What Causes Blue Smoke in a Lawn Mower?

Description

• Blue smoke emitting from the motor indicates either the wrong oil used or oil has found a path into the combustion chamber by way of a leak. The smoke itself is not dangerous but it does indicate a potentially serious problem.

Oil

• Using the incorrect grade oil causes blue smoke and a sputtering motor. Mowers calling for 4-cycle oil cannot function properly on 2-cycle oil. A 4-cycle motor is larger and must have the oil and fuel separated. A 2-cycle motor uses a mixed balance of fuel and oil. Drain all oil from the machine and refill with the proper oil. Run the mower to burn off any old oil; it will burn blue smoke until all the old oil is out.

  Placing too much oil in the crankcase causes the overflow to leak into the engine and spark plug. Fill the oil reservoir only to the line indicated on the dipstick.

Angles

• Tilting the mower sideways at angles greater than 15 degrees causes the oil to spill over into the crankcase and foul the spark plug. When the spark plug attempts the spark, it ignites the excess oil causing blue smoke. To avoid this problem, tilt the mower so the spark plug is facing upward. This ensures oil doesn't escape the crankcase and flood the spark plug or other engine parts.

Airflow

• Excess air entering the crankcase or breather tube can cause a high oil mist. This mist is excess oil brought back through the breather tube and through the air filter. The result in either case is blue smoke. Causes include blockage in the breather tube, holes in the crankcase or breather tube, or damage to the piston and rings. Inspect the crankcase and breather for damage. The breather tube is behind the air filter. The crankcase is in the motor.

Riding Mowers

• Riding mowers have added issues that cause blue smoke compared to walking mowers. Riding mowers have engines similar to automobiles with cylinders, rings and head gaskets. Burning oil in riding mowers can be a result of worn cylinders and rings or blown head gaskets. Any of these require multiple hours of engine work. If you have not successfully worked on engines before, take the mower to a repair shop to avoid further engine damage.