White Smoke From
Lawn Mower

White Smoke from your mower looks pretty serious, but usually it's a simple fix.

 

So what causes white smoke from a lawn mower? This commonly happens after you've tipped your mower over to clean it, or you've over filled it with engine oil. But there are other possible causes:

 

  • 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 lawn mower is a tractor mower, white smoke usually means too much oil, carburettor fault, or blown head gasket. (more on this below)

mower tipped over

Tip Over

Tipping the mower on its side for maintenance or cleaning causes oil to move into the cylinder.

Disco

When you fire up the mower the oil in the cylinder burns causing white smoke.

It will clear after a short while.


mower smoking
mower tipped over

Tip Up

I hook the handlebars under a bench.

Turing your mower to the side is fine as long as you turn it with the carburettor side up.

Tip Over


mower maintenance

Tipping the mower on its side for maintenance or cleaning causes oil to move into the cylinder.

Disco


mower smoking

When you fire up the mower the oil in the cylinder burns causing white smoke.

It will clear after a short while.

Tip Up


mower smoking

I hook the handlebars under a bench.

Turing your mower to the side is fine as long as you turn it with the carburettor side up.

A Case Of Too Much Oil

Lawn mower engines usually take a little over half a quart (.6lt), and 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 - shows you how dip your oil, oil types, quantity, 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, makes life soooo easy.

 

Too Full


mower oil level

Too much oil will cause other problems, smoke, poor running, no running, leaks, engine damage.

Check out "How to check oil level".

Drain


mower oil drain

Drain excess oil and check and top up if needed.

mower oil check

Too Full

Too much oil will cause other problems, smoke, poor running, no running, leaks, engine damage.

Check out "How to check oil level".

Drain

Drain excess oil and check and top up if needed.


mower oil drain

Gas in The Oil

If your oil level is overfull and smells of gas, it's likely you have a failed carburettor seal. Don't run the engine as the oil is too thin and offers no protection. Change the oil after making repairs to the carburettor.

 

If you think this sounds like your problem, go ahead and replace the carburettor, it's faulty. I wrote this complete guide to help you check your "Carburettor gas flow".

 

Need a tune-up, I wrote an easy to follow "Lawn mower tune-up" guide, including pictures and maintenance chart. 

 

Gas Smell


mower oil level

The oil level will be overfull and stink of gas.

Carburettor


mower carburetor

The fix is to replace the float needle and seal, or replace the whole carburettor.

Consider fitting a fuel tap, and don't forget to change the oil.

mower oil check

Gas Smell

The oil level will be overfull and stink of gas.

Carburettor

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.


mower oil drain

White Smoke & Dies

This is a sure sign that oil has made its way into the carburettor, and is blocking the gas feed jet. Most times repeated starting and running of the engine will clear the oil. However, if you are unable to run the engine long enough, you may need to clean the carburettor. 

 

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 "Carburettor Cleaning" where I show you how to remove and clean your carburettor.

 

Carburettor Repair Tools

This page lists all the likely tools you'll need to clean or replace your carburettor,

Carburettor Tools
mower plug clean

Clean Plug

Remove the plug and clear the cylinder by turning over the engine with the pull cord.

Clean the plug before refitting.

Clean Plug


Mower plug

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 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 with fresh gas, run the engine to clear the system. If the engine fails to start, no problem, check out my guide - "carburettor cleaning".

 

Head Gasket Failure

A failed head gasket is much less likely but it 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 oil in the cylinder and white smoke, clearing the breather pipe is a simple fix. 

 

Failed or worn piston rings is the end, sadly a rebuild is needed. A new engine is most likely a cheaper option and it comes with a guarantee. 

 

 

Head Gasket


mower head gasket mower head gasket

Head gasket failure can cause white smoke.

This will only be an issue with ohv (over head 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 where it fails, it will suck oil into the cylinder and blow gases into the crankcase. 

Mower Head gasket Mower Head gasket

Head gasket

Head gasket failure can cause white smoke.

This will only be an issue with ohv (over head 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 where it fails, it will suck oil into the cylinder and blow gases into the crankcase. 

Engine Wear


Mower engine test

Last on the list is engine wear or ring damage.

A compression test will confirm if you have internal damage.

This condition is rare.

Wear

Last on the list is engine wear or ring damage.

A compression test will confirm if you have internal damage.

This condition is rare.


Mower engine test

Related Questions

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 the causing the smoke.

 

Lawn mower smokes on start up? 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 carburettor float needle
  • Crankcase breather fault
  • Valve seals hard
  • Head gasket failure
Auto Technician and Writer at | Website

John Cunningham is an Automotive Technician and writer on Lawnmowerfixed.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.