Lawnmower Stops Running When Hot, Causes and Fixes


Most lawnmower owners are no mechanics. When their mower suddenly stops working, they will bring their machine to a repair shop. But most of the time, the problems aren’t too big and can be solved at home. Lawnmowers shutting down when they are hot is one of those common problems. Your lawnmower starts normally but will stop running as soon as it gets hot. This can be after ten to twenty minutes of running. There are multiple possible causes for this problem that each have their own fix.

Why does your lawnmower stop running when it gets hot:

  • Reason 1: Engine overheating; is the most common cause behind this problem. It happens when the fins get covered with a buildup of debris, blocking engine heat dissipation.
  • Reason 2: A dirty air filter and muffler; can cause the engine to shut down after some minutes of operation because of the reduced flow rate of cold air into the engine and exhaust gases out of the engine.
  • Reason 3: Dirty carburetor; a dirty carburetor won’t get enough fuel during operation, causing the running engine to shut down.
  • Reason 4: Vapor lock; occurs when heated gases get trapped in the fuel tank. It can also cause the engine to stop running.
  • Reason 5: Gas and oil problems; old gas and engine oil problems can also cause the engine to stop running after some operation minutes.
  • Reason 6: Moisture in the fuel lines
  • Reason 7: Spark plug issues
  • Reason 8: Loose Bolts

You do not need to panic if your mower stops running when hot. Let your mower cool down first, then start with the diagnosis. Once you know the cause, you can start with the fix. This blog post will take you through the entire process, and we will discuss the causes mentioned above in more detail.

Reason 1: Lawnmower Engine Overheating

Generally, all mechanical processes result in the production of heat as a by-product because of friction. An enormous amount of heat is produced in a lawnmower engine due to the compressed gas’s combustion in the cylinder. Additionally, heat is generated due to friction between moving parts. For this reason, the lawnmower engine gets heated to very high temperatures only minutes after starting. This temperature keeps increasing during the mower’s operation in an overheating engine until it is too hot to work correctly. It stops working, and it will not start again until it cools down.

● Fins:

Most of the heat dissipated by the engine gets dissipated to the surrounding air by the heat transfer mode of “convection.” Heat transfer to the environment by convection is directly proportional to the hot surface area (the engine block in this case). If the hot surface area is increased, heat dissipation will increase, and overheating will be avoided. This increase in the area is achieved by using fins.

To prevent an engine from overheating, fins are molded on the exterior surface of the engine block. When a lawnmower is in use, grass clippings and dirt and debris stick to the fins. If the fins are not cleaned regularly, a thick layer of debris will get accumulated on them. This layer will act as a blanket, reducing heat transfer from the fins to the environment, resulting in the engine’s overheating.

● Fix Lawnmower Engine Overheating

If your mower stops running after some minutes of operation, let it cool down and then check the fins. If they are covered with debris, clean them with a brush until their entire surface is exposed. If dirty fins were the cause of the problem, your mower wouldn’t stop running again.

Reason 2: Air Filter and Muffler

Your lawnmower engine takes in the cool fresh air in each cycle and expels hot exhaust gases produced by combustion. An interruption in the inflow of cool air and the hot exhaust gases’ outflow can cause the engine to overheat and eventually stall.

The incoming cool air passes through the air filter before reaching the engine. Dirt particles in the air can clog the air filter. Lawnmower air filters usually get dirty pretty quickly and must be cleaned regularly.

The hot exhaust gases are removed from the engine through the muffler. If the muffler is dirty, exhaust gas’s outflow rate will decrease, increasing the engine temperature. Over time, the engine will get overheated and stall.

● Fix Air Filter and Muffler

If you are faced with your mower stopping when hot, check the air filter and the muffler. Clean them if they are dirty. Both of these can be cleaned with water and a brush. Make it a habit to clean the two components regularly. Replace them if they are too dirty to be cleaned.

Reason 3: Dirty Carburetor

The carburetor’s job is to mix air and fuel in the correct balance and provide the mixture for combustion in the cylinder. A faulty carburetor will fail to prepare the correct mixture, leading to improper or incomplete combustion in the cylinder. Improper combustion means that sufficient power will not be produced in the engine, causing the mower to stop running.

