If your lawn mower engine is blowing out white smoke like a locomotive, then your engine is burning off something that it shouldn’t be. The causes will differ depending on whether you have a 2-cycle or a 4-cycle mower engine. However, it’s still the same issue, but it’s reacting in different ways. So, let’s look at both of these different engines and determine why your lawn mower engine is blowing white smoke.
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White Smoke From Lawn Mower Engine (Overview)
White smoke from a lawn mower engine signals that the engine is burning oil. With 2-cycle engines, this is a sign of too much oil in the fuel mixture. With 4-cycle engines, this indicates that oil has made its way from the crankcase to the top of the cylinder.
What to Do When Lawn Mower Starts Smoking?
The best thing to do if your lawn mower engine is smoking is to switch off the engine and investigate what is happening. Depending on the cause of the smoke, your lawn mower engine may need repairing. If you continue running your lawn mower engine and carry on cutting, the potential damage could get significantly worse. So, kill the engine, let the lawn mower cool down, then start your diagnosis.
2-Cycle Lawn Mower Smoking
White smoke coming from a two-cycle engine simply means that your fuel-to-oil mixture is off and there is too much oil in the mix. While this won’t cause any major problems with your mower engine, it will mean that you could mess up a few parts, like the spark plug, spark arrestors, and potentially the air filter.
4-Cycle Lawn Mower Smoking
Four-cycle engines lubricate themselves very differently from two-cycle engines. Rather than adding oil to the fuel, a 4-cycle engine stores oil in the lower part of the engine called the sump/crankcase. So if your lawn mower is blowing white smoke, the engine oil has managed to travel from the bottom of the engine to the top, the cylinder head. Now, there are lots of ways that this can happen. Here are a few examples.
Common Causes of 4-Cycle Lawn Mower Blowing White Smoke
- Tipping the lawn mower over the wrong way
- Using the lawn mower on steep inclines
- Too much oil in the engine
- A blown head gasket
- Broken piston rings and damaged cylinders
Tipping a Lawn Mower Over the Wrong Way
There are two ways to tip over a lawn mower, the right way and the wrong way. If you tip your lawn mower over the wrong way, say when removing the blade, oil can run out of the bottom of the engine. This means the engine oil can flow into the cylinder head, through the carburetor, and out of the air filter. If this happens, your lawn mower engine burns off the oil when you start it, creating white smoke.
Using a Lawn Mower on Steep Inclines
Most mowers should only be used on inclines of about 15° before the engine oil starts leaking out of the bottom of the engine. So, just like tipping a lawn mower over the wrong way, oil makes its way to the top of the engine and produces white smoke as it burns off.
Too Much Oil in the Engine
A pretty common issue when it comes to changing lawn mower oil is using too much. Now you probably wouldn’t think a little extra oil would be a problem. But this isn’t true. You see, the oil has to go somewhere. The excess oil creates pressure in the crankcase, forcing the oil into locations where it shouldn’t be. For example, oil can pass the pistons, break gaskets, and even push past the valves and end up in the fuel delivery system. So, not only is the oil making white smoke, but the excess oil is damaging the engine. Stop the engine immediately if you notice your lawn mower is smoking to avoid additional damage.
A Blown Head Gasket
Head gaskets can suffer damage in several ways. For example, a loose cylinder head, overheating, or low oil, to name a few issues, can cause the head gasket to become damaged. As a result, the oil that passes through the oil passages in the engine can move through the damaged gasket and into the top of the engine. However, oil getting past the gasket isn’t the main issue. Instead, the problem is that the oil in the crankcase is getting lower and lower. Eventually, no oil will be left, and the engine will overheat, which could lead to irreparable damage.
Broken Piston Rings and Damaged Cylinders
To convert the combustion power in the cylinder head, the seal between the pistons and the cylinders must be nice and tight. Not too tight as to cause an issue but tight enough to maintain the seal. Your lawn mower uses piston rings to create this seal. Unfortunately, the seal will release if the piston rings or the cylinder are worn or damaged. As the seal is lost, the oil has a new path to move into the top of the engine and get burned during the combustion stroke.
The piston rings or cylinders can get damaged in several ways, such as lack of oil, engine overheating, and general engine wear. To check, remove the cylinder head and inspect the pistons, or you can conduct a compression test. If the engine compression is low, you likely have an issue with the seal. If you catch this early enough, you can probably replace the rings.
However, if the cylinders are damaged, then it will be too late. Now you could re-bore the cylinders, but this is an expensive process and not really something you would do to a lawnmower. Why? Well, the cost to re-bore would usually be more than the cost to replace an engine.
How to Stop White Smoke From Lawn Mower (6 Solutions)
So what can you do if your lawn mower is blowing white smoke? Well, here are six different things you can try. Let’s take a closer look.
