Why Is Oil Coming Out Of My Lawnmower Exhaust?

Upgraded Home Team
by Upgraded Home Team
Lawn mower engines are relatively complex, especially for new homeowners. One common issue is when oil comes out of your lawn mowers exhaust. Whether it be a bad carburetor or dirty valves, let’s take a look at the simple solutions to oil coming out of your mower’s exhaust.

Because your lawnmower’s engine is relatively complex, it can sometimes encounter problems. One particular issue with gas-powered mowers is that oil can start leaking into the exhaust. If this does happen, you have to correct the problem ASAP.

The most common reasons why oil is coming out of your lawnmower exhaust are fluids shifting from tilting the lawnmower on its side, an overfilled crankcase, or a damaged piston ring. You can fix the issue easily once you identify the problem, which can be challenging.

In this article, we will discuss each potential problem in-depth and provide solutions to get your mower back to normal.

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Top Reasons for Oil In Your Lawnmower Exhaust

Damaged Piston Ring

The pistons are an integral of your lawnmower’s engine, whether you have a push model or a riding mower. The pistons help distribute air and gasoline for combustion, which is what makes your machine run smoothly. Because the pistons are always moving, they need oil to stay lubricated.

A piston ring is a rubber seal that contains the oil and prevents it from leaking. So, if the ring is damaged or worn, the liquid can seep out. Usually, piston rings will last for years, but they can crack or break down because of environmental factors or neglect. As oil leaks from the piston, it will mix into the gasoline and come out of the exhaust pipe.

The Fix: Replace the Piston Ring

If you’re handy with an engine, you should be able to locate the pistons and remove them. Be sure to check your owner’s manual for reference, just in case there are any unique steps. Each piston has multiple rings, so you will want to remove each one and inspect it.

There is a special tool available to make this process easy. It’s called a ring expander. However, if you don’t want to purchase a new item for this project, you can usually use needlenose pliers. Since you’re replacing the rings, don’t worry if you damage the material.

Once the old rings are off, be sure to clean the grooves and wipe away any excess oil. Then you can put the new rings on and replace the piston. Oil leaks can happen from multiple pistons, so make sure that you check them all before starting the machine again.

Dirty Air Filter

This issue is not very common, but it can happen. The air filter allows oxygen to get into the engine, where it mixes with the gasoline to create combustion. If the filter is so dirty that air can’t pass through it well, the mixture may have too much gas. When that occurs, the excess fluid will get into the muffler and escape through the exhaust. So, while it might look like an oil leak, the liquid is actually gasoline.

The Fix: Replace the Air Filter

Typically, this problem happens when the filter is filthy. If it just has some dirt and debris inside, the source is likely the carburetor. However, it’s always a good idea to replace the air filter regularly, as doing this will help the engine work more efficiently. Sometimes, you can simply clean the filter, but that is only a temporary fix.

Worn Valves

There are two valves within your mower’s engine; one for intake and one for exhaust. Over time, these valves can wear down, which doesn’t allow the cylinders to stay sealed. When that happens, oil can seep into the exhaust. Usually, the fluid will go into the cylinders, but it can find its way into the valves as well.

The Fix: Repair or Replace the Valves

In most cases, the valves have simply knocked loose, which means that you have to tighten them in place. You will want to look at your owner’s manual to locate each valve for a visual inspection. The problem may be with either valve, so check them both before starting. Sometimes, the valve itself may be stuck, which means that you have to loosen it to start working properly.

This project is relatively complicated, so you will want to follow a step-by-step guide to learn how to adjust or replace these parts. Also, make sure that they are genuine original pieces since they will last a lot longer than OEM components. However, keep in mind that if your mower is old, you might have to do some searching to find the right model number.

Overfilled Crankcase

The crankcase is the section that houses the crankshaft, which is an integral part of your engine. When you pour oil into your lawnmower, most of it winds up in the crankcase to lubricate the shaft. However, because lawnmowers have such small motors, you don’t need much oil to fill them. If you have overfilled the tank, the excess will get pushed into the cylinder, where it can make its way to the exhaust valve.

The fluid will also get into your carburetor, which is how it winds up in your exhaust pipe.

The Fix: Clean Your Carburetor and Never Overfill the Oil

The first way to identify this problem is to check the oil dipstick. If it shows too much fluid, you will want to drain the excess into a drip pan. Fortunately, this process is relatively easy on a lawnmower, since there is often a single valve that connects to the oil tank.

Another thing you will want to do is access your carburetor to clean it. While extra oil will often burn off, it could leave a sticky residue that will make your carburetor work less efficiently. Refer to your owner’s manual to see where it is. You can also find more detailed instructions on cleaning the carburetor here.

To prevent this problem in the future, be sure to add a little bit of oil at a time. Check the dipstick and wipe it off to ensure an accurate reading. Your owner’s manual should also tell you how much oil should go into the engine. We recommend finding a plastic measuring cup and marking the correct amount to make it easier during your next oil change.

Leaks From Tipping the Mower

If you have a push mower, you will have to tip it on its side from time to time. You may lift the machine to clean out the cutting deck or remove the blades. When you tilt the lawnmower in the wrong direction, oil and gas will leak into the engine. When this occurs, it can spill into the exhaust valve or get into the carburetor.

The Fix: Always Be Careful When Tipping Your Mower

As a rule, you should keep the oil and gas intake on the upper side of the machine when tipping it. Doing this will help prevent leakage. Also, you can elevate your mower and tilt it from the back to avoid any potential spills. You can find out more about how to do that here.

Dangers of Oil in Your Lawnmower Exhaust

Generally speaking, engine fluids should always remain in the engine. If they start leaking to other parts of the lawnmower, you can experience problems, such as:

  • White Smoke – If oil gets onto various components, it will burn off as the motor heats up, creating white smoke. This can be an environmental hazard and could damage your lungs if you breathe it in.
  • Dead Grass – As oil drips onto your lawn, it will seep into the soil. Over time, drips will create dead patches of grass, which can make your yard look patchy and rough.
  • Hard Starting – If the problem is too much oil in the tank, you’ll notice that it’s harder to start the engine. This can also occur if there are issues with the valves.
  • Fire Hazard – Not only will oil leak onto the grass, but it will get on any surface where the mower sits. This fluid is flammable, so it could create a fire hazard in your garage or shed.

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Related Questions

Should I take my mower into a shop if I notice oil in the exhaust?

If you’re unsure whether you can fix the issues we’ve listed above, then it will be better to take your machine to a professional workshop.

Can I prevent oil from leaking into the engine?

As long as you perform routine maintenance on your mower and keep it clean, oil leaks shouldn’t be a problem.

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Upgraded Home Team
Upgraded Home Team

We are a team of passionate homeowners, home improvement pros, and DIY enthusiasts who enjoy sharing home improvement, housekeeping, decorating, and more with other homeowners! Whether you're looking for a step-by-step guide on fixing an appliance or the cost of installing a fence, we've here to help.

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