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Bad gas in a lawnmower, how to fix it? With Helpful Tips

Owning a house with a lawn requires you to take care of it. You have to mow your grass occasionally to let it stay healthy and visually pleasing. You can use various lawnmowers to make the mowing job easy. But what if your lawnmower sputters or does not start after you have not used it for some time? Before panicking or calling a mechanic, do a check-up of your lawnmower. One of the more prevalent problems is bad gas. This article will cover all topics related to bad gas and provide a clear insight into what one should do if one’s lawnmower’s gas goes bad.

Bad gas in a lawnmower, how to fix it:

If your lawnmower is sputtering and not starting after you have not used it for some time, there is a big chance that the gas has gone bad. After confirming the fuel problem, you must drain the fuel tank’s old or remaining gas. The easiest way to do it is by using a siphon hose. Check the carburetor and clean all its parts if the mower still does not work properly. The mower should work after filling the tank with new gas.  

Now that the basic procedure of fixing bad gas in a lawnmower has been explained let’s get into the details of this topic.


How to tell if your lawnmower’s gas has gone bad?

If your mower makes thumping noises or has difficulty starting, you should check if the gas in your lawnmower is still ok. There are various methods for how you can do this:

● Smell Test:

You can tell if the gas has gone bad simply by smelling it. Move your mower to somewhere without cars, bikes, or machines using gas so your nose does not confuse the smells. Get the tank’s lid off and smell the gas inside. Gasoline has a very particular smell.

The gas has gone bad if the smell is sour, rotten, and strong. Otherwise, your lawnmower has another problem hindering the mower’s start-up process.

● Appearance Test:

Bad gas has a very different appearance from fresh gas. Put some gasoline from the tank and pour it into a transparent container. Keep some fresh gas with you for comparison too. If the tank’s gas has a darker color than the fresh gas, it has most likely gone bad.

Comparison with fresh gas makes the assessment easier, but even if you do not have fresh gas, bad gas can easily be recognized because of its darker color.  

Why does gas go bad?

The first cause depends on gas volatility. The volatility of gas allows it to burn. The lightest chemicals evaporate first, leaving the incombustible gas, which becomes a gummy, varnish-like substance with time.  This substance can clog up fuel lines.

Keeping old gas in the tank for a long time can worsen the gas. The old gas has hydrocarbons that react with oxygen to produce new compounds, resulting in oxidization. This produces a gum-like substance that can clog gas lines, fuel filter, and carburetor.

This is most harmful to carburetors because fixing them can be expensive, and they would not run properly until their insides are cleaned. This happens for empty tanks, too, because some residue gas remains in the tank. This is why it is important to empty the tank before filling the new gas properly.

Sometimes the fluctuating temperature can cause gas condensation, and when the resulting water finds its way into fuel tanks, it causes problems. 

Effects of bad gas on a lawnmower:

As mentioned earlier, bad gas can clog up the fuel lines, resulting in the engine not running properly. But other than that, bad fuel is also said to kill your engine. Water in the engine caused by the condensation of old gas often contributes to rust formation and corrosion.

Rust can make the engine stop working unless the mower is taken to a mechanic and properly cleaned. In extreme cases, you have to get the engine replaced too. Water in the engine also hinders the engine’s lubrication. So, it is best to avoid bad gas, and if there is some in your engine, get it fixed as soon as possible.

Fixing bad gas in a lawnmower:

You have to drain out the previous gas from the tank and replace it with a fresh one for the mower to start running.

– Equipment needed:

  • Gloves
  • Safety Glasses
  • Siphon hose
  • Carburetor cleaner
  • Rags or towels
  • Plastic container
  • Tarp
  • Crescent Wrench
  • Lawnmower’s user manual

– How to fix bad gas in your lawnmower step by step:

● Step 1. Park the lawnmower straight:

Set the lawnmower at a place from where its underside is easily accessible. Make sure that the mower does not tip. Tipping can cause the fuel to move from the fuel tank to other motor parts, creating many new problems. If you must tip it at all costs, do it backward instead of the side to avoid problems.

● Step 2. Use a tarp:

Spread a tarp under the lawnmower to avoid getting the ground dirty with oil spills. Also, follow the safety precautions, like wearing safety glasses and gloves.

