What’ll Happen if You Put the Wrong Gas in Your Lawn Mower

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For one reason or another you may have found yourself with your mowers tank full of the wrong type of gas. It’s a pretty coming thing to happen and its easily done, but what are the real consequences of doing this? Have you completely ruined your lawn mower? or is there any easy way back from making a fuelling mistake?

Lawn mowers filled with the wrong gas type may knock loudly, smoke excessively or experience damage depending on the engine and the type of fuel used. In some cases, the engine will stop altogether.

If you accidentally filled your lawn mower with the wrong fuel type, it’s not the end of the world so don’t lose hope! You can probably recover it with the information provided here.

Let’s discuss the several situations you may face and what you can do about it.

Putting the Wrong Grade of Gas in a Lawn Mower

The most common mistake that people make when filling up their lawn mower is using the wrong grade of fuel. Gas stations usually offer different grades of gasoline such as regular, mid-grade and premium gas.

Each of these grades has a different octane rating which measures the fuel’s ability to resist “knocking” or “pinging” during combustion. Knocking or pinging is a sign that the fuel is detonating prematurely which can cause engine damage.

Using gasoline with a lower octane rating than recommended by the manufacturer can cause your lawn mower’s engine to knock or ping which can damage the engine over time.

Pretty much any lawn mower will not “require” premium gas. Premium gas is generally designed for higher powered engines so you won’t benefit and it will just cost you more. I’ve written more about that here.

Putting the Incorrect Fuel Mixture in Your Lawn Mower (Fuel to Oil Ratio)

There are two main types of gas combustion engines used in lawn mower. The two cycle (2-stroke) and four cycle (4-stroke). In a 2-stroke engine the oil has to mix with the gas before it gets to the cylinder so the fuel is pre-mixed before it goes in the tank. This is not the case with 4-stroke engines where

Here are some cases of putting the incorrect fuel mix into lawn mowers and what the results may be:

2-Stroke Mixed Gas in a 4-Stroke Engine

The oil in the mixed gas can gum up the works and clog your carburetor, which is not good news. Your mower might start to run rough, stall out, or even refuse to start altogether.

The 2-stroke mixed gas burns hotter than the straight gas your mower engine needs, which means it can cause some serious overheating issues. Overheating can warp metal parts and cause serious damage that might not be repairable.

Regular Fuel in a 2-Stroke Engine

In a 2-stroke engine, the gas it’s supposed to be burning has oil already mixed in with it. It’s kind of similar to trying to run your car without oil… It’s not going to be good for the engine.

Mixed Gas in a 4-Stroke Engine

This likely won’t hurt your lawn mower’s engine. It may make the mower smoke because it’s going to be burning oil.

Mixed Gas in a Diesel Engine

If you’re trying to run mixed gas in a diesel motor, the engine probably won’t even start.

If the amount of fuel in your lawn mower is insufficient for what it was designed to use, the engine won’t be able to lubricate itself enough. It’s just the same as trying to run your car without oil. It’s going result to overheating and possibly some internal damage. Your engine could potentially seize too. If you have too much oil its not usually too much of a problem because the extra oil will usually just burn off.

Putting Ethanol Fuel in Your Lawn Mower

Ethanol is an alcohol based fuel that is made by the fermentation of certain plants

Gasoline usually always has some out of ethanol but too much can be a problem. In recent years, ethanol has been added to the fuel at the gas stations to reduce Co2 emissions and this has been a problem in the small engine industry.

The ethanol in the fuel absorbs moisture in the air and in the fuel tanks. Once it absorbs, the water and then ethanol separates and this mix is highly corrosive.

Putting Diesel in Your Gas Powered Lawn Mower

In most lawn mowers (gas powered ones) it’s not going to be a good idea to put in diesel. Diesel is a completely different type of gas and it’s made to run for a different type of engine. A few of the larger lawn mowers run on diesel but it’s not the norm for most.

If you accidentally put diesel into your gas powered lawn mower, it probably won’t start if the tank was empty. If it does start, it’s going to smoke and lose power quickly. It may even put out flames which can certainly be dangerous

If you put diesel in your gasoline lawn mower, you must turn off the engine and drain it as quick as you can. You must not try to continue to use the lawn mower thinking you’re going to burn off the fuel. 

Below is a video of a guy who tries to add 100% diesel to his gas powered lawn mower and it doesn’t even start. See what happens once he tries to add some gas into the mix…

Putting Unmixed Gas in Your Two-Stroke Lawn Mower

Using unmixed gasoline in your two-stroke lawn mower can cause serious damage to the engine. The two-stroke engine relies on an oil-fuel mixture to provide lubrication to the internal parts, and the oil helps keep the engine cool.

If you use unmixed gas, the engine will not receive the lubrication it needs and can quickly become overheated, potentially leading to catastrophic engine failure.

Additionally, using unmixed gasoline can lead to carbon build-up in the engine which can also cause damage or reduce its performance.

