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Why Your Lawn Mower is Smoking (Black, Blue or White Smoke)

The first thing you should do when you notice smoke coming from your mower is to stop the engine to avoid potential engine damage.

A lawn mower is smoking due to a plugged air filter, low engine oil, too much engine oil, a bad engine gasket, a blown seal, an oil spill, or problems with the valve train or piston ring.

You should start with the simple items to check before moving on to the more complicated diagnostics.  Wait for the engine to cool and remove the spark plug boot(s) before making repairs.

lawn mower is smoking

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Follow all safety instructions provided in your equipment operator’s manual before diagnosing, repairing, or operating. Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.

Reasons a Lawn Mower is Smoking (Black, Blue or White Smoke)

1. Clogged Air Filter

If my engine was smoking the first Item I would check is the air filter system. This is the easiest thing to check so why not start there? 

  • First, remove the air filter cover. The cover is always located above the carburetor so that it can pull in clean air. Depending on the type of mower you have, the cover usually has a single screw or two knobs holding it down.  
  • Some air filters will be along the side of the engine next to the carburetor. 
  • Pull the filter out being very careful not to let any dirt fall into the intake pipe when you remove the filter. 
  • Inspect the filter and check to see if it is full of dirt and grass. If it is, the engine will not be able to breathe. Instead of pulling in the fresh air, a clogged filter will bring in air from the engine crankcase or the internal part of the mower engine. 

    The air that comes from the engine can pull up oil with it. The burning of this oil can create smoke. 
  • If your air filter is clogged, you need to replace it. If the air filter doesn’t look too bad, knock the dirt out of it and reinstall it. I explain the cleaning procedures here.

2. Insufficient Engine Oil Level

If the air filter is not the problem, the next thing you are going to check is the engine oil. Now you may be asking yourself why you are checking the oil. If your engine oil is lower or higher than the manufacturer’s recommended oil level it can result in a smoking lawn mower. 

Low Engine Oil Level 

If your engine oil is low, you could be burning up the engine due to extreme heat causing lawn mower engine to smoke. In this case, the engine is having a meltdown and burning up the small parts in the engine.

You can try to add fresh oil to see if the smoking stops, but most of the time, at this stage, it is often too late for this simple solution. 

High Engine Oil Level 

Having too much oil can cause too much crankcase pressure which causes oil to get back into the cylinder or even up to the air intake through the valve train.  
In this case, the oil is getting into the cylinder and begins to burn off when the engine is running at full speed creating smoke. You need to drain some oil out of the engine to resolve this problem. 

Read more about the engine damage from adding too much oil to your lawn mower.

3. Piston Ring Problem 

After checking the easier troubleshooting steps and not finding any problem with the air filter or oil level, the next thing to do is check for oil on the spark plug. To do this, you will need a socket to remove the spark plug.  

You may need a 3/4″ or 5/8″ socket to remove the plug. Inspecting the plug isn’t going to tell us a lot about the type of engine problem you have but pulling out the spark plug and checking to see if there is excessive oil buildup on it indicates you have a larger internal engine problem

If you are mechanically inclined, you could check for internal engine damage. I recommend bringing your mower to a small engine dealer repair shop if you are not. If you find a lot of oil on the spark plug you may have a piston ring or valve train problem.  

Either problem will result in you having to take the engine apart. A spark plug with oil build-up can indicate there is a ring problem and a score inside the cylinder wall.

If the cylinder has a score in it, the engine can bring up the oil to the combustion chamber and begin burning it.  

This will create your lawn mower to begin smoking while in use. You may have to replace the engine in this situation. 

4. Valve Train Problem 

Sometimes having a valve train problem is not that bad of a problem to have. The only way to detect this is by removing the cylinder head or performing a leak-down test. This should be completed by a small engine mechanic.  

The cause of a valve train problem is the result of the valve getting burned by being overheated. The edges of the valve begin to fall apart due to the heat. This is what is often referred to as a burnt valve.  

You can also see a burnt valve or timing problem if the muffler is glowing red from heat while the engine is running. In this case, you will need to replace the valve and grind the seat.

The seat is where the valve contacts the engine block to complete the combustion chamber.  

Both the valve and the seat will have to be cut at certain angles in order to make the valve seat correctly. 

5. Bad Mower Engine Gasket 

A bad engine gasket may leak oil onto the muffler and make the lawn mower engine smoke as it burns. You will need to find the bad gasket and replace it.

This may sound easier than it actually is. It becomes increasingly more difficult depending on the location of the bad gasket. 

Difference Between White, Blue and Black Mower Smoke

Sometimes the color of the smoke can give you an idea of what kind of engine problem you are dealing with. It is best to go through the checklist above to narrow down the issue, but here is a quick list of items that could be causing the smoke. 

Blowing Black Smoke

This color usually is due to the engine running too rich. The ratio of fuel to air is incorrect with more fuel being burned than air.

If you see black smoke coming from the mower, the first thing to check is the air filter. A plugged air filter must be cleaned or replaced. If the filter isn’t the issue check for another air flow restriction. 

Blowing Blue or White Smoke

This color smoke is due to burning excess oil. It is best to follow the steps above to identify the root cause starting with checking the engine oil level followed by checking for damage to the piston rings, valve train, or engine gasket. 

It is essential to continue to check your engine oil level and air filter before each mowing use to avoid large expensive repairs. For a checklist to perform before each mowing, I explain it in my Lawn Mower Checklist

Complete Regular Engine Oil Changes 

It may seem like a lot of work to change your engine oil, but changing your oil is necessary. To keep your lawn mower running and not shorten the life of the mower’s engine, you must change the engine oil regularly. 

Read more about the damaging effects of skipping your oil change, Engine Damage is Likely if You Skip Mower Oil Changes

Still Having Problems with Your Lawn Mower?

Lawn mower ownership doesn’t come without its frustrations. Own a mower long enough, you are bound to run into many lawn mower problems including starting, smoking, leaking, cutting, and overheating.

For mower troubleshooting, check out my guide Common Lawn Mower Problems: Solved.