● Step 3. Cleaning:

First of all, clean the surrounding areas of the gas tank. When the outer portion is cleaned, remove the fuel tank lid and wipe out the filter’s insides using rags, towels, or any other cloth you might have.

Step 4. Container and siphon hose:

Place a container on the ground to catch the gas. Use a siphon hose to drain the gas from the tank to the container. Make the siphon hose create a vacuum to drain the gas from the tank by squeezing the bulb several times. Try to drain all the gas from the tank.

Step 5. Check if all gas is drained:

Wipe the residue with a towel or a rag to ensure it is empty now and no bad gas remains in it.

Step 6. Refill and check if the lawnmower runs well:

Refill the gas tank and start the lawnmower. If the mower works after that, you are good to go, but if it runs for a while and dies, it means the carburetor is clogged and needs to be cleaned.

● Step 7. Find the Carburetor drainage bolt:

Then, place the container beneath the carburetor and look for a drainage bolt on the float tank’s bottom.

● Step 8. Remove the drainage bolt:

Remove the drainage bolt and let the fuel run out of the float tank into the container under it. The drainage bolt can be easily removed by using a crescent wrench.

● Step 9. Remove the Carburetor:

Remove the whole carburetor, spray all carburetor parts with carburetor cleaner, and reattach it to the engine.

● Step 10. Reassemble all the parts:

Put together all parts again and fill the fuel tank with fresh gas. Spray some carburetor cleaner into the filter, too, for easy start-up of the system. Start the lawnmower and allow it to sputter for some minutes before it starts running smoothly.

● Step 11. Change the oil:

This is an extra step for those who own 4-stroke lawnmowers. In that case, change the oil too, for the mower to work properly. Consult your lawnmower’s user manual to see what type of lawnmower you have.

Precautions for future use:

Most of the bad gas issues stem from old gas. Avoid storing gas in tanks for more than a month or two. If, for some reason, gas cannot be replaced and has to be stored, make sure to use a fuel stabilizer for it because it prevents the oxidation of gas, which results in gas going bad.

Also, whenever you change your lawnmower’s fuel, empty the tank. Wipe out the residue with towels, rags, or whatever cloth you have so the tank is completely free of any old gas. That way, there would be no gas to get oxidized and clog the fuel lines.

To avoid gas condensation, keep the mower maintained at a steady temperature. If you keep it outside in the open, where it can be exposed to any temperature change, there is a high chance it will cause bad gas.


Bad gas is one of the primary reasons for a lawnmower’s defective function. It is usually the first thing you should check when a mower malfunctions. It is usually caused by old gas, which oxidizes over time and damages the whole system. It can hinder fuel flow because it clogs up fuel lines.

It is better to get it immediately fixed to prevent it from causing further problems. You must drain the bad gas from the engine and wipe it clean. Only then should you add fresh gas and start it again.

Changing your fuel at least once in two months is essential so the fuel does not grow old and starts causing problems. It is also necessary to keep your lawnmower where it does not get exposed to significant temperature changes, as temperature changes can cause many problems.

Repairing a Lawnmower with Poor Fuel Quality

– Lawnmower Engine Struggles to Start

One of the most common signs of bad gas in a lawnmower is the engine struggling to start. When you attempt to start the engine, it may take several attempts or not start at all. This may occur due to the accumulation of water or debris in the fuel, making it difficult for the engine to ignite properly.

– Poor Lawnmower Performance

Another symptom of bad gas is poor lawnmower performance. This may manifest as uneven or slow cutting, sputtering, stalling, or lack of power. The deteriorated fuel may be unable to generate the necessary energy to propel the lawnmower forward and maintain its regular functions.

– Unpleasant Smell from Exhaust

Bad gas may produce an unusual, pungent odor when it burns in the engine. This smell will come from the lawnmower’s exhaust as the tainted fuel is combusted.

• How to Diagnose Bad Gas in a Lawnmower

– Inspecting Fuel Tank

Look into the fuel tank using a flashlight. Observe if there is any visible debris or water in the bottom of the tank. In the case of water, it will sink to the bottom, forming a separate layer below the fuel.

