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White smoke from a lawnmower? What the pros say

Every lawnmower owner experiences odd-colored smoke from their mower’s exhaust at some point. Among the different colored smoke, your mower may produce white smoke, which is the most common. It is natural to get worried when you see any smoke coming from your mower’s exhaust, especially if you have a 4-stroke mower because 4-stroke mowers aren’t meant to smoke from their exhaust. Most of the time, white smoke from your mower’s exhaust will go away on its own, but other times, it might not go away, indicating that something is wrong with your mower.

The most probable cause of white smoke is that your lawnmower’s engine burns oil. This can happen due to any of the following reasons:

  • Tipping the mower on its side or using the mower at a steep slope
  • You have overfilled the mower with engine oil
  • Damaged head gasket
  • Piston ring failure
  • The petrol you used in your mower had engine oil in it

Some causes of engine oil entering the combustion chamber of your mower’s engine can be resolved by simply letting the mower idle for a while until the smoke clears out. While other causes might require a little more effort to fix, and you may need help from a professional small engine mechanic to stop white smoke from your mower’s engine.

In this article, I will explain the possible causes of white smoke from your mower’s exhaust and how you can fix them.

Tipping the mower or using it at a steep angle

If you tip a mower the wrong way or use it at a very steep angle, it can cause the engine oil to make its way into the cylinder and start burning. This is one of the most common reasons why lawnmowers produce white smoke.

People tip the mower to its side or back the wrong way so they can take a look under the deck or replace the blades. Or the lawnmower might have been used at a surface that is too steep. Both of these situations may result in the cylinder filling up with oil and white smoke being produced from the exhaust.

Fix: Tipping the mower or using it at a steep angle

If your mower has started smoking after being tipped over or using it at a steep angle, you can get rid of the white smoke by simply letting the mower run on idle until it burns up the oil that has gotten into the combustion chamber. To know the correct way to tip the mower on its side, read the owner’s manual of your lawnmower.

Similarly, I recommend avoiding using your mower at a surface with a slope of more than 15 degrees, as too steep can cause excess engine oil to enter the cylinder and start burning, resulting in a cloud of white smoke. Most lawnmowers should be fine if tipped on the side for maintenance with the carburetor and air filter facing upwards.

Overfilling the engine with oil

Engine oil is your friend if you want to make your lawnmower’s engine last as long as possible and keep it running smoothly. Changing the engine oil after regular intervals is part of the normal maintenance routine for your lawnmower. But the important part of every oil change is to avoid filling the mower’s engine with oil and, simultaneously, not to put in less oil than needed.

While a lawnmower engine with less than a normal amount of oil will face a lot of friction due to lack of lubrication, if you have excess oil in your mower’s engine, the oil will enter the cylinder and start burning along with the fuel. This results in white smoke from the exhaust, and sometimes excess engine oil can start coming out of the exhaust muffler.

If you have too much oil in your mower, it will start leaking, and when the engine gets hot, it will produce white smoke as the oil burns. Excess engine oil will not just cause the mower to produce white smoke but can also drown the crankset and hinder its motion. In most cases, a mower overfilled with engine oil won’t start, or it will take too long to start up and will be difficult to idle because of burning engine oil in the combustion chamber.

Fix: Overfilling the engine with oil

Preventing accidental overfilling of engine oil is fairly simple, and it is best to avoid filling excess oil in your mower in the first place. When it is time to change the engine oil, refer to the user manual of your mower to find out the exact amount of engine oil your lawnmower needs. Normally a lawnmower will not need more than 20 ounces of engine oil.

Once you have put engine oil in your lawnmower’s engine, use the dipstick to ensure the oil level is within the indicated mark on the dipstick. If you have already put too much engine oil in your mower, then the only solution is to remove the excess oil. You will have to drain all the engine oil from the engine and then pour in the recommended amount.

