What Would Cause My Lawn Mower to Leak Oil From the Engine?

The oil sits in a reservoir close to the engine. On most four-stroke lawn mowers, 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.

Before you can stop an oil leak on a lawn mower 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 buildup of oil and dirt. Then, run the engine again for several minutes. Stop the engine and inspect it closely for the leaking oil.

The carburetor on most lawn mowers 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 buildup. Replace the gasket underneath the breather cover after cleaning it.

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.