For these reasons, it is important to always use the correct oil-fuel mixture in your two-stroke lawn mower. Consult your owner’s manual or the manufacturer’s website to ensure you are using the right mix of oil and gas.

Putting Incorrectly Mixed Fuel in Your Lawn Mower (Fuel to Oil Ratio)

It is not recommended to put incorrectly mixed fuel in your lawn mower as it can damage the engine. If you do use the incorrect fuel to oil ratio, you can cause significant damage to the engine, including corroding and damaging the spark plugs and other internal components of the motor. 

Additionally, the incorrect fuel to oil ratio can cause the engine to overheat, leading to engine failure. It should also be noted that any fuel that is left over for more than a few weeks will separate and will need to be mixed again. If you do not mix the fuel to the proper ratio, it can cause damage to the engine.

Mixing Unleaded and Premium Gas in Your Lawn Mower

Mixing different types of gasoline in your lawn mower can also cause problems. For example, if you mix unleaded and premium gasoline, it can cause the engine to run poorly.

This is because the different types of gasoline have different octane ratings, and mixing them can cause the fuel to detonate prematurely. It can also cause the fuel system to become clogged, which can lead to damage or malfunction.

What Gas SHOULD You Use in Your Lawn Mower

It is essential to use the correct type of fuel in your lawn mower to ensure that it runs effectively. The best fuel to use in a lawn mower is regular unleaded gasoline with an octane rating of 87.

It is also important to check the owner’s manual for the manufacturer’s recommendations on fuel type and octane rating.

When to Drain and Replace the Gas

  • If you put unmixed gas in a two-stroke lawn mower gas tank, you should drain the tank immediately and replace with the correct fuel. Trying to run the engine without oil can result in severe engine damage.
  • If you put diesel in your gas-powered lawn mower, you should drain the tank immediately and replace with the correct fuel. Trying to use diesel in a gas-powered engine can lead to internal damage.

When to Run It Anyway

  • In some cases, you may not be able to drain and replace the gas in your lawn mower
  • For example, if you are in the middle of mowing your lawn and realize that you have put the wrong gas in the tank
  • In this case, finish mowing your lawn and then drain and replace the gas as soon as possible
  • Running the mower for a short period with the wrong gas will not cause severe damage, but it is essential to address the problem as soon as you can.

Draining a Mowers Gas Tank

Let’s say you’ve put the wrong fuel in your mower’s gas tank.

In order to do drain the fuel, you’ll first want to loosen the clamps and then remove one end of the fuel line. You’ll need to catch the fuel which will come out pretty fast so much sure you have something to catch it in that can hold all the fuel. Once you’re all done you can replace the hose, make sure the clamps are tight and you should be good to go.


Unless you’ve decided to go electric, you’re going to need fuel to run your mower. If you’re not familiar with the different types of engines in a lawn mower and what type of fuel they require, it can see a little overwhelming but its actually pretty easy once you learn.

Just make sure you know your dirt bike’s recommended fuel type and stick with it as much as possible. You can add higher octane fuel if you so choose, but don’t go lower than the suggested rating. Doing this will help you avoid engine knocking and a really bad day due to a destroyed engine.

The first thing you need to do is figure out what your lawn mowers recommended fuel type is and stick with that as best as you can. As previously mentioned, its fine to choose a higher quality fuel with a higher octane number than recommended but its not usually necessary for lawn mowers. J

Just make sure don’t use a lower octane rating than suggested if you’re going to have a really bad time with a wrecked engine.

If the worst happens and you do put in the wrong type of fuel, you need to make sure you drain it and put in the correct fuel type before you run it too long to cause damage to the mower. Don’t feel bad! We’ve all been there before so just know that you’re not alone.

Frequently Asked Questions

What to do if you accidentally put diesel in your lawn mower?

If you accidentally put diesel in your lawn mower, it is important to drain the fuel tank as soon as possible. Diesel is not suitable for use in a lawn mower engine, as it can cause damage to the fuel system and the engine.

You should also have a professional check your mower for damage, and have them flush the fuel system before refilling it with the correct gasoline.

Can the wrong gas damage your engine?

Yes, using the wrong gas or fuel can damage your engine. If you put gasoline with a lower octane rating than recommended by the manufacturer, it can cause engine knocking and decreased performance.

Additionally, using gasoline with a higher octane rating than recommended can also cause damage to the engine. And using the wrong fuel, like diesel in a gasoline engine, can cause damage to the fuel system and engine.

Does it matter what gas I put in my lawn mower?

Yes, it matters what gas you put in your lawn mower. It is important to use fresh, clean gasoline with the correct octane rating as instructed by the manufacturing company.

Additionally, you should never use diesel or other types of fuel in a lawn mower engine.

Other Helpful Resources

Should You Use Regular or Premium Gas in Your Lawn Mower? – Check out this article if you want to learn more about the differences between using regular and premium gas in lawn mowers!

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