– Examining Fuel Filter

Inspect the fuel filter to check for any obstructions. If the filter is clogged, it is a strong indicator of bad gas in your lawnmower. A clogged filter will prevent proper fuel flow, mimicking symptoms of stale fuel.

– Perform Fuel Test

Perform a simple fuel test by taking a sample of the fuel from the tank and pouring it onto a clean, white surface or paper towel. Observe the color and consistency of the fuel. Fresh gasoline should be clear and slightly yellow or light amber tint. The fuel may be contaminated if it appears dark, murky, or has a strong odor.

• How to Fix Bad Gas in a Lawnmower

– Drain and Clean the Fuel Tank

If you suspect that your lawnmower has bad gas, the first step is to drain the fuel from the tank. To do this, locate the fuel line that connects the fuel tank to the engine and disconnect it. Place a container below the fuel tank and allow the tainted gasoline to flow out.

Once the tank is empty, clean it thoroughly to remove any residue or debris. This can be done using a fuel tank cleaning solution or white vinegar and water.

– Replace the Fuel Filter

After draining and cleaning the fuel tank, it is also a good idea to replace the fuel filter. This ensures that any debris removed from the tank does not clog the fuel system or reach the engine. A new filter will provide optimal fuel flow, ensuring proper engine function.

– Clean Carburetor

Contaminated fuel can also affect the carburetor of your lawnmower, causing it to become clogged or function improperly. To clean the carburetor, follow the manufacturer’s instructions for your lawnmower model.

Common cleaning methods include using carburetor cleaner spray, soaking the carburetor in a cleaning solution, or disassembling and manually cleaning the individual components.

– Add Fresh Fuel and Fuel Stabilizer

Once the fuel tank, filter, and carburetor are clean, fill the tank with fresh gasoline. As a precautionary measure, add a fuel stabilizer to the gas according to the manufacturer’s instructions. Fuel stabilizers help prevent gasoline degradation over time, ensuring optimum engine performance.

• Preventing Future Issues with Bad Gas

– Store Gasoline Properly

To prevent future issues with bad gas, store fuel properly in an approved container, away from direct sunlight or extreme temperatures. Use a fuel stabilizer to prolong the life of the gasoline and ensure optimal engine performance.

– Use Fuel within a Reasonable Timeframe

When possible, use stored gasoline within 30 days of purchase. The older the fuel is, the more likely it is to become contaminated or degrade.

– Turn Off the Fuel Valve when not in Use

If your lawnmower has a fuel valve, turn it off when it is not in use. This prevents fuel from sitting in the carburetor for an extended period, reducing the risk of clogging or degradation.

– Regular Maintenance

Perform regular lawnmower maintenance, such as checking and replacing the fuel filter, cleaning the carburetor, and inspecting the fuel tank for debris. This ensures optimal engine performance and reduces the likelihood of issues related to bad gas.

For more information on maintaining your lawnmower, visit the US Department of Energy’s manual on small engine maintenance for expert guidance and advice.

Turn off the lawnmower and remove the spark plug for safety.
Drain the bad gas from the lawnmower’s fuel tank into an appropriate container.
Remove and clean the carburetor to remove any deposits caused by the bad gas.
Replace any damaged or clogged fuel lines and filters.
Refill the lawnmower with fresh, high-quality gasoline.
Reattach the spark plug and start the lawnmower to ensure it runs properly.

Recognizing Lawn Mower Issues Due to Bad Gas

Lawn mowers are essential tools for maintaining a well-kept yard. However, issues with bad gas can lead to several problems, affecting your mower’s performance and life span.

• Engine Trouble

– Difficulty Starting the Engine

One of the primary symptoms of bad gas in a lawn mower is an engine that is difficult to start. Gasoline can go stale if left to sit for too long, leading to a less efficient combustion process.

If the engine struggles to turn over or requires multiple attempts to start, bad gas might be the culprit. In this situation, emptying the mower’s fuel tank and refilling it with fresh gasoline is recommended.

– Inconsistent Engine Performance

Bad gas can also cause fluctuations in the performance of the lawn mower’s engine. This might manifest as the engine revving or cutting out unexpectedly during use. If the mower is properly maintained and the engine continues to perform poorly, bad gas could be the issue.