Although overly filling a lawnmower engine with oil isn’t as bad as underfilling it as most lawnmower engines use the splash lubrication method. A small amount of oil is systematically splashed on different engine components to lubricate them, while the rest stays at the engine’s bottom. 

Here you can find good oil for your Lawn Mower.

Damaged or leaking head gasket

While the first two causes of white smoke were not something that would cause worry or require you to do a serious repair, a leaking or blown head gasket is one of the more serious reasons your lawnmower might be producing white smoke.

A Head gasket is a seal used to keep the engine closed off, and it is located in the area that joins the cylinder with the rest of the engine. If your lawnmower has a damaged head gasket, then the white smoke won’t go away if you run the mower for a while as it does if the mower is tipped on the side or overfilled with engine oil.

 When the head gasket is not sealing the cylinder head properly, oil can leak into the cylinder from the crankcase. It doesn’t take a very big crack in the gasket to suck oil into the combustion chamber since there is a lot of compression in the cylinder that sucks oil from the crankcase, even if there is a tiny leak in the head gasket.

As the oil starts to enter the cylinder, it can start burning and cause white smoke, and the number of smoke increases if the engine keeps running since more oil is being sucked into the combustion chamber through the damaged gasket. In addition to consistent white smoke from the exhaust, you can hear the air being blown from the leaking gasket and loss of compression, which causes low power output from the mower.

If a lot of oil is rushing into the cylinder, the engine may fail to start or stop running after a while. Moreover, you will notice oil around or under the head gasket and on the body of the lawnmower that is leaking from the damaged head gasket seal.

Fix: Damaged or leaking head gasket

A Head gasket is found in an overhead valve (OHV) engine, and if it is damaged or starts leaking, the only way you can fix it is by putting a new head gasket in because gaskets are not repairable once they become damaged.

A head gasket is not very expensive, and you can get a new, good, quality one for a few dollars. If you have worked on a small engine before, you can easily replace your lawnmower’s head gasket by yourself by following these simple steps:

Step 1: Get a replacement head gasket

Purchase a replacement head gasket. Your local dealer can get you the correct one or find one online.

Here you can find head gaskets for your Lawn Mower.

Step 2: Remove the cylinder head

Before you start, you will have to remove the sparkplug wire and the bolts that hold the cylinder head onto the engine block using a socket.

Step 3: Remove the gasket

You will find the damaged gasket on the engine block where the cylinder head connects. Take the gasket off and use a tool to scrape off any hard-to-remove pieces of the gasket but be careful not to damage the smooth surface when scraping off the debris.

● Step 4: Add the new gasket

Place the new gasket on the engine block and place the cylinder head back in place before bolting it back onto the cylinder head and putting the sparkplug wire on the sparkplug. You can purchase head gaskets here.

● Remark: check if you have an overhead valve engine

Only overhead-valve engines have gaskets, so before you take your lawnmower’s engine apart, ensure that your lawnmower has an overhead-valve engine. When you have an overhead valve engine, you will see the valves and cylinder head above the combustion chamber. The valves are below the combustion chamber if your engine is a flathead engine.

Failed piston rings

Of all the causes of white smoke in lawnmowers, a  failed piston ring is probably the worst one, and it is normally seen in older, worn-out lawnmowers or if the mower is poorly maintained. Piston rings control the amount of engine oil and supply a minimal amount of oil to the piston to move smoothly inside the cylinder.

Piston rings of your mower’s engine also remove excess oil from the combustion chamber and flow it back into the engine block. But when these piston rings fail, there is nothing to stop the engine oil from entering the combustion chamber and burning up to produce white smoke.

Piston rings may get damaged due to any number of reasons, but the most common ones include:

  • An unclean or worn-out air filter may allow dust and other particles to enter the combustion chamber and damage the piston rings.
  • Not replacing the engine oil on time or dirty engine oil will cause poor lubrication and damage to the piston rings.

If your mower is burning up a lot of oil and producing white smoke as it runs with little to no power, it can have bad piston rings.

