Your Push Mower Engine Overheats (Troubleshoot 7 Causes)

Your engine is smelling hot and it may even begin smoking. Continuing to run your mower without finding and fixing the root cause can permanently damage the engine.

A push mower engine may overheat with a low engine oil level, the wrong type of engine oil, a clogged air filter, or plugged cooling fins.

Overworking the engine by running the push mower with a plugged mower deck, dull mower blade, or at a ground speed that is too fast for long, thick, or wet grass may also cause the engine to overheat.

Follow the safety precautions listed in your push mower operator’s manual. Stop the mower, remove the ignition key, wait for the engine to cool, and remove the spark plug boots before performing repairs.

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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.

This is Why Your Push Mower Overheats

1. Using the Wrong Engine Oil

The engine oil doesn’t only keep the parts lubricated, it also helps keep the engine cool. All engine oils are not the same so it’s important to choose the right oil for a small engine.

The air-cooled engine used on a lawn mower requires oil that differs from the oil used in a liquid-cooled engine.

Gas-powered small engines on a push mower use air-cooled engine oil with a high concentration of zinc. Zinc is an additive used as a cooling agent.

Regular motor oil, like that used in a car, does not include much zinc because it uses water in a liquid to cool the engine.

The chart below that shows the type of oil that works best in different outdoor temperatures.

Use this chart and consult with your operator’s manual for the manufacturer’s recommendation for oil as these recommendations will vary from these recommendations.

Most manufacturers of small engines used on push mowers will recommend using SAE30 or 10W-30 engine oil in your lawn mower engine. However, you may need to change your oil viscosity to 20W-50 when operating in higher ambient temperatures.

SOLUTION: Drain the wrong engine oil and add the correct air-cooled engine oil.

2. Low Engine Oil Level

The engine requires proper lubrication so the internal engine parts can move freely. When the engine oil level is low, friction builds that creates heat in the engine.

The oil will become thicker when you continue to run an engine with a low oil level. This can cause significant engine damage.

It’s important to check the engine oil prior to using the mower. Add a little oil if you find it is low.

If the push mower is consistently low on oil, look for the following:

• Oil Leak – Look over the engine area to find signs of an engine oil leak. Once you find the cause of the leak, replace the gaskets from which the oil is coming.
  You will also want to inspect your filter if your mower uses one. Make sure your oil filter is sealing properly.
• Burning Oil – Check for a plugged air filter. A plugged air filter will cause oil to burn from the engine making the engine work harder. You can clean the air filter or replace it is very dirty and unable to be cleaned.
• Using Oil – An engine can be using oil due to overheating while the engine is in use. It can also be an indication of a valve or ring problem. You need to take your mower to a repair center for an engine diagnosis.

SOLUTION: Check the engine oil level and correct it if it’s a little low. If it is the cause of your engine smelling hot, complete a full oil change to ensure you are running good clean oil. A heated oil may have thickened.

If you continue to experience problems after completing an oil change, running your push mower with low engine oil may have caused extensive damage.

Take your lawn mower to a small engine dealer or mechanic for repair. The technician will need to run additional tests to determine the extent of the damage.

3. Engine Cooling Fins are Damaged or Clogged

The engine cooling fins are responsible for pushing air to the exterior of the cylinder head and engine block to keep it cool.

• The top of the engine has a fan, or a flywheel with fins that act like a fan, on it. It pulls fresh cool air in and then blows it down toward the bottom of the engine.
• The air passes through all of the hot parts of the engine and then blows the hot air out of the bottom.

The cooling fins can plug with dirt or the fins can get damaged resulting in a failure to dissipate heat from the engine. This can cause the engine to overheat.

SOLUTION: Remove the engine cover and clean out the cooling fins. If you find any damaged fins, you must replace them. Remove dirt from the engine cover and around the engine.

Going forward, make this part of your annual maintenance.

Washing the engine on your push mower can further add to the problem by pushing dirt and water into the engine area. When washing your mower do not spray water into the engine to avoid compounding the problem.