In this case, it is prudent to replace the old gasoline with fresh fuel and see if the engine performance improves.

– Engine Stalling

Bad gas might be responsible if the lawn mower’s engine is frequently stalling during operation. Poor-quality gasoline can build residue in the carburetor or clog the fuel lines, leading to engine stalling.

To address this issue, a thorough cleaning of the carburetor can be helpful, along with replacing the gasoline.

• Poor Fuel Economy

Another symptom of bad gas in a lawn mower is poor fuel economy. This occurs when the engine consumes more fuel than usual, reducing the overall efficiency of the mower. Stale or contaminated gas requires the engine to work harder, increasing fuel consumption.

Switching to fresh gasoline and ensuring the mower is properly maintained, including regular oil changes, can help to improve fuel economy.

• Foul Smell or Smoke

– Unpleasant Odor

Bad gas in a lawn mower can produce a foul smell during use. Gasoline has a natural odor, but old or contaminated fuel can emit a more pungent and unpleasant smell. If a strong gasoline smell is present during mower use, it is a sign that the fuel may need to be replaced.

– Smoke Emissions

Unusual smoke emissions from the lawn mower during use can also indicate a problem with the fuel. For example, black or blue smoke signals that the gasoline is burning too rich, or oil leaks into the combustion chamber.

White smoke can signify water or coolant entering the combustion process. In either case, it is advisable to check the fuel, replace it with fresh gasoline, and perform any necessary equipment maintenance.

• Loss of Power

When a lawn mower runs on bad gas, it can experience a loss of power. The engine might seem sluggish and struggle to perform at its normal level, making it harder to cut grass efficiently. This can be attributed to stale fuel or residue build-up reducing the engine’s combustion efficiency.

Addressing the fuel issue by replacing the gasoline and cleaning the carburetor can help restore the mower’s power.

• Prevention and Maintenance

To prevent the occurrence of bad gas in a lawn mower, several maintenance steps can be taken:

  • Store gasoline in a properly sealed container to inhibit evaporation and contamination.
  • Add a fuel stabilizer to the gasoline, extending its shelf life and keeping it fresher for longer periods.
  • Do not leave gasoline in a lawn mower’s fuel tank for extended downtime. It is better to store the mower in an empty tank if it will not be used for an extended time.
  • Replace fuel filters regularly to ensure proper flow and prevent fuel contamination.
  • Perform routine cleaning and maintenance of the mower to improve its overall performance and efficiency.

In conclusion, bad gas can lead to numerous symptoms in a lawn mower, including engine trouble, poor fuel economy, foul smells, and loss of power. Regular maintenance and fuel management can help prevent these problems and keep your lawn mower running smoothly for years.

For more information on lawn mower maintenance and troubleshooting, visit The Lawn Institute to access resources and expert advice.

Effective Fuel Treatments for Contaminated Gas

Bad gas, also called contaminated fuel, is a common problem among vehicle owners. It can cause various issues in the engine, leading to poor performance and even damage.

• Causes of Bad Gas

Several factors can contribute to the contamination of fuel in your engine. Some of the most common causes include:

– Water Contamination

Water can find its way into the fuel tank, leading to poor combustion and potential damage to the engine. Water contamination can occur due to condensation, entry through a leaking fuel system, or even during refueling from a contaminated fuel source.

– Dirt and Debris

Dirt and debris can enter the fuel system through various sources, such as a dirty fuel tank, a dirty fuel filter, or an unclean fueling nozzle at the gas station. These contaminants can clog the fuel system and lead to poor engine performance.

– Ethanol and Fuel Separation

Ethanol, a common additive in gasoline, can absorb moisture from the air. Over time, the ethanol and water can separate from the gasoline, leading to a phenomenon known as phase separation. This can cause drivability issues, as the engine may consume the separated fuel mixture instead of pure gasoline.

– Fuel Degradation

Gasoline can degrade over time, especially if left unused for a long period. Degraded fuel can lead to varnish and gum deposits, clogging the fuel system and causing poor engine performance.

