Lawn Mower: Blows White Smoke From Exhaust

White smoke coming from your lawnmower’s exhaust could indicate a problem with the piston rings. The piston rings form a seal between the piston and the cylinder wall. There are three rings on a typical lawnmower engine piston. The upper most ring is the compression ring. Its primary job is to seal the combustion chamber so combustion gases and heat aren’t allowed past the piston and into the crankcase. It also slightly wipes the cylinder wall so excess oil doesn’t end up in the combustion chamber. The middle ring is called the wiper ring. The wiper ring spreads the engine oil to a consistent thickness to provide lubrication to the piston. It is designed so it wipes excess oil back down into the crankcase so the oil doesn’t end up in the combustion chamber. The lower ring is the oil ring. The oil ring controls the application of oil to the cylinder wall. Its design allows excess oil to flow past it and back into the crankcase. An engine that is consistently and properly maintained will last for years if not decades. In normal operation the piston rings don’t actually make contact with the cylinder wall. There is an extremely thin coating of oil between the two – often only a few molecules thick. With this tiny margin of error, it is important that the engine is properly maintained to prevent damage to the piston, its rings, and the cylinder. Wear to the piston rings or the cylinder wall will allow oil to leak past the rings into the combustion chamber. The oil will be burnt during combustion, which produces white exhaust smoke. A common problem with lawnmowers is piston ring wear caused by neglected or missing air and fuel filters. When dirt enters the combustion chamber it acts like sandpaper, quickly wearing away at the piston rings and the cylinder wall. The wear will allow oil past the rings causing it to burn in the combustion chamber and produce white smoke. Ignoring the engine oil is another maintenance issue that can lead to oil in the cylinder and white exhaust smoke. The right amount of clean engine oil is critical to maintaining a healthy engine. Old oil, low oil, and dirty oil can all lead to white smoke in the engine’s exhaust. When oil is used past the engine manufacturer’s recommended interval it begins to break down and can no longer provide the amount of lubrication that new oil provides. Over time old oil will lead to ring and cylinder wear. Low oil in a small engine is a recipe for disaster. At best, low oil will cause accelerated engine wear. At worst, it can quickly cause critical engine components to seize such as the piston to the cylinder or the connecting rod to the crankshaft. Most walk-behind lawnmowers don’t have pressurized oil systems with oil filters. Instead, the engine oil is kept clean by frequent oil changes, usually every 50 to 100 hours of use. If the engine oil is used beyond the recommended time between changes, it will begin to wear the rings and cylinder. Dirty oil is full of metal particles from normal engine wear as well as any dirt that isn’t caught by the air or fuel filters. The longer the oil is left in the engine, the more concentrated these abrasive particles become. Repairing an engine that is blowing white smoke from the exhaust is not a simple repair. The engine will require a complete teardown to access the piston rings. Ring damage usually goes hand-in-hand with cylinder damage. The cylinder will need to be honed at a minimum, and likely will need to be replaced as well. A repair of this scope will require advanced skills, special tools, and engine service manuals. If you are up to the challenge you can save hundreds of dollars by doing it yourself. If this seems a little too complex you might consider letting a professional shop overhaul the engine for you or simply replacing the engine.