A mower that runs hot is bad for both the engine and the environment. I’m a mechanic, and fixing a hot mower is usually a really easy fix.
Lawnmowers typically overheat when the oil level is low.
However, there are other possible reasons:
- Cooling fins blocked
- Wrong spark plug fitted
- Wrong gas type
- Vacuum leak
- Damaged flywheel cooling fan
In this post, you’ll learn why mowers overheat and how you can fix it.
Reasons Mowers Overheat
An overheating mower isn’t always easy to spot, so congrats on your powers of observation; you’ve likely saved your mower engine from irreversible damage.
A low oil level is, as you know, the most common reason for the higher operating temperature. That said, trapped debris in the cooling fins causes plenty of heating issues too.
Since low oil level is the most likely reason and luckily easier to check, we’ll begin there.
Low Oil Level
Oil has many functions. Number one is, of course, to lubricate; it does so by coating components in a layer of protective oil which serves to reduce metal-on-metal friction.
Friction, as you know, causes heat, and excessive heat is an engine killer. So oil’s number one job is to keep friction (heat) to a minimum. Bathing critical components in oil that serves to transfer component heat away.
Oil also contains additives to help neutralize corrosive acids inside the engine. The acids that naturally form as a byproduct of the internal combustion process are corrosive and, if not naturalized, would happily eat away at component surfaces.
When the oil level is low, you are effectively asking the available oil in your engine to work two or three or more times as hard. And oil will do that for a time, but it degrades quickly, and when heat overcomes the oil, your engine seizes (fuses together) and can’t be repaired.
That critical minimum level of oil is known, and engine manufacturers fit a dipstick so operators can keep a tight eye on it.
The oil level in this picture is way too low and risks engine failure. If you are unsure of how to check your oil, go ahead and check out the oil checking video below.
Debris Trapped in Cooling Fins
Internal combustion engines turn gas and oxygen into heat. The heat energy is used to turn the crankshaft.
The problem is, engines aren’t super efficient, meaning not all the heat energy is used; most of the potential energy is actually lost in the form of heat.
The lost heat energy, however, must be managed. Otherwise, the engine will seize solid, which means the internal components get so hot they fuse together. When this happens, the engine is beyond repair.
Large engines such as cars or even some large tractor mowers employ a coolant solution that’s circulated around the engine, which serves to carry the heat away. The heated coolant is passed through a radiator where ambient air cools the coolant, and the cycle starts over.
Small engines manage to heat a little differently; they use cooling fins.
What are cooling fins? – Cooling fins are metal fins cast into the engine casing.
The fins increase engine surface area, and that helps dissipate engine heat to the atmosphere quickly.
How to diagnose – To diagnose this as the likely cause of the problem. We’ll need to remove some components.
We’ll need to remove the engine blower housing (engine cover). Some mowers will require tank removal also.
The process is as follows:
- To remove the blower housing, you’ll need to remove the aesthetic plastic engine cover first (usually Philips screws)
- Then go ahead and remove the blower housing (usually a few bolts)
- Check the cooling fins for matted debris
The fix – Using a paintbrush and a suitable plastic or wooden probe, remove the packed debris (old dried grass & dust) from between the fins.
Compressed air works great, but remember to place a rag over the carburetor opening (if exposed). Job Done!
Wrong Spark Plug
Spark plugs come in various heat ranges. A spark plug should get hot enough so as to burn off contaminates that naturally collect on the electrode but not so hot that it causes the plug to pre-ignite the mix. (fuel mix combustion without a spark)
If your engine is fitted with a plug with a heat range that’s too high, it will cause pre-ignition, and that will also lead to an overheating engine.
While on the subject of spark plus, it’s worth noting a poorly gapped spark plug may also cause overheating. Cleaning and gapping the plug will fix the issue. I’ve covered cleaning and gapping the spark plug, and you can check it out here, “Gapping spark plug,” or check out the video below.
How to diagnose – Remove the plug wire and check the spark plug type code printed on the white spark plug ceramic insulator.
Check the recommended plug type for your mower, which you’ll find in your owner’s manual or on your engine maker’s website.
The fix – Go ahead and replace the spark plug
Wrong Gas Type
Blended gas is commonplace now, but for small engines, blended gas can be problematic. Mower manufacturers are playing catch up in terms of design and materials used in their fuel systems.
Standard E10 and E15 are generally Ok to use in your mower engine. However, you’ll need to use a fuel stabilizer if you intend to store the gas in the engine for more than a month. If you need help using a gas stabilizer, you can check out the video below.
E85 is not OK to use in small engines, it causes them to run lean, and a lean running mower engine runs hot.
How to diagnose – You can’t tell what type of gas you have. Gas just looks like gas.
The fix – If you suspect that you may have filled the tank with E15, go ahead and drain the tank and fill it with E5 or E10.
A mower engine runs best with 14.7 parts air (oxygen) to 1 part gas. The carburetor is tasked with measuring incoming air and adding the correct amount of gas precisely.
However, the carburetor can only do this if all incoming air passes through the carburetor. Air that sneaks in, i.e., bypasses the carburetor, isn’t measured. In effect, the mix contains too much air, and that means the engine is running lean, and a lean running engine runs hot.
Unmetered air is known as a vacuum leak, and a common symptom is an irritating engine surge, which may also be described as an engine that revs up and down by itself. I’ve included a video below of a repair for a common B&S Classic engine surge.
So what causes a vacuum leak? Vacuum leaks are usually caused by a faulty carburetor gasket, but there are many other possibilities, and here they are:
- Head gasket
- Worn valve seat
- Valve too tight
- Loose spark plug
- Faulty carburetor
- Cracked carburetor manifold
How to diagnose – Diagnosis can be tricky because of the aforementioned extensive list of possibilities. That said, here’s a mechanics hack for locating vacuum leaks.
Use a can of carb cleaner and have the mower engine idle (IMPORTANT – Keep fingers and toes away from the deck – When the engine is running, the blade is spinning)
Spray the carb cleaner around the carburetor gasket and manifold; if the engine surges, you’ve found the source of your vacuum leak.
The fix – Replace the gaskets.
Damaged Cooling Fan
Small engines need a little extra help on top of the cooling fins to help move the heat away from the engine. To do that, they employ a fan fitted to the flywheel. So long as the engine is running, the fan is moving cool air across the engine.
The fans are made from plastic and can become brittle with constant heating cycles and age. Some blades may break up and lose their ability to move air.
How to diagnose – You’ll need to remove the plastic engine cover and the blower housing. I’ve covered it already above.
With the housing removed, check the fan for missing blades.
The fix – Replace the blade as necessary.
You may find the following pages helpful:
- About the Author
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John Cunningham is an Automotive Technician and writer at Lawnmowerfixed.com.
He’s been a mechanic for over twenty-five years and shares his know-how and hands-on experience in our DIY repair guides.
Johns’s fluff-free How-to guides help homeowners fix lawnmowers, tractor mowers, chainsaws, leaf blowers, power washers, generators, snow blowers, and more.