Why Is Blue Smoke Blowing out of My Lawnmower?

Incorrect Oil

• The use of an incorrect oil for the engine can be the reason for the appearance of blue smoke from the exhaust. An oil that is too thin slips past the piston rings and winds up in the combustion chamber. The oil fouls the air-fuel mixture, and blue smoke is exhausted. Too much oil in the engine will also force excess oil into the combustion chamber. A properly performed oil change can rectify either situation.

Oil Changes and Angles

• It might be necessary to tilt your mower to evacuate the old oil as a step in the oil change process. Keep the spark plug side of the engine high, if possible, when dumping old oil. Any smoking after an oil change should clear quickly and only requires a follow-up check of the oil level. Operating a mower at angles steeper than 15 degrees can also force oil into the combustion chamber, and it should be avoided.

Head Gasket

• The head of an engine is the portion beyond the piston's stroke. The head requires lubrication and cooling from the engine oil. The head gasket keeps oil from entering the combustion chamber. A defective gasket can allow oil into the air-fuel mixture, causing blue exhaust smoke. Combustion gases could also enter the crankcase, and either condition is detrimental to engine function and longevity. Lack of proper tools or experience could mean you need professional assistance to replace the gasket.

Piston Rings

• Rings are fitted to the piston to provide a smooth, tight seal with the engine cylinder. The rings are lubricated by the crankcase but form a secure seal at the cylinder walls. Scores in the cylinder wall can allow oil to slip by the rings and enter the combustion chamber. Similarly, worn rings will permit oil to pass by at each piston stroke in the cylinder. This condition is sometimes referred to as "blow-by," and mechanical repair is required.