4. Plugged Air Filter

The air filter is used to make sure the engine receives clean air to keep dirt from causing engine wear.

When the filter isn’t cleaned and kept in good condition, it can become so plugged with dirt and debris that the engine is starved of air.

This can cause the push mower engine to run hot and overheat because it has to work harder to breathe.

SOLUTION: Remove the air filter to inspect its condition.

Replace and clean the air filter: I recommend replacing the air filter annually and then cleaning it several times throughout the mowing season to keep it in good condition.

You may have to replace or clean it more often if you are operating your push mower in dusty conditions or use the mower more than the average homeowner.

Clean a push mower paper air filter:

• Remove the air filter from the housing.
• Wipe out any dirt remaining in the housing. Be careful to not let any dirt fall into the air intake.
• Tap your filter against a solid surface. What you are trying to do is knock as much dirt out of the filter that will come loose and fall out.
• Hold your air filter up to a light source and make sure you can still see light shine through the paper element. If you can, go ahead and reuse your air filter as long as it isn’t damaged.  If you can’t, it’s time to buy a new one.
• Reinstall the air filter and attach your air filter housing cover.
• If your air filter also uses a foam pre-filter, wash the filter with mild detergent and water. Rinse and allow to air dry. DO NOT APPLY OIL to the pre-filter.

Clean a push mower foam air filter:

• Remove the air filter from the housing.
• Wipe out any dirt remaining in the housing. Be careful to not let any dirt fall into the air intake.
• Wash the filter with a mild detergent and water to remove dirt.
• Rinse until the soap is removed. Squeeze the excess water out of the filter and lay it flat to dry.
• Once the filter is dry, use filter oil or engine oil to lubricate the filter. Get it wet enough to be lightly saturated with oil, but not too much where it is dripping oil. Squeeze the filter to remove excess oil.
• If the foam filter is excessively dirty and has dark spots or tears, you need to replace it. 
• Reinstall the air filter.

Don’t be cheap and try to make the air filter last as long as possible. I am very cost-conscious and always spend money on maintenance items.

The money I spend on the air filter is far less expensive than an engine repair or replacement due to debris reaching the engine cavity.

5. Mower Deck Full of Debris

Your mower deck must be regularly scraped clean to remove any buildup of grass clippings, dirt, and other debris.

This buildup reduces the open area under the deck that is used to create air movement for a nice cut. It also causes the engine to have to work harder to rotate the blade through the debris.

SOLUTION: Scrape the mower deck with a deck scraper. A wire brush or putty knife works well too.

Frequently check the mower deck’s condition to keep it free of debris.

While there are products on the market to coat the underside of the mower deck that can help reduce buildup. However, they are not miracle products and you will still experience some buildup.

To reduce clumping and buildup under the deck, avoid cutting wet grass and always run your mower at full throttle.

6. Dull Blades

Dull mower blades will magnify the problem of a plugged mower deck.

Not only will the engine have to work harder to turn the blades through debris under the deck, but also having to turn dull blades through the debris can put a strain on the engine causing it to overheat.

SOLUTION: Mower blades must be checked regularly and sharpened. For the average homeowner, mower blades should be sharpened twice a season or about every 25 hours.

7. Overworking the Engine

It’s best to evaluate your mowing conditions so you can determine how to get the best performance from your mower.

Cutting a lawn with long, thick, or wet grass can put the engine under load. This may cause it to overheat.

SOLUTION: When cutting long, thick, or wet grass, you must operate the mower at a slower ground speed. Follow these tips to reduce the load on the engine and obtain a nice cut:

• Avoid cutting wet grass.
• Double-cut or triple-cut long grass. This process includes setting the mower deck height at its highest setting. Make the first cut. Then lower the deck height a little and make a second cut.
• Run the mower at a fast engine speed at full throttle.
• Reduce how fast you move the mower when cutting long, thick, or wet grass.