What causes a lawn mower to overheat and shut down?

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asked May 6 in Lawn Mowers/Garden Tractors by 98pstarget (1,370 points)
What causes a lawn mower to overheat and shut down?

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answered May 10 by Wendell (33,580 points)
The causes of a lawn mower overheating and shutting down include clogged fins on the engine head, clogged fins on the engine flywheel or other debris around the engine.

Other causes of a lawn mower overheating are not enough air flow getting to the engine.

If your lawn mower is the type that has coolant then the coolant level could be low.

Be sure to clean the vents and fins of an air cooled engine.

Take a stiff-bristled brush and give the air vents a thorough going over.

If they were completely clogged up with dry debris, this could have been the sole cause of your lawn mower overheating.

Make sure you regularly check on the condition of these vents going forward.

White smoke from a lawnmower means that your lawn mower engine is burning oil.

Blue or white smoke is most often an indication of the lawn mower burning oil which could be because the lawn mower engine is overfilled or you tipped the lawn mower over and got oil into the carburetor.

Another reason your lawn mower could be burning oil is worn piston rings which allow the oil to get into the combustion chamber and then is burnt off.

The causes of the white smoke can vary although it's pretty common for mowers to produce white exhaust smoke if there's too much oil, for instance, in the crankcase or low-quality oil grades have been used.

The other most common of white smoke from a lawn mower engine is because of worn out or damaged cylinder/piston rings.

A lawn mower engine typically reaches temperatures of 250 F when under load.

The exhaust side however of a lawn mower engine can reach temperatures as high as 400 F.

A lawn mower engine can overheat if it does not get enough air flowing around it.

The lawn mower engine uses air to cool the engine and if the engine is dirty or has debris clogging the fins of the flywheel or the engine head etc the engine can overheat.

Lawn mower engines are cooled through air moving around the engine.

The lawn mower engine is an air cooled engine so it needs to stay clean and free of dirt, grass etc to keep the air flowing over and around the engine.

The flywheel also has a type of fan built into it and as it spins it creates air flow and if those fins are clogged the air won't be able to flow so the engine could overheat.

A lawn mower can stop running when it gets hot because of the engine overheating or also because of vapor lock in the fuel lines and fuel tanks due to the gas cap or gas tank not venting properly.

An often overlooked, trigger for an engine stall occurs when the heated gases inside the fuel tank can't vent properly.

The pressure causes the gas flow to reverse and move out of the carburetor, effectively shutting the engine down.

A small engine, such as a lawnmower engine, generates a lot of heat.

If the engine cannot dissipate that heat, it will likely stop running and will not restart.

After the engine cools, you may be able to fire it up again, but it probably will bog down and stop after it overheats once more.

Gas can sit in a lawn mower for around 5 to 6 months before it goes bad.

Although in some cases gasoline can start going bad in as little as 30 to 60 days depending on the conditions.

The polymers (varnish) will tint the color of the fuel while evaporation causes the lighter ignition vapors to disappear.

The result is a progressively darker, heavier, stickier and less volatile fuel.

When dealing with degraded fuel, do what you can to remove it from tanks and fuel systems.

Fuel varnishing is a gummy residue that clogs the carburetor.

It's caused by the deterioration of fuel that sits in the engine too long, and it happens in a surprisingly short period of time.

Depending on fuel quality and storage conditions, gasoline can deteriorate in as little as 30 days.

When storing your lawn mower the gas should be drained out or used up before the lawn mower is stored.

The most common cause of lawn mowers not starting after being stored is bad gas that was left in the gas tank and carburetor.

The gas eventually gums up the fuel jets and main jets in the carburetor which then means the carburetor needs cleaned or even replaced.

Overtime gasoline starts going bad and turns to varnish which is not good.

So always run the lawn mower out of gasoline before you store it longer than a month.

Gas contains volatile compounds that allow it to burn.

As these compounds evaporate, gas becomes less combustible and eventually turns into a gummy varnish-like substance.

When this happens, gasoline no longer smells like gasoline; it smells like pungent varnish.

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