Why Do I Have Gas In My Oil In My Lawn Mower? (Solved)

View of lawn mower from behind with garden shed in background

If you detect the unmistakable odor of gas when you enter your shed to get your lawn mower out you are inevitably left with a few questions.

Most notably why do I have gas in my oil in my lawn mower!?

And then probably your next question will be will it harm the engine?

Either that or how do I fix the issue of gas in the oil of my mower?

Well luckily for you, we are tackling all of that and more in this article…

Why Do I Have Gas In My Oil In My Lawn Mower?

It is probably down to the needle valve on the carburetor getting stuck and allowing more gas through than it should, which then seeps into the oil. Cleaning, rebuilding or replacing the carburetor will usually solve the problem. Other reasons include poor compression, a dirty air filter or because the mower has been in storage for the winter.

Gas in a Lawn Mower Oil Tank: How Did It Get There?

Gas managing to find its way into the oil tank of a lawn mower does happen more commonly than you might think.

Sometimes it might be obvious immediately, whereas other times you might just notice a gas smell coming from the oil of your lawn mower.

Either way, it can usually be pinpointed to a few things.

Cause #1: The Carburetor Is Often the Problem

If a lawn mower starts smelling like gas, or if you notice gas has got mixed in the oil tank of your lawn mower it often indicates a problem with the carburetor.

Usually, something is causing the needle and seat in the carburetor to leak.

Just to explain the carburetor’s job is to inject fuel into the pistons of your lawn mower. This is regulated by a needle valve.

If that needle valve gets stuck, and something stops it from sealing against the seat, then the engine will continuously be flooded with the extra gas that is being allowed through. This will then drain into the oil.

Sometimes this is down to the needle or seat being worn, or sometimes dirt can be the cause and get stuck behind the needing holding it open.

If you have a riding mower it is also worth checking the solenoid in the float bowl. 

The solenoid shuts off the flow of gas when the ignition is switched off.

If the design is faulty it might not shut off, which results in gas leaking down the cylinder wall and into the crankcase.

>>> Find out how to fix this issue <<<

Cause #2: It Could Be Down to Poor Compression

You should also check the compression of your mower, to determine if the cause is blow-by.

Blow-by is when a mixture of air/fuel or gas finds its way between the pistons and cylinder wall of an engine and into the crankcase.

Blow-by will lead to a loss of compression in the engine.

The compression of your mower should be somewhere in the 80 to 110 PSI range.

If it is below that, and particularly if it is below around 60PSI, that indicates gas vapors are bypassing the pistons and getting into the oil.

If your mower is experiencing blow-by it will require major work, and you are best off taking it to a mechanic to get their expert opinion.

RELATED ===> Buyers Guide: The Best Lawn Mowers on the Market

Cause #3: It Has Been In Seasonal Storage

Frosted window in winter

More often than not you will notice gas in the oil tank of a lawn mower after the mower comes out of seasonal storage.

The gas can seep in during the colder winter months. 

Adding a stabilizer to the gas before packing it up for storage can prevent this, but if you are noticing it happen year after year, then you might want to try a different approach.

Before storing it away run the engine dry and add an ounce or two of non-ethanol fuel. Then run it dry one more time before leaving it for the winter.

Cause #4: You Left the Key On

This only applies if you have a riding mower, but be extra vigilant about making sure you don’t leave the key turned on overnight.

If you do sometimes gas will leak into the engine cylinder, it will slip past the rings of the engine and fill the engine oil case.

Leaving the key of a riding mower turned on overnight can also damage the alternator, so that is another reason to check it is turned off.

Cause #5: The Air Filter is Dirty

A quick and easy check to make is the air filter.

If it is dirty, then the level of air flowing into the engine will be restricted, and consequently, the engine will run rich and gas can get into the oil.

Cleaning or replacing the air filter is quick and easy and can solve the problem.

Cause #6: It Is Gas Related

It seems obvious, but check everything gas related in your mower:

  • Gas Lines: Check them for leaks and a buildup of residue.
  • Gas Cap: Check the mower’s gas cap and gasket to see if there are any cracks and if it is secured properly.
  • Gas Tank: Check your gas tank to see if it is damaged or has any leaks.

RELATED ===> Why Is There Oil In My Lawn Mower Air Filter?

Will Gas in Oil Harm the Engine?

The short answer to this is it depends on how much gas has got into the oil and how quickly you notice it.

If it is only a tiny amount and you notice it quickly it shouldn’t do any significant damage.

But even a small amount will dilute the oil and make it thinner. This will affect engine lubrication, and could ultimately lead to engine failure.

That is why if you notice gas in the oil tank of your lawn mower, or even the smell of gas in and around the oil tank of your lawn mower, you should act immediately to resolve the issue.

Talking of which…

How to Fix the Problem of Gas in the Oil of Your Lawn Mower

As we have established, when gas is getting into the oil of your mower the most common cause is an issue with the carburetor.

Notably a faulty needle valve and seat.

If it looks like dirt is preventing the needle valve from sealing against the seat then simply removing the carburetor and giving it a thorough clean can solve the problem. Try spraying it with some brake cleaner and scrubbing it with an old toothbrush.

You can also buy a carburetor rebuild kit, that will take you step-by-step through taking apart your carburetor and putting it back together. Or it may need a new needle, seat or fuel float.

Sometimes the easiest bet is just to buy a completely new carburetor. They are reasonably cheap, often around $20 or so.

When you have carried out the cleaning or repair work on the carburetor then don’t forget to drain and replace the contaminated oil.

Fill the gas tank also and before you start mowing the next time you use it, check the oil level to make sure it isn’t higher and also to make sure there is no smell of gas.

I Accidentally Put Gas in the Oil Tank of My Lawn Mower, What Should I Do?

So this is a slightly different scenario from a bit of gas somehow getting mixed in with the oil in your lawn mower.

We all have momentary lapses on occasion, and let’s say yours came when you were topping up your lawn mower with gas.

Say for some reason you were daydreaming and thinking about what you were going to do that evening, your weekend plans, or something else entirely.

And then you look down and realize you had filled the oil tank with gas!

First things first, don’t panic! 

Second thing, don’t start the engine!

Then disconnect the spark plug, and locate the oil drain plug.

You can use a turkey baster to ‘suck’ the gas out or leave the tank upside down/tilted over a container so it will drain out.

RELATED ===> Can You Use Bar And Chain Oil In A Lawn Mower?

Either way make sure you get as much of the gas out as absolutely possible and then leave the tank out overnight in an open area, so any fumes can escape and the tank can dry.

If the following morning the tank doesn’t look like it has dried, add some new oil in and then drain that and again allow it to dry for 24 hours.

Repeat this process until you are sure you have rid of any residue of gas. Then change the oil filter and add new oil.

Turn the mower on and run it at idle for a while to see how it responds.

As long as you didn’t try to run the mower with gas in the oil tank, everything should be fine.

If not, a trip to a mechanic might be in order…

Final Thoughts

Why Do I have Gas in My Oil in My Lawn Mower infographic

If you notice gas seems to be getting into the oil in your lawn mower then the infographic above shows the potential causes, but by far the most common cause is a faulty carburetor.

Usually the needle valve has got stuck and is letting more gas than it should be through, which floods the engine and then drains into the oil.

Replacing or rebuilding the carburetor should solve the problem.

A small leak of gas into the oil should not harm the engine, but you should still deal with the problem as soon as you notice it.

If you accidentally put gas in the oil tank of your lawn mower this does have the potential to be more serious.

But again if you notice it immediately and thoroughly drain and replace the gas and add in fresh oil there should be no long-lasting effects.

Spread the love

Leave a Comment