• Symptoms of Bad Gas

Several signs can indicate that you are dealing with bad gas in your vehicle. Some of the most common symptoms include:

  • Hard starting or stalling
  • Poor acceleration and performance
  • Decreased fuel efficiency
  • Engine knocking or pinging
  • Check engine light is coming on

If you experience any of these symptoms or suspect your vehicle has bad gas, it is essential to address the issue immediately to prevent further damage to your engine and fuel system.

• Gas Treatment Options for Bad Gas

If you suspect that you have bad gas in your vehicle, there are several gas treatment options available to help resolve the issue:

– Fuel Additives

Fuel additives are chemicals designed to address specific problems in the fuel system. They can help remove water, clean the fuel system, and break down deposits. Some popular fuel additives in the market include the following:

Water Removers

Water removers, such as HEET, are designed to eliminate water from the fuel system. These products work by binding with the water molecules and allowing them to burn off through combustion, helping prevent issues caused by water contamination.

Fuel System Cleaners

Using a fuel system cleaner, such as Chevron Techron, can help remove deposits and dirt from the fuel system. These products can improve fuel efficiency, engine performance, and overall drivability.

– Draining and Refilling the Fuel Tank

In severe cases, it may be necessary to drain the fuel tank completely and refill it with fresh, clean fuel. This can ensure you remove all the contaminated fuel from your vehicle and start with a clean slate. Remember to dispose of the drained fuel responsibly, following your local regulations.

– Replacing Fuel Filter and Other Components

If your vehicle has an old or contaminated fuel filter, it may exacerbate the problems caused by bad gas. Replacing the fuel filter and other affected fuel systems components, such as the fuel pump or injectors, can help to resolve the issue and improve engine performance.

– Regular Maintenance

One of the best ways to prevent issues related to bad gas is through regular vehicle maintenance. This includes regularly changing the fuel filter, monitoring the fuel quality you purchase, and using fuel stabilizers if you expect your vehicle to sit for extended periods without being used.

• Conclusion

Bad gas can cause numerous problems for your engine and affect the overall performance of your vehicle. Understanding the causes and symptoms of bad gas will allow you to identify when dealing with this issue and choose the right gas treatment solution to resolve it.

Remember that preventative measures, such as regular maintenance and using high-quality fuel, can help minimize bad gas in your vehicle. By staying vigilant and addressing any fuel-related issues promptly, you can help to ensure that your vehicle remains in optimal condition for the long run.

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  1. says:

    It’s important to regularly maintain your lawnmower to avoid issues like bad gas.

    • Allard Vdhooft says:

      Thank you for emphasizing the importance of maintaining your lawnmower to prevent issues like bad gas. It’s crucial for optimal performance and longevity.

  2. Renee Stone says:

    I never realized how important it is to empty the gas tank of a lawnmower.

    • Allard Vdhooft says:

      Thank you for sharing your experience with draining the gas tank of a lawnmower. It’s a crucial step in maintenance to ensure the mower runs smoothly. Glad you found the information helpful!

  3. Julie Meyer says:

    Is there a way to prevent bad gas in lawnmowers?

    • Allard Vdhooft says:

      “Drain the fuel tank, clean the carburetor, and refill with fresh gas to fix bad gas issues in lawnmowers. Thanks for asking, Julie!”

  4. Brianna Austin says:

    Can contaminated gas cause permanent damage to a lawnmower’s engine?

    • Allard Vdhooft says:

      “If your lawnmower is sputtering or not starting, contaminated gas might be the culprit. Drain the old gas, clean the carburetor, and fill it with fresh gas. This should fix the issue. Good luck!”

  5. Lisa Scott says:

    I had no idea that bad gas could lead to poor fuel economy in lawnmowers.

    • Allard Vdhooft says:

      “Thank you, Lisa! It’s important to keep an eye on the fuel quality in lawnmowers. Let me know if you have any other questions or topics you’d like me to cover.”

  6. Julian Gray says:

    I never thought that bad gas could clog up fuel lines in a lawnmower.

    • Allard Vdhooft says:

      “Bad gas can indeed cause issues in lawnmowers. It’s important to drain old gas, clean parts, and refill with new gas for optimal performance. Good luck with your lawnmower!”

  7. Hilda Walker says:

    I didn’t know that bad gas could cause so many problems in a lawnmower.

