Why Is Oil Coming Out of My Lawn Mower Exhaust?

Lawn mower owners may occasionally experience a peculiar and concerning issue: oil coming out of the mower’s exhaust. This problem not only affects the performance of your lawn mower but can also damage the engine over time. This comprehensive guide will help you understand the causes and solutions for oil leakage from the lawn mower exhaust. We will also discuss ways to maintain your lawn mower for optimal performance and longevity.

Lawn Mower

Basic understanding of lawn mower components

Before diving into the causes of oil coming out of the exhaust, it is essential to understand the key components of a lawn mower engine and how they function.

Overview of lawn mower engine components

  • Engine block: The main structure that houses the engine’s components.
  • Piston and cylinder: The piston moves up and down within the cylinder, generating power through combustion.
  • Exhaust system: Transports and expels combustion byproducts, such as gases and smoke, away from the engine.
  • Carburetor: Mixes air and fuel in the proper proportions for combustion.
  • Oil system: Distributes oil to lubricate and cool the engine’s moving parts.

Role of engine oil in lawn mowers

Engine oil plays a critical role in the performance and health of your lawn mower engine. Some of the primary functions of engine oil include:

  1. Lubrication: Reduces friction between moving parts, preventing wear and tear.
  2. Cooling: Absorbs and dissipates heat generated by the engine.
  3. Cleaning: Removes dirt and debris from engine components.
  4. Sealing: Helps seal the space between the piston rings and cylinder walls to maintain proper compression.

Causes of oil coming out of lawn mower exhaust

There are several potential reasons for oil leakage from your lawn mower’s exhaust. Let’s explore the most common causes.

Overfilled engine oil

Overfilling the engine with oil can lead to several problems, including oil coming out of the exhaust. When there is too much oil in the engine:

  • The excess oil can be forced into the combustion chamber.
  • The overfilled oil can generate excessive pressure, pushing the oil past seals and gaskets.

Worn or damaged piston rings

Piston rings are essential for sealing the gap between the piston and the cylinder wall. When piston rings are worn or damaged:

  • They fail to prevent oil from entering the combustion chamber.
  • The oil can mix with fuel and air, resulting in oil leakage through the exhaust.

Incorrect oil type or viscosity

Using the wrong type of oil or oil with an incorrect viscosity can also contribute to oil leakage from the exhaust. When you use the wrong oil:

  • The oil may not provide adequate lubrication and protection.
  • The oil can be more prone to leaking past seals and gaskets.

Poorly functioning crankcase breather

A crankcase breather allows the escape of harmful gases and pressure from the engine. When the crankcase breather is not functioning correctly:

  • Pressure builds up within the crankcase, forcing oil into the combustion chamber.
  • This increased pressure can lead to oil leakage through the exhaust.

Malfunctioning head gasket

The head gasket forms a seal between the engine block and the cylinder head. When the head gasket is faulty:

  • Oil can leak into the combustion chamber.
  • The compromised seal can result in oil coming out of the exhaust.

Diagnosing the issue

To resolve the problem, you must first diagnose the root cause. Here are some steps you can follow:

  1. Visual inspection of the exhaust system: Check for visible signs of oil leakage from the exhaust pipe and surrounding components.
  2. Checking oil levels and viscosity: Ensure that the oil

level is within the recommended range and that the oil is of the correct type and viscosity for your lawn mower. 3. Inspecting the piston rings and head gasket: Look for signs of wear or damage to the piston rings and head gasket, which may indicate a need for replacement.

  1. Examining the crankcase breather: Check the crankcase breather for blockages or damage that may impair its function.

Solutions to fix the issue

Once you have diagnosed the cause of oil leakage, take the appropriate steps to fix the problem:

  1. Draining excess engine oil: If your engine oil is overfilled, drain the excess oil to bring the level within the recommended range.
  2. Replacing worn or damaged piston rings: If the piston rings are worn or damaged, replace them to restore proper sealing and prevent oil leakage.
  3. Using the correct oil type and viscosity: Ensure that you are using the appropriate oil for your lawn mower, as specified by the manufacturer. This will help maintain proper lubrication and reduce the likelihood of leakage.
  4. Repairing or replacing the crankcase breather: If the crankcase breather is malfunctioning, clean or replace it to restore proper ventilation and prevent pressure buildup.
  5. Fixing or replacing the head gasket: If the head gasket is damaged or faulty, repair or replace it to reestablish a proper seal between the engine block and cylinder head.

Preventative maintenance tips

Regular maintenance can help prevent oil leakage and extend the life of your lawn mower engine. Here are some essential tips for keeping your engine in top shape:

  • Regularly checking oil levels: Periodically check the oil level to ensure it is within the recommended range. Overfilling or underfilling can cause issues with engine performance and longevity.
  • Changing engine oil as per manufacturer recommendations: Follow the manufacturer’s guidelines for oil change intervals to maintain engine cleanliness and lubrication.
  • Periodically inspecting engine components: Regularly inspect the piston rings, head gasket, and other engine components for signs of wear or damage that may lead to oil leakage.
  • Using the correct oil type and viscosity: Always use the appropriate oil for your lawn mower to ensure optimal performance and protection.
  • Ensuring proper engine ventilation: Maintain a clean and functioning crankcase breather to prevent pressure buildup within the engine.


In summary, oil leakage from a lawn mower exhaust can be caused by several factors, including overfilled engine oil, worn or damaged piston rings, incorrect oil type or viscosity, a poorly functioning crankcase breather, and a malfunctioning head gasket. Diagnosing and fixing the issue is crucial to maintaining your lawn mower’s performance and preventing further damage. Regular preventative maintenance, such as checking oil levels, changing engine oil, and inspecting engine components, can help keep your lawn mower running smoothly and efficiently for years to come.


  • Nathan Collins

    Having spent years working in the landscaping industry, Nathan Collins has cultivated a wealth of knowledge about the natural world. He is committed to helping others appreciate the beauty in their backyards, whether it's through identifying rare rocks and minerals or crafting the perfect landscape.

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