Why Is My Lawn Mower Oil Contaminated with Gas?

You’re out there, enjoying a sunny afternoon, mowing your lawn, when suddenly, your lawnmower sputters and dies. You check the oil, and instead of clear, golden liquid, you find a murky, brownish mixture – a horrifying mix of oil and gasoline. This is a problem, and one that can potentially lead to serious engine damage. So, why is your lawn mower oil contaminated with gas, and what can you do about it? This article will delve into the causes of this issue, explain how it can affect your engine, and offer solutions to prevent and resolve this problem.

What Causes Gas to Get in Oil?

The culprit behind gas mixing with your lawnmower’s oil is often a faulty carburetor. The carburetor is responsible for mixing air and fuel in the correct proportions, ensuring your engine runs smoothly. But when it malfunctions, things can go wrong.

Common Causes of Carburetor Malfunction

1. Worn-Out Float Valve

The float valve, a critical component of the carburetor, is responsible for regulating fuel flow. When it wears out or gets stuck, it can allow excess fuel to flow into the engine, leading to a mixture of gas in the oil.

2. Clogged Jets

The carburetor contains tiny jets that regulate the fuel flow. These jets can become clogged with dirt, debris, or varnish buildup, resulting in a richer fuel mixture and potential oil contamination.

3. Improper Carburetor Adjustment

A carburetor needs to be carefully adjusted to ensure the correct air-fuel ratio. Incorrect settings can lead to a rich fuel mixture, causing excess fuel to seep into the crankcase and contaminate the oil.

4. Cracked or Damaged Fuel Lines

The fuel lines connecting the fuel tank to the carburetor can crack or become damaged over time. This can lead to fuel leakage, which can find its way into the engine and contaminate the oil.

5. Leaky Fuel Pump

The fuel pump is responsible for delivering fuel from the tank to the carburetor. If the fuel pump develops a leak, fuel can seep into the crankcase and mix with the oil.

How Does Gas in Oil Affect Your Lawn Mower Engine?

Gas in oil can have severe consequences for your lawn mower engine.

  • Reduced Lubrication: Gasoline doesn’t provide the necessary lubrication for your engine’s moving parts. The oil is diluted by the gas, reducing its effectiveness and leading to increased wear and tear.
  • Corrosion: Gasoline is corrosive to engine components. When it mixes with the oil, it can damage the engine’s internal parts, leading to costly repairs.
  • Premature Engine Failure: The combined effect of reduced lubrication and corrosion can lead to premature engine failure, requiring a replacement engine.

Diagnosing and Solving the Problem

  1. Inspect the Fuel System: Start by examining the fuel lines for cracks or leaks. Pay attention to the fuel pump and check for any signs of fuel leakage.
  2. Check the Carburetor: Remove the carburetor and inspect it carefully for any signs of wear, damage, or clogging. Clean the jets and replace worn-out or damaged parts.
  3. Adjust the Carburetor: If the carburetor needs adjustment, consult your lawnmower’s manual for the correct settings.

Preventing Gas in Oil

  • Regular Maintenance: Perform regular maintenance on your lawnmower, including cleaning the carburetor and replacing worn-out parts.
  • Use Fresh Fuel: Store your gasoline in a sealed container to prevent moisture contamination and use fresh fuel whenever possible.
  • Avoid Overfilling the Tank: Overfilling the fuel tank can cause fuel to leak into the crankcase.
  • Proper Storage: When storing your lawnmower for extended periods, empty the fuel tank to prevent fuel from degrading and causing problems.

When to Seek Professional Help

If you’re not comfortable diagnosing or repairing the problem yourself, it’s best to take your lawnmower to a qualified mechanic. They can diagnose the issue accurately and repair it efficiently.


Gas in oil is a serious problem that can damage your lawnmower engine. It’s important to address this issue promptly to prevent further damage. By understanding the causes, diagnosing the problem, and taking preventative measures, you can keep your lawnmower running smoothly and avoid costly repairs.


Why Is My Lawn Mower Oil Contaminated with Gas?

This is a common issue, but it can be difficult to diagnose and requires some mechanical knowledge to resolve.

How do I know if my lawn mower oil is contaminated with gas?

You can tell if your lawn mower oil is contaminated with gas by its appearance and smell. Contaminated oil will have a gasoline-like odor and may be discolored, often appearing milky or cloudy. If you suspect contamination, it’s best to check the oil level and inspect the oil for any abnormalities.

What causes gas to get into the lawn mower oil?

There are a few common causes of gas contamination in lawn mower oil. One possibility is a faulty carburetor, which can allow gas to leak into the crankcase. Another possible culprit is a worn or damaged fuel line. A third common reason is a clogged air filter, which restricts airflow and forces excess fuel into the engine.

What are the risks of using contaminated oil in my lawn mower?

Contaminated oil can cause significant damage to your lawn mower engine. The gas in the oil can act as a solvent, thinning the oil and reducing its ability to lubricate engine parts. This can lead to wear and tear on the engine, including piston damage, bearing failure, and even engine seizure.

What should I do if I find gas in my lawn mower oil?

The first step is to drain the contaminated oil and replace it with fresh oil. It’s also a good idea to check the spark plug and clean or replace it if necessary. Next, you need to find the source of the contamination. You may need to consult a mechanic for assistance in this step.

Can I use my lawn mower if the oil is contaminated?

It’s not advisable to operate your lawn mower with contaminated oil. Doing so could result in serious engine damage. It’s best to address the issue immediately and avoid further use until the problem is resolved.

What should I do to prevent gas from getting into the lawn mower oil in the future?

There are a few things you can do to prevent gas from getting into your lawn mower oil. First, make sure you are using the correct fuel mixture. Second, ensure that the carburetor is properly adjusted. Finally, regularly check your fuel lines and replace any worn or damaged parts.

How do I fix a faulty carburetor?

Repairing a faulty carburetor is a complex task that often requires specialized tools and knowledge. If you’re not comfortable working on your own mower, it’s best to consult a qualified mechanic. A mechanic can diagnose the problem and make the necessary repairs.

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