By: Author John Cunningham. Published: 2019/01/31 at 10:17 am
White Smoke from your mower looks pretty serious, but usually, it’s a simple fix.
A mower commonly blows white smoke for two reasons:
- Mower tipped over incorrectly
- Engine overfilled it with oil
Other possible causes include:
- Head gasket failure
- Faulty crankcase breather
- Worn out engine
Mower tipped over is the most common reason for white smoke. Usually, the customer turns over the mower to clean the deck or unclog the chute. This allows oil to enter the cylinder, and when the mower is restarted, the oil burns and turns your yard into a 70s disco. Sometimes, oil will also leak from the muffler.
The fix is simple: check the mower for oil and let the engine idle until the smoke clears. If your lawnmower is a tractor mower, white smoke usually means too much oil, a carburetor fault, or a blown head gasket. (more on this below)
The diagnosis and fix are covered here in this post, but if you need video help, check out the Mower blowing white smoke video. It walks you through the process step by step – diagnosing, oil removal, carb check & repair, and compression testing for a blown head gasket; it’s all covered.
Tipping the mower on its side for maintenance or cleaning causes oil to move into the cylinder.
When you fire up the mower, the oil in the cylinder burns to cause white smoke. It will clear after a short while.
I hook the handlebars under a bench. Turing your mower to the side is fine as long as you turn it with the air filter side up.
Turn the air filter side up for greater access and avoid the white smoke.
A Case Of Too Much Oil
Lawnmower engines usually take a little over half a quart (.6lt), so it’s easy to overfill them. Lots of my customers add oil without ever checking the level. They do this thinking it won’t do the engine any harm.
Wrong! Too much oil can damage the engine, as most operate a splash lubrication system. When the oil level is above the splash paddles, they don’t work efficiently.
Don’t damage your engine needlessly. I wrote the complete guide – which shows you how to dip your oil, oil types, quantity, and how to drain oil; you can check it out here How to check oil level.
The white smoke is the engine burning off all the excess oil. The fix – drain the excessive oil and idle the engine until the smoke clears. This may take 5 minutes or so.
Depending on your mower type, draining the oil can be a pain in the ass. I got this Briggs and Stratton oil extractor on Amazon; it makes life soooo easy. You can check it out here on the “Small engine repair tools page.”
Too much oil will cause other problems, such as smoke, poor running, no running, leaks, and engine damage. Check out “How to check oil level.”
Drain excess oil and check and top up if needed.
Gas In The Oil
If your oil level is overfull and smells of gas, it’s likely you have a failed carburetor seal. Don’t run the engine, as the oil is too thin and offers no protection. Change the oil after making repairs to the carburetor.
If you think this sounds like your problem, go ahead and replace the carburetor; it’s faulty. I wrote this complete guide to help you check your “Carburetor troubleshooting.”
You may prefer a step-by-step video guide on carburetor cleaning; it includes removing, stripping, cleaning, rebuilding, and refitting your carburetor.
Carb cleaning is included in the video library of common lawn mower problems; all guides are easy to follow.
Carb cleaning is covered in this guide titled “Mower engine surging.” It’s so-called because surging and stalling are common symptoms of a dirty, faulty, or contaminated carburetor.
You may also need a tune-up; I wrote an easy-to-follow “Lawn mower tune-up” guide, including pictures and a maintenance chart.
The oil level will be overfull and stink of gas.
The fix is to replace the float needle and seal or replace the whole carburetor. Consider fitting a fuel tap, and don’t forget to change the oil.
White Smoke & Dies
This is a sure sign that oil has made its way into the carburetor and is blocking the gas feed jet. Most times, repeated starting and running of the engine will clear the oil. However, if you cannot run the engine long enough, you may need to clean the carburetor.
First, try removing the plug and turning over the engine a few times to expel oil from the cylinder. Clean or replace the plug and try starting again. If this fails, you can check out “Carburetor Cleaning,” where I show you how to remove and clean your carburetor.
Remove the plug and clear the cylinder by turning over the engine with the pull cord. Clean the plug before refitting.
Oil In The Gas Tank
In addition to overfilling with oil and tipping the mower on its side, putting oil into the gas tank accidentally is very common. Using the trimmer 2-cycle mix in the mower gas tank is a common mistake, too; it won’t produce the dramatic white smoke like engine oil, though.
The fix – drain the gas tank and, refill it with fresh gas, run the engine to clear the system. If the engine fails to start, no problem, check out my guide – “Carburetor cleaning.”
Head Gasket Failure
A failed head gasket is much less likely but will produce lots of smoke. Unlike previous solutions, it’s a little more work. A head gasket is a metal and graphite material.
It’s fitted between the cylinder block and the cylinder head of an engine. Its job is to seal the combustion chamber.
High crankcase pressures and oil leaks are a sign of head gasket failure; you may also hear a slight puffing noise as compression escapes from the cylinder. The fix – replace the head gasket.
A blocked crankcase breather will cause the oil in the cylinder and white smoke; clearing the breather pipe is a simple fix.
Failed or worn piston rings are the end; sadly, a rebuild is needed. A new engine is most likely a cheaper option and comes with a guarantee, but a new mower might make more sense at this stage.
Head gasket failure can cause white smoke. This will only be an issue with OHV (overhead valve) type engines and will also depend on where the gasket fails.
OHV engines are usually well marked with OHV on the front engine cover. When it fails, and depending on where it fails, it will suck oil into the cylinder and blow gases into the crankcase.
Last on the list is engine wear or ring damage. A compression test will confirm if you have internal damage. This condition is rare. Check out the “Compression test video” here.
Why is my electric lawn mower smoking? If your electric lawn mower is smoking, you must unplug it before attempting any further investigation. The motor has most likely burnt out. If, on the other hand, your electric mower uses a belt to drive the blade, it’s possible that just the belt is causing the smoke.
Does the lawnmower smoke on startup? This is generally associated with an old mower; it’s a sign of engine wear. But other possible causes include:
- Wrong oil type
- Wrong plug type
- Leaking carburetor float needle
- Crankcase breather fault
- Valve seals hard
- Head gasket failure
- About the Author
- Latest Posts
John Cunningham is an Automotive Technician and writer at Lawnmowerfixed.com.
He’s been a mechanic for over twenty-five years and shares his know-how and hands-on experience in our DIY repair guides.
Johns’s fluff-free How-to guides help homeowners fix lawnmowers, tractor mowers, chainsaws, leaf blowers, power washers, generators, snow blowers, and more.