Why Is My Lawn Mower Blowing White Smoke?

Smoke from a lawnmower that isn't accompanied by impaired engine performance often looks worse than it is. If the smoke is white or blue, it means the engine is burning oil, and while that could mean internal engine problems, it usually doesn't if the smoke is white. The smoke from burning oil tends to turn bluish as the amount increases -- if the smoke is white, it normally goes away while the engine runs.

Seals on the valves and pistons separate the combustion chamber in a 4-cycle engine from the oil crankcase. In addition, the head gasket isolates the entire valve assembly. When the engine is operating and the pistons and valves are moving, these seals keep the lubricating oil in the crankcase, but it's normal for them to wear out. When they do, small amounts of oil may get into the combustion chamber and burn along with the fuel. As long as the amount is small, the smoke from this oil combustion is white. It turns bluish as the seals continue to wear and the amount of burning oil increases.

Oil can get into the combustion chamber in other ways. For example, if you turn over your push mower to service the blade, oil can drip directly into the combustion chamber. The same thing can happen while you're operating your mower on a hill with a slope greater than 15 degrees or as a result of overfilling the crankcase with oil. The smoke that occurs is usually short-lived -- it goes away if you keep the engine running and burn off the oil. The color of the smoke may be bluish at first, but it usually turns white when most of the oil has burned away.

Most lawn mower engines have a breather tube that directs air from the combustion chamber to the exhaust port, and this tube also collects oil spray from inside the engine and directs it back to the crankcase. If an air leak reduces some of the pressure inside this tube, oil can spray out onto the muffler, where it burns and produces white smoke. This won't affect engine performance unless there's enough oil to foul the air filter. If there is, the filter restricts air, incomplete fuel combustion produces black smoke, and the engine runs rough and may stall. A severely-soaked air filter often signifies a blown head gasket, which requires immediate repair.

If your lawnmower is new and it starts producing white smoke, the best course of action is to allow it to keep running to burn off the oil. If you stop the engine, it will just smoke again when you restart it. If your lawnmower is older, a constant stream of white smoke is a sign that the engine needs servicing. You can keep using the mower, as long as your neighbors don't complain, but eventually, the engine will begin to lose compression and power. It's always a good idea to check the air filter; if you see oil on it, it's an indication that you need to replace the breathing tube or the head gasket.