Oil Coming Out of Exhaust Lawn Mower

You’re working on your lawn one fine day when you suddenly notice oil coming out of the exhaust. While it’s not unusual, it should be a major concern. Leaking oil can cause engine startup issues, trigger garage fires when the mower is under storage, and can also cause dead patches of grass if it comes into contact with your lawn. 

Oil coming out of your lawn mower’s exhaust indicates problems with the internal parts. The problem likely stems from faulty piston rings, the reciprocating cylinder, the exhaust pipe, or the exhaust manifold.

We’d like you to keep your lawn pristine and lawnmower in peak shape. Therefore, we get to the root of the problem and highlight some restorative measures. 

Why is oil coming out of the lawnmower’s exhaust?

The top causes of oil coming out of the lawnmower exhaust include worn-out piston rings or intake valve, an overfilled crankcase, or tilting the mower the wrong way. A clogged air filter and a carburetor that’s not adjusted may also cause oil to leak into the cylinder before being ejected via the muffler.

The causes of oil coming out of lawnmower muffler include:

1. Worn-Out Piston Ring

The piston rings regulate the amount of oil between the piston and the cylinder wall. They close the gap between these two engine components, preventing blow-by and oil from leaking into the cylinder. As such, if any of the piston rings are worn out due to environmental elements, the oil may leak into the cylinder before finally being pushed out of the exhaust/muffler.

2. Lawn Mower Tilted the Wrong Way

When doing blade maintenance (such as blade repair or replacement) on a mower, you need to position the mower on its side to access the blade. However, a lawnmower should only be tilted 15 degrees in one direction, with the carburetor and air filter facing up. Tilting a mower in the wrong direction and letting it stay in that position for long might lead to oil leaking out of the crankcase and into the cylinder due to gravity.

3. Worn-Out Intake Valve

When the intake valve is worn-out, it doesn’t properly seal the cylinder and may lead to oil ending up inside the cylinder before finally being ejected via the muffler.

4. Overfilled Crankcase

In 4-stroke engines, the oil is usually stored in the crankcase where it’s needed for shaft lubrication. However, the small crankcase can only hold about 1 quart of oil. As such, if you fill it with more oil than it can hold, the excess oil is pushed into the cylinder and eventually out of the mower via the exhaust system.

5. Clogged Air Filter

Sometimes, the fluid coming out of your mower’s muffler isn’t oil. It’s gasoline fuel. This happens when the air filter becomes clogged by dirt and debris and cannot let in enough air to facilitate complete combustion inside the cylinder. The unburnt fuel is then pushed out of the cylinder during the exhaust stroke and comes out via the exhaust system.

Note: Unburnt fuel coming out of the muffler is technically referred to as flooding. It’s easy to confuse it for oil since it reacts with carbon and comes out of the muffler as a dark fluid as it’s being ejected. Still, you can differentiate fuel from engine oil by its characteristic smell.

6. Imbalanced Carburetor

You may sometimes confuse unburnt fuel coming out of the exhaust for oil. Apart from a dirty air filter, an imbalanced carburetor can also cause incomplete fuel combustion. Since the carburetor is out of adjustment, the air-gasoline mix drawn into the engine becomes flawed, resulting in incomplete combustion and the unburnt fuel coming out through the exhaust system.

7. Faulty Engine Components

Certain engine components can also cause oil leaks if they’re damaged. For instance, oil can leak into the cylinder if the head gaskets are blown or if the engine block is cracked. Head gasket seals usually prevent fluid leakage. However, while it’s rare, they sometimes wear out and can cause oil to leak out and into the cylinder. 

Note: Look out for blue-ish smoke when you run the engine. It’s a sign the oil leaking out of your lawnmower exhaust is due to gasket failure.

How to fix a lawnmower spitting oil out of the muffler

Oil dripping out of the lawnmower’s muffler can cause engine hard start, burn turf grass, and pose a fire hazard. This oil also burns off as white smoke when the engine heats up, posing an environmental hazard. It’s, therefore, important to fix the underlying issue as soon as you notice oil coming out of your mower’s exhaust.