2-Cycle Lawn Mower Smoking White (The Solutions)
First, you should remove the fuel from your lawn mower. If you have a small lawn mower, you may be able to tip the lawn mower over and pour the fuel out into a fuel container. If you can’t use this method, then you can use a fuel pump or fuel siphon to assist you.
Next, add the correct fuel-to-oil mixture to your mower. When it comes to the correct fuel mixture, check your owner’s manual for the precise ratio. Typically it’s around 32:1, fuel-to-oil.
Finally, once you have replaced the fuel, you should check to see if you need to clean the lawn mower air filter. This issue will affect your lawn mower’s performance and can even make your lawn mower hard to start.
4-Cycle Lawn Mower Smoking White (The Solutions)
As you have read, quite a few things can make a 4-cycle lawn mower blow white smoke, certainly more reasons than a 2-cycle lawn mower. So let’s review the solutions for each potential cause and figure out how to stop your lawn mower from smoking white.
Tip Lawn Mower Over the Right Way
When it comes to tipping a lawn mower over the right way, you just need to remember that you always need to keep the air filter facing up. That’s all there is to it. If the air filter is facing up, then oil can’t pour into it. Good old gravity!
Use Your Lawn Mower Within its Limits
I know not many people read manuals, but they are full of important information, including your mower’s recommended limits. So, check for the maximum slope your lawn mower can run on before any issues arise. Unless you like a white smoking lawn mower, you might find that your yard is over and above your mower’s limits. So, what can you do about this? Well, the only real option is to trade out your lawn mower for one suitable for your yard.
Remove Excess Oil
If you have a small lawn mower, you likely have the option of pouring the excess oil into an oil pan. But with larger mowers like riding mowers, you’ll either have to use an oil removal pump/oil extractor or drain the oil through the oil plug in the bottom of the crankcase, as if changing the oil and oil filter on your riding mower. Check out my article on how to remove oil from a lawn mower to learn more. To avoid this mistake in the future, I recommend adding oil in small quantities and checking your lawn mower’s oil between pours.
Replace the Head Gasket
The idea of replacing a head gasket on a lawn mower seems like a daunting idea. But it’s actually pretty easy. While it’s an involved process that requires you to strip down the top of the engine, as long as you take your time and remember to put everything back correctly, you should have no problem.
Here’s an outline of the steps to give you a rough idea of what is required to replace a lawn mower head gasket. This example is for a Kohler Courage engine that you will find on many mowers like Cub Cadet and Toro.
Lawn Mower Head Gasket Replacement (Overview)
- Remove the valve cover.
- Remove the spark plug ignition cable.
- Remove the exhaust manifold bolts.
- Remove the air intake manifold bolts.
- Remove the cylinder head mounting bolts.
- Remove the valve rocker arms.
- Remove the push rods.
- Remove the old head gasket.
- Clean the cylinder head’s mating surface.
- Clean the cylinder’s mating surface.
- Place the new head gasket onto the cylinder.
- Place the cylinder head onto the engine.
- Install the cylinder head mounting bolts to the required torque specifications.
- Insert the push rods.
- Connect the rocker arms.
- Gap the valves.
- Install the valve cover.
- Attach the spark plug ignition cable.
- Attach the exhaust manifold.
- Attach the air intake manifold.
What You’ll Need to Replace a Lawn Mower Head Gasket
- Replacement Head Gasket (Engine Specific)
- Socket Wrench Set
- Torque Wrench
- Screwdriver Set
- Feeler Gauge
Replace the Pistons Rings
Replacing the piston rings is a few stages harder and more involved than replacing a head gasket. For example, you need to remove the engine, the pulleys, and the clutch and disconnect the electrical throttle cables, fuel lines, etc. It’s a long list of things.
Once you remove the engine, you’ll need to follow the same steps as removing the cylinder head to access the top of the cylinder. Then you’ll need to remove the crankcase cover to access the bottom of the crank to get to the bottom of the piston.
Still following? Ok, so after accessing the bottom of the piston, you’ll need to remove the connecting rod from the crankshaft. This will allow you to slide the piston out of the cylinder. Next is the easy part. Remove the old piston rings, including the oil, second and top rings, and install the replacements. Now for the hard part. Put everything back together the same way that you took it apart.
Now you are probably thinking, “I’ll just buy a new lawn mower.” Well, if you’re not familiar with engines and how they work, then this would probably be a good idea. But if you have a big expensive lawn mower, it’s worth having a professional technician do the work.
Black Smoke From Lawn Mower Engine
So what does it mean if your lawn mower is smoking black? A lawn mower blowing black smoke usually means the engine is running rich. In other words, the engine is receiving too much fuel and not enough air. Now this tends to be a much simpler problem than a lawn mower blowing white smoke and a lot easier to resolve. Here are some potential causes of a lawn mower making black smoke.
Common Causes of Lawn Mower Smoking Black
- Choke is On
- The Air Filter Is Clogged
- Carburetor is Incorrectly Adjusted