    • Allard Vdhooft says:

      Thank you, Hilda! Bad gas in a lawnmower can definitely cause issues. It’s essential to drain and replace the old gas to keep your mower running smoothly.

  8. Steve Morales says:

    Can using fuel stabilizers help prevent bad gas in lawnmowers?

    • Allard Vdhooft says:

      Using fuel stabilizers can indeed help prevent bad gas in lawnmowers. It’s important to address bad gas promptly to ensure your lawnmower’s optimal performance. Drain and refill the tank with fresh gas, and consider using a stabilizer for future use.

  9. Cecil Stanley says:

    This article is very informative about the effects of bad gas on lawnmowers.

    • Allard Vdhooft says:

      Thank you, Cecil, for your feedback on the effects of bad gas on lawnmowers. I’m glad you found the article informative. Let me know if you have any more questions!

  10. Adrian Bradley says:

    Is it possible to clean a lawnmower’s carburetor without professional help?

    • Allard Vdhooft says:

      Yes, it is possible to clean a lawnmower’s carburetor without professional help. Follow the steps in the article to drain bad gas and clean the carburetor for better performance. Good luck!

  11. Tyler Stone says:

    I will definitely follow the steps provided here to fix bad gas in my lawnmower.

    • Allard Vdhooft says:

      Thank you for reading and following the steps to fix bad gas in your lawnmower, Tyler. I hope your lawnmower runs smoothly now! Best of luck with your lawn care.

  12. Dan Pierce says:

    Thank you for explaining the importance of regular maintenance to prevent bad gas issues.

    • Allard Vdhooft says:

      Dan, thank you for highlighting the significance of regular maintenance to avoid issues with bad gas. It’s essential to address these problems promptly to maintain your lawnmower’s functionality.

  13. Willie Davidson says:

    This article has motivated me to take better care of my lawnmower to avoid bad gas.

    • Allard Vdhooft says:

      Thank you, Willie! Taking care of your lawnmower is crucial for its longevity. Regular maintenance and ensuring good fuel quality will keep it running smoothly.

  14. Bob Jacobs says:

    How often should I change the gas in my lawnmower to avoid bad gas?

    • Allard Vdhooft says:

      To avoid bad gas in your lawnmower, change the gas every 30 days and use a fuel stabilizer. Follow these steps to fix bad gas issues: Drain old gas, clean the carburetor, and refill with fresh gasoline. Good luck!

  15. Stacey Simpson says:

    This article provides great step-by-step instructions on fixing bad gas in a lawnmower.

    • Allard Vdhooft says:

      Thank you, Stacey! I’m glad you found the step-by-step instructions helpful for fixing bad gas in a lawnmower. Happy mowing!

  16. Tina Fuller says:

    I appreciate the detailed explanation of how to diagnose bad gas in a lawnmower.

    • Allard Vdhooft says:

      Thank you, Tina! I’m glad you found the explanation helpful. Let me know if you have any more questions about lawnmower maintenance. Happy mowing!

  17. Genesis Gomez says:

    What are the signs of bad gas in a lawnmower?

    • Allard Vdhooft says:

      “Check the gas tank for dark, foul-smelling gas. Drain and refill with fresh gas. Clean the carburetor if needed. Your lawnmower should run smoothly afterward. Hope this helps!”

  18. Isobel Silva says:

    I appreciate the tips on how to clean the carburetor of a lawnmower.

    • Allard Vdhooft says:

      Thank you, Isobel! I’m glad you found the tips helpful for cleaning the carburetor of your lawnmower. Feel free to reach out if you have any more questions or need further assistance. Happy mowing!

  19. Tommy Mckinney says:

    How do I know if my lawnmower’s carburetor needs cleaning?

    • Allard Vdhooft says:

      Hi Tommy, if your lawnmower is sputtering or having trouble starting, it might be due to bad gas. Try draining and cleaning the fuel tank, replacing the fuel filter, and adding fresh gas. This should help resolve the issue.

  20. Virgil Reid says:

    Great tips on preventing future issues with bad gas in lawnmowers.

    • Allard Vdhooft says:

      Thank you, Virgil! I’m glad you found the tips helpful in fixing bad gas issues in lawnmowers. Happy mowing!