Fix: Failed piston rings

Unfortunately, repairing failed piston rings is not simple; it will require a complete engine rebuild, and the whole engine will need to be opened up. If your lawnmower has bad piston rings, the head cylinder will also have signs of damage and need to be replaced.

This kind of repair requires special tools and a lot of expertise, and it is best to leave it to a professional repair shop or install a brand-new engine since rebuilt engines don’t tend to work as well as new ones.

Here you can find lawnmower piston rings.

Oil in the fuel of a 4-stroke mower

If you own a 4-stroke lawnmower, mixing oil in your mower’s fuel is a big no. Many modern-day lawnmowers come with 4-stroke engines because they are more reliable and last much longer than 2-stroke engines. Besides being reliable, 4-stroke lawnmowers have fewer maintenance needs, and unlike a 2-stroke lawnmower, you don’t have to mix oil in its fuel.

If the fuel you put inside your lawnmower has oil in it, then as the lawnmower starts running, it will burn the oil that has been mixed in the fuel and produce a lot of smoke. And if you keep running the engine with engine oil in the fuel tank, it can lead to damage and premature engine wear.

Fix: Oil in the fuel of a 4-stroke mower

Depending upon how much oil got into the fuel tank, you might be able to get rid of oil in the fuel tank by simply running the mower till it burns off the contaminated gas and then filling it with fresh, clean gas in it. If you are certain that very little oil went inside the mower’s fuel tank, it will get dissolved in the gas and get used.

Of course, you will get white smoke from your mower’s exhaust until all the oil has been burnt. But if you are unsure how much oil got mixed with the fuel and want to be on the safer side, then you will have to remove the fuel from the fuel tank and carburetor before filling it with new fuel. You can remove oil-mixed fuel from your lawnmower by following these simple steps:

● Step 1: Locate the fuel line

Locate the fuel line that exits the fuel tank and feeds fuel into the engine.

● Step 2: Remove the fuel line

Remove the fuel line from where it exits the fuel tank by undoing the clip that holds the fuel line in place and pulling the fuel line off. Or you can follow the fuel line where to the shut-off valve and remove the fuel line where it connects with the shut-off valve.

A shut-off valve is a small knob that allows you to stop the flow of fuel to the engine, and it is pretty simple to locate.

● Step 3: Collect the spilled gas

After the fuel line is removed, the fuel will start pouring out of the gas tank, and you can collect it in a pan to avoid spilling it since spilled gasoline can be a fire hazard, so you would want to prevent any spillage on or around the lawnmower.

● Step 4: Drain the carburetor

As the fuel tank is emptied, go ahead and locate the carburetor of your lawnmower to drain fuel from it. Lawnmower carburetors have a carburetor bowl underneath the carburetor, containing a small screw or nut that can be used to empty the carburetor.

Simply undo the screw or nut on the bottom or side of the carburetor bowl and let the fuel drain into a pan.

● Note 1: Do not tilt the lawnmower

Avoid tilting the mower to drain the fuel out of the mower because this can result in engine oil spilling into the cylinder or the carburetor.

● Note 2: Check for stuck oil

If you notice the oil is stuck inside the mower’s fuel tank, you will have to remove the fuel tank and clean it using dishwashing soap or liquid. Pour the soap inside the fuel tank and shake it well so the oil stuck inside it is removed.

Use a can of carburetor cleaner to clean the carburetor by spraying the carburetor cleaner on it.
You can find a good carburetor cleaner here.

Final remarks

White smoke coming from the lawnmower should never be ignored as it can potentially be a sign of a serious problem with your mower’s engine that needs immediate attention. Using the details provided in this article, you can easily pinpoint the exact cause of white smoke from your mower, and the troubleshooting process will be much simpler. But one thing is common among all the causes of white smoke from your mower; it mostly occurs due to your lawnmower’s engine somehow burning engine oil.