My son just called me pretty upset his lawn mower stopped running. He’s a new homeowner and has received a lot of hand-me-down yard tools including a lawn mower.
They aren’t in the best condition. He can’t complain too much when the equipment he receives is free, but he still does when they stop working!
A mower will stop running when it isn’t getting air, fuel, and spark required for combustion.
This may be due to old fuel, a clogged air filter, a dirty carburetor, clogged fuel lines, a bad fuel cap, or insufficient engine oil.
With so many reasons a lawn mower can stop running, I have listed possible running problems starting with the easiest and most common issues and ending with the tougher, harder-to-fix, issues. Hopefully, your problem is in the “easier to fix” category.
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Follow all safety instructions provided in your equipment operator’s manual before diagnosing, repairing, or operating. Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.
9 Reasons Your Lawn Mower Stopped Running
1. No fuel in the Gas Tank
So, when I asked my son if he checked for gas in the gas tank, he just sighed and wondered if I thought he wasn’t very smart. He said, of course, he checked the fuel level. I asked because it is such a simple solution that it is often overlooked.
Fill your fuel tank with unleaded gas with an octane rating of 87 or higher. You can find this information on the label on the gas station’s fuel pump.
87 octane is commonly known as regular gas. Make sure your gas contains no more than 10% ethanol content.
2. Bad or Stale Fuel
Now that you have confirmed you have fuel in the gas tank, you need to figure out if you have bad gas that degraded and broke down becoming less effective.
Gasoline only has a shelf life of about 30 days so it’s important to use gas within 30 days before it starts to degrade.
If you are unable to use your fuel within 30 days, add a fuel additive to stabilize your gas. I like a product called Sea Foam that can be purchased online or at your local automotive store.
You can read more about the advantages of using Sea Foam in my article.
Much of today’s fuel contains ethanol which is an environmentally friendly corn-based product. While gas with ethanol is okay to run in your vehicle, it is not good for your lawn mower engine.
Ethanol attracts moisture that can collect in your fuel system.
This moisture, when evaporated, can gum up your fuel system and cause you to have problems with your lawn mower running.
This is why small engine manufacturers recommend using fuels with no more than a 10% ethanol level. A lower level of ethanol content or fuel without ethanol is best for the longevity of your fuel system.
Drain your fuel tank and fill it with fresh gas. Add Sea Foam fuel treatment to your gas. Sea Foam acts as a fuel system cleaner and stabilizer.
3. Dirty Air Filter
An air filter prevents lawn mower engine damage by preventing dirt and debris from entering the engine intake. While the air filter acts as the first line of defense to protect the engine, if it becomes plugged, it can hurt the engine.
When the engine isn’t able to get clean air because the filter is plugged, it will search for air wherever it can find it. This may cause your engine to overheat and pull air and oil out of the crankcase.
Check your air filter regularly and clean or replace your air filter as needed.
Clean Paper Air Filter Element
Remove the filter from the air filter housing. Use a clean cloth to remove any dirt left in the housing. Be careful not to let any dirt enter the air intake.
Tap the filter against a solid surface to knock the loose dirt from the filter. Hold the filter up to the light to verify you can see light through the filter. If you are unable to see light you must replace the air filter.
Clean Foam Filter Element
You may have a foam filter. Remove the filter from the air filter housing making sure you don’t knock any dirt into the intake.
Wipe out any dirt left in the housing with a clean cloth. Wash the foam filter with dish soap removing any dirt and oil on the filter.
Rinse the filter until all soap is removed and let air dry. Use filter oil to lightly coat the filter. Squeeze the filter of any excess filter oil. You do not want your filter to be over-saturated with oil.
If your air filter has dark spots on it or has become dry and brittle, you need to replace your foam air filter.
4. Clogged or Broken Fuel Cap
Fuel caps must have a vent. Without your fuel cap properly venting, the fuel tank will form a vacuum and fuel will be unable to flow from the tank.
You can check if your cap is causing your lawn mower to stop running by removing the cap and letting your lawn mower run. If it runs well, replace the cap and continue to let the engine run to see if the engine starts running sluggish or turns off.
If this happens, chances are your fuel cap is bad and you need to replace it with a new one if you are unable to unclog the vent.
5. Dirty and Clogged Carburetor
Gummy and hard deposits from running bad fuel through your lawn mower can buildup up in the carburetor. A carburetor regulates the amount of fuel and air required to create combustion.
A dirty carburetor is not able to allow fuel to flow through the stem to the carburetor jet.
Spray carburetor cleaner into the air intake and start your engine. If it doesn’t stay running, you will need to clean your carburetor or replace it. Read this article for steps to clean your lawn mower carburetor.
6. Plugged Fuel Filter or Fuel Lines
Much like deposits left by fuel can clog parts of your carburetor, it can also clog your fuel filter and fuel lines. If your fuel filter is dirty and clogged, you must replace it.
Use clamps to stop fuel flow and remove fuel lines to check for fuel flow. You can clear clogged lines by spraying carburetor cleaner into the line and then using compressed air to blow the line to remove clogs.
When fuel lines become dry and cracked, or you are unable to remove a clogged line, they can easily be replaced.
Visit your hardware store or lawn mower dealership with the correct diameter and length of fuel hose required.
7. Too Much Engine Oil in the Crankcase
Adding too much oil to your lawn mower can cause your engine to smoke and stop while mowing. The smoke can clog your air filter causing your engine to look elsewhere for air and causing your lawn mower to stop running.
Too much oil in your lawn mower can cause significant damage including internal engine damage and the possibility of having to replace your engine
Remove excess engine oil until you are at the correct oil fill line on the dipstick following your manufacturer’s guidelines. Use one of these methods to remove engine oil.
- Drain Plug – Loosen the drain plug and then quickly tighten it back up to only allow a small amount of oil to drain.
- Remove Engine Oil Filter – You can drain a little oil out of the air filter. Spin off the filter. Have a cloth ready to collect oil.
- Oil Evacuator – Vacuum the oil through a tube placed in the oil fill.
- Turkey Baster – This simple kitchen tool can easily suck a small amount of oil. Do not reuse it for food purposes after using it with your engine oil.
8. Too Little Engine Oil in the Crankcase
When your engine oil is low, your engine doesn’t have enough lubrication to run sufficiently. Friction begins to build, creating heat that can melt internal engine components. This heat can prevent your lawn mower from running.
Look over your engine for oil leaks. You can try to change the engine oil and filter, but when the engine overheats and stops running chances are adding engine oil isn’t going to help.
You probably have bigger problems with internal engine damage.
You will need to bring your mower to your local small engine repair shop so your engine can be looked at for internal engine damage.
9. Engine Overheats
Anytime an engine overheats your lawn mower stops running. This can include using the wrong type of oil, clogged engine fins or housing, or your engine working harder due to cutting conditions.
Read more about lawn mower overheating causes in “7 Things That Can Cause a Lawn Mower to Overheat”.
Still Having Problems with Your Lawn Mower?
Lawn mower ownership doesn’t come without its frustrations. Own a mower long enough, you are bound to run into many lawn mower problems including starting, smoking, leaking, cutting, and overheating.
For mower troubleshooting, check out my guide Common Lawn Mower Problems: Solved.