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What would cause my lawn mower to leak oil from the engine?

Updated February 21, 2017

The engine on a lawnmower requires a constant stream of oil into the crankcase and piston. If oil leaks out of the tank or pump before it reaches the engine, the engine is at risk of severe damage. Stop the lawnmower immediately if you suspect an oil leak, troubleshoot the problem and fix the leak.

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Causes of Leak

The oil sits in a reservoir close to the engine. On most four-stroke lawnmowers, a small oil pump delivers oil from the reservoir to the crankcase and piston. Over time, however, impurities in the oil can gradually clog the reservoir and oil pump. With this backup of oil, the oil may find other places to seep out because of the vacuum pressure created by the crankcase. Another common cause of oil leaks is cracks when the seals heat and cool.

Locating the Source

Before you can stop an oil leak on a lawnmower engine, first locate the source of the leak. Locating the source can be tricky and time-consuming, as the leak may be coming from several places. A good way to tell is to isolate the leak into a side of the engine. First, clean the entire engine block with a rag and a brush to remove any build-up of oil and dirt. Then, run the engine again for several minutes. Stop the engine and inspect it closely for the leaking oil.

Leak Near Carburetor?

The carburettor on most lawnmowers sits near the top half of the engine, on the same side as the fuel tank and oil tank. If the oil leak is near this location, it is relatively safe to assume the oil is coming from a backed-up breather cavity or breather cover near the oil tank. Take the breather cover off the engine and clean the cover with a rag. Use a wire to poke the breather hole clean of any oil or dirt build-up. Replace the gasket underneath the breather cover after cleaning it.

Leak Near Base?

If the oil leak is coming from the bottom of the engine, near the base, it is likely because of a problem in the oil tank or oil sump. Pull the oil dipstick out of the engine and reinsert it firmly. Restart the engine and check for an oil leak. If oil continues to leak, remove the oil tank and sump and replace the gasket underneath. Clean the oil tank thoroughly and check for any small holes in the bottom.

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About the Author

Currently based in Minneapolis, Minn., Eric Blankenburg has been a freelance journalist since 2000. His articles have appeared in "Outside Missoula, Outside Bozeman," "Hello Chengdu" and online at GoNomad.com and various other websites. He holds a Bachelor of Arts in creative writing from the University of Montana.

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