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

Causes of Leak

• A lawnmower will leak oil if the oil can't travel to the crankcase. After the oil leaves the tank, it travels through a breather cavity, a suction hose and into the pump, where it is distributed into the crankcase. Any place along this route can be compromised. Dirt, oil impurities and other foreign debris can clog this system, creating a backup of oil. If an oil leak is noticed, stop the engine immediately as damage may occur quickly in the crankcase. Find the source of the leak before using the mower again.

O-Ring

• The oil's filling tube reaches up slightly out of the oil tank and extends downward into the tank. This filling tube often requires a small, rubber O-ring to create an airtight seal with the fill tube and the oil tank. If this O-ring comes loose or gets lost, oil may start leaking, often near the base of the engine. Pull the oil dipstick out of the fill tube and press it down again firmly to make sure it's properly installed. If the leak continues, the source lies elsewhere.

Breather Cavity

• The breather cavity on a lawnmower works to allow some of the built-up pressure from combustion to escape so the crankcase seals don't fail. However, if the crankcase seals start failing due to a clogged breather cavity, oil will start leaking out of the cracks. Remove the breather cavity, which looks like a small cover with a hole in it, from the engine. Insert a small metal wire through the breather hole to clear out any debris. Check the condition of the crankcase seals to ensure they aren't cracked.

Oil Sump

• The oil sump sits at the bottom of the mower's engine and collects the leftover and unused portions of the oil after it goes through the crankcase. This oil sump can get clogged up with debris or old, dried-out oil. When this happens, the oil will have nowhere to go and a backup will start causing a leak elsewhere in the engine. So a problem in the sump may produce a leak anywhere in the oiling system. The sump lies just below the engine and can be difficult to reach, so it's better to leave this to a professional.