter hose and place one end in the gas tank. This one doesn’t need to touch the gas. 
  4. Blow into the end of this hose, adding air to the tank. The goal is to increase the air pressure in the tank. During this step, be careful not to inhale the gas fumes while blowing into the hose, as it may take more than one attempt to get the gas flowing. As you blow air into the tank, the gas will begin to siphon out of the tank and into your container. 
  5. Though the air pressure is important, gravity is what really helps this method work. So, keep the container on the ground. 

Should You Drain the Gas or Use a Fuel Stabilizer?

You should drain bad gas. However, a fuel stabilizer can help prevent gas from going bad when you know you will leave it for an extended period. Unfortunately, there is a common misapprehension that fuel stabilizers can fix expired gas, but this is not the case. 

The benefits of a fuel stabilizer only happen when you mix it with fresh gas and allow it to preserve the gas. You can’t fix already bad gas with a fuel stabilizer. 

When To Drain the Gas

Draining the gas from your lawn mower can help you prepare for winter or eliminate already bad gas. If you want to ensure that the gas in your lawn mower doesn’t go bad and become sludgy, you should drain it. 

Otherwise, you’ll be stuck with a sludgy mess that can stick to the fuel tank and carburetor. 

When To Use a Fuel Stabilizer

Using a fuel stabilizer is not a fix for already bad gas or bringing back those evaporated hydrocarbons, but it can help prevent your gas from going bad. No matter how hard you try, there’s no way for a stabilizer to bring gas back to its previous consistency. 

For the stabilizer to work, you need to add it to the fresh gas that you recently added to your lawn mower. 

Once you put it in with the gas in your lawn mower, you’ll need to ensure it mixes properly. Otherwise, the fuel stabilizer won’t be able to mix with the gas, which can affect how well the stabilizer works and where it reaches. 

Allow the fuel stabilizer to mix properly by putting it in with fresh gas, then let the lawn mower engine run for a few minutes. This will allow the combination to fully mix, giving your lawn mower gas the best chance of surviving for an extended period. 

You only need to let your lawn mower run for about ten minutes before the stabilizer mixes.  

Final Thoughts

Bad gas in a mower is typically easy to detect, but it can be a major issue for the engine. Ignoring this problem or not realizing it right away can lead to your engine no longer working or needing a new mower altogether. 

Therefore, I recommend testing your gas before starting it after leaving it off for a while. Also, consider using a fuel stabilizer if you plan to leave your mower off for an extended period.

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