Possible remedies for a lawnmower that’s spitting oil out of the muffler include:

  • Replacing worn-out piston rings
  • Replacing the air filter
  • Repairing/replacing damaged intake valve
  • Adjusting and cleaning the carburetor
  • Repairing faulty engine parts

Replace Damaged Piston Rings

To replace worn-out piston rings, you have to access the pistons and remove them. If you’re unsure of how to disassemble your lawnmower engine, refer to the brand manufacturer’s instruction manuals for detailed guidelines on the same.

After accessing the pistons, remove the rings using a ring expander or needle-nose pliers and inspect them to determine the faulty ones. Then, wipe off excess oil from the piston ring grooves and install the replacement rings. Finally, replace the pistons and reassemble your engine housing.

Repair or Replace Faulty Intake Valve

A faulty intake valve can cause improper sealing, leading to oil leaking into the cylinder and exhaust system. You need to fix or replace the valve. Repairing or replacement of engine valves is a complex job. Engine failure is easy if you do a shoddy job. It’s a smart move to pay a professional to fix the faulty intake valve.

Drain out Excess Oil

Draining excess oil out of the mower helps if the oil leakage is from an overfilled crankcase. Afterward, ensure that you don’t overfill the crankcase with oil again. Engine manufacturers usually give guidelines on how much oil should be poured into the engine at a time.

Replace Dirty Air Filter

If flooding is causing fuel to come out of your muffler and the underlying problem is not the carburetor, replace the dirty air filter.

Remove the dirty air filter from the filter housing and wipe the housing clean. Dispose of the old air filter and install the new replacement filter. Finally, reattach the casing.

Adjust and Clean the Carburetor

If the oil coming out of your lawnmower’s muffler is unburned gasoline caused by carburetor imbalance, adjusting and cleaning the carburetor will fix the issue. To adjust the carburetor on a riding lawnmower, adjust the idle speed mixture, the high-speed mixture, and the carburetor choke linkage.

Note: The simplest way to clean the carburetor is to spray a carb cleaner into it. Cleaning a carburetor by disassembling it is a more complex process and you may have to call in the pros.

Repair Damaged Engine Components

For oil leaks caused by bad gaskets or a cracked engine block, you should be able to repair or replace these parts by yourself if you have some basic engine troubleshooting skills. However, if you’re unfamiliar with mower engines, it’s best to hire a professional technician to fix the engine parts for you.

To repair a head gasket that’s blown or leaking, reinforce the gasket seal using a gasket sealer product. These products work by drying up to form a tight seal that plugs up the gap within the original gasket seal.

Meanwhile, to fix a cracked engine block, we recommend applying a radiator and block sealer product. These products are specially formulated to form a tight bond with the metallic engine block, essentially sealing up cracks in the block.

Clean the Spark Plug

If the oil that’s coming out of your mower’s exhaust is due to leakage caused by tilting the mower on the wrong side, it’s likely that the oil has leaked into the air filter, carburetor, and spark plug. To avoid potential startup failure issues, you not only need to drain out the excess oil but also replace the air filter and clean both the carburetor and the spark plug.

Car cleaner spray is a great solution to clean the carburetor. Remove the spark plug before cleaning it using a brake cleaner. Finally, wipe off the cleaner residue using a dry, clean cloth and replace the spark plug.

How to clean oil from lawnmower muffler

To effectively clean oil from your lawnmower muffler, remove the muffler by turning it in an anticlockwise direction and pulling it off the engine. Then, spray carburetor cleaning spray into the rear opening of the muffler before spraying the cleaner into the muffler’s external holes as well.

Next, shake the muffler thoroughly with the rear hole facing down. This will get rid of all dirt and debris inside it. Once the carb cleaner dries out, reattach the muffler back by turning it in a clockwise direction.


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