Lawnmower carburetors get dirty and clogged because of the sticky gum-like material produced if gas is let to sit in the tank for too long.

The internal mechanism of a carburetor comprises an assembly of needle valves, spray jets, springs, and floats. The buildup of the sticky gum-like material can clog these components hampering their smooth operation.

● Fix Dirty Carburetor

You can clean the carburetor yourself with help from online tutorials. Repairing a malfunctioning carburetor is not that simple and better left for the technicians. If the carburetor is faulty beyond repair, replace it with a new one.

Reason 4: Vapor Lock

Vapor lock is when hot gases get trapped in the fuel tank and can cause engine stall. In most lawnmowers, the fuel tank caps have small holes that allow hot gases to escape from the fuel tank. If a fuel tank cap is dirty, these vent holes get filled, blocking the outflow of hot gases. When this happens, the pressure inside the fuel tank rises, causing the heated gases to flow in reverse and move out of the carburetor, immediately shutting the engine.

● Fix Vapor Lock

If your lawnmower’s fuel tank cap is dirty, clean it, or replace it with a new one. Open vent holes will ensure reduced pressure in the fuel tank, preventing the vapor lock condition.

Reason 5: Gas and Oil Problems

Gas sitting in the tank of a stored mower will go bad after some days because of oxidation and evaporation. Gas sitting in the tank produces sticky material that clogs the fuel lines and the carburetor. We have already discussed the adverse effects of a clogged carburetor. Before you start a mower that had been stored, make sure that its gas tank was empty and fill it with fresh gas. If there was old gas sitting in your mower, you might have to clean the fuel lines too. In general, check the gas before starting your mower to see if the gas level is enough for the mower to keep running. If the gas level is too low, the mower will start alright but stop running some minutes later.

● Fix Gas and Oil Problems

If the oil level is too low, the engine will overheat and stall. So, ensure that there is enough oil in the crankcase before starting the mower. Using the right oil and changing the oil regularly is also crucial. We recommend you to change the oil after every 25 hours of operation.

Reason 6: Moisture in Fuel Lines

Water in the fuel lines or the gas tank can also cause your lawnmower to stop running. This is because moisture does not mix with gasoline and causes problems with the ignition if it enters the engine.

● Fix Moisture in Fuel Lines

Water can contaminate the fuel either because of condensation in the fuel tank and fuel line or moisture entry into the fuel line because of a loose gas cap. If water has contaminated the fuel, the only remedy is to drain the fuel and put fresh gasoline in your lawnmower. It would be even better if you clean the fuel tank before putting in fresh gasoline.

Reason 7: Faulty Spark Plugs

A lawnmower with a faulty spark plug will either not start at all or stop running some minutes after starting. Spark plug faults include dirty and corroded electrodes, broken porcelain housing, and carbon buildup at the firing end.

● Fix Faulty Spark Plugs

You can clean dirty spark plugs. You can use sandpaper or a file to remove the carbon buildup, but a broken spark plug needs to be replaced. When replacing a spark plug, ensure that the new spark plug matches the old one in the plug gap. Spark plugs are not expensive, and you can easily find one at part shops.

Reason 8: Loose Bolts

After a long time of use, the bolts that hold various engine components together can become loose. Loose bolts create tiny spaces that can enlarge because of metal expansion as the engine heats up. Through these gaps, the engine can draw in excess air. This messes the air-fuel ratio and causes the engine to stall.

● Fix Loose Bolts

We recommend you to check the bolts that connect the intake manifold to the engine block and the bolts that hold the carburetor in place regularly. Tighten these bolts whenever they become loose.

Final Remarks:

Malfunctioning engine parts, problems with gas and oil, insufficient cooling, etc., can cause the engine to stop some minutes after starting. If you are not familiar with the causes behind a stalling engine, you might get worried and rush to a technician’s shop. However, with basic info on this topic, you’ll be able to diagnose your mower and fix this problem yourself. As you can see from the information we have provided in this blog post, most of these problems are not too serious, and their fixes are not too difficult. So, stop worrying, let the engine cool, then start working. 

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