Why Your Lawn Mower Is Smoking and How to Fix It

Some people enjoy taking care of their lawns. To many, a well-groomed landscape is a source of pride and a testament to hours of hard work and sweat. However, when it’s time to mow the grass, the last thing you want is a smoking lawn mower.

What causes a lawn mower to smoke?

A smoking riding lawn mower during a race at the Saco Pathfinders Snowmobile Club
A smoking riding lawn mower | Gregory Rec/Portland Press Herald via Getty Images

Family Handyman says that “leaking oil from the exhaust is the usual cause of blue, white or black smoke from a lawn mower.” Burning engine oil creates a lot of smoke that comes from various sources. 

The most common and harmless sources of stray engine oil occur during lawn mower maintenance. For example, changing the engine oil often results in spilled oil on the mower’s engine, and tipping your mower on its side to clean out grass clippings often leads to oil seeping into the combustion chamber. However, mowing grass on steep hills may also lead to oil seeping into the lawn mower’s combustion chamber while running. 

Bob Vila notes that lawn mowers often produce black smoke when the engine runs too ‘rich.’ This ‘rich’ condition occurs when the engine’s air/fuel mixture contains too much fuel. Hard starting and rough idle are other rich air/fuel mixture symptoms. A clogged or excessively dirty air filter is the leading cause of a lawn mower’s rich air/fuel mixtures. 

What should you do if your lawnmower is smoking? 

If spilled oil during an oil change or while filling the crankcase is the source of the oil smoke, or if it’s caused by tipping the mower to clean out grass clippings or perform other maintenance, it’s best to let the engine run to burn off any excess oil. Of course, you should still clean up as much spilled oil as possible during the maintenance task before attempting to start the engine. 

If you have not performed lawn mower maintenance recently and your lawn mower is running rough and spewing black smoke, it is probably time to do some maintenance. Shut off the engine and allow it to cool down, then locate the air filter and determine if it’s composed of paper or foam. Foam filters are reusable after cleaning, but if you find them caked with grass clippings and dirt or damaged, it is better to replace them with new ones. If your lawn mower uses a paper air filter, it’s best to change them annually or anytime they become filthy.

Is it better to repair or replace a lawn mower?

Lawn mowers are expensive machines but are relatively simple to use and should provide reliable service for decades with minimal maintenance. Maintenance tasks like changing the engine oil, cleaning or replacing dirty air filters, replacing the occasional spark plug, and keeping the blades sharp are about all they require besides fresh fuel in the spring.

However, if your lawn mower continues to smoke after you’ve exhausted these tips, you may need professional help from a small engine repair shop. Repairing a well-maintained lawn mower could provide additional years of worry-free operation. Yet, on the other hand, the repair technician may uncover evidence of catastrophic failure that warrants buying a new one. 

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