Lawn Mower Blowing White Smoke → Diagnosis & Repair

Reading time: 4 min

The color of exhaust smoke can tell you quite a bit about the overall health of an internal combustion engine. If your lawn mower is blowing white smoke, it indicates that the air/fuel ratio may be off, or water has gotten into the engine, or that oil is getting into the combustion chamber.

This guide serves to help you diagnose the white smoke coming from any mower. Small engine’s work the same whether it’s a push mower, zero turn, or rider. They are air cooled, which rules out coolant leaking into the combustion chamber like a car would.

Here are some of the most common reasons that a lawnmower will blow white smoke.

 

1. Too Much Oil

Mower White Smoke

Overfilling the crankcase with oil can cause oil to enter the combustion chamber that would otherwise not have gone there. This is one of the most common reasons that you might have white smoke coming from your mower. It also happens to be super easy to check.

How to Fix

Check the oil level to confirm that the level is too high. If it’s higher than the full line on the dipstick, it’s too full. Once you’ve confirmed this, Drain the oil out of the engine, and replace it with the correct type of oil in the correct quantity that the mower calls for.

 

2. Mower Stored on Side

If the mower has been stored on its side for any period of time, the oil will exit the crankcase and enter into the combustion chamber. This applies to an oil change as well.

How to Fix

If your engine is showing just a little white smoke, letting it burn off should be the only thing you need to do to fix it. Once you burn it off, remember if you tilt it for any reason, try to keep the spark plug facing upwards to keep oil from entering the combustion chamber again.

 

3. Oil in Gas

Oil in Gas White Smoke Lawn mower

This can happen when you fill the mower up with a gas can that has been designated for two stroke fuel. Two stroke fuel (like that which would go into a gas powered trimmer) has oil poured into it already.

How to Fix:

If this has happened to your mower, go ahead and drain, replace the gasoline. Once the gas in the fuel line has gone through the engine, the problem should clear up.

 

4. Mowing on a Steep Incline

When you mow on a steep grade (15° or more), it effectively does to your mower what overfilling it with oil does. It allows oil to exit the crankcase that ordinarily would not have.

How to Fix

If you have to mow on a steep grade, there is something that you can do about this problem. Briggs and Stratton recommend that you keep the spark plug pointed uphill to help keep the oil where it needs to be.  Here are our top picks for best mower for hills.

 

5. Bad Crankcase Breather

A crankcase breather takes the gasses that are not wanted for the combustion process and removes them from the crankcase before they can enter into the combustion chamber. Here’s more on how these work.

How to Fix

Replacing the crankcase breather is the only option that you’ll have when this happens. They really aren’t that expensive, or hard to replace. What you don want to do is make sure that it isn’t any of the more obvious and more common reasons listed above before you replace a part.

Here’s a great video that shows you exactly how you would want to go about replacing a crankcase breather.

 

6. Bad Piston Rings

White Exhaust Mower

Every piston fits in the cylinder wall with virtually no wiggle room. But, that wiggle room is enough for oil to get into the combustion chamber. To solve this problem, piston rings close up the rest of that gap.

They wrap around the piston. When they go bad, oil comes up and there’s white smoke in the exhaust.

The same problem can happen when the cylinder is worn. A worn cylinder happens after a lot of use, usually with no oil at all.

How to Fix

If the piston rings have gone bad, we recommend an entire engine rebuild. The logic here is that you are going to be pulling most of the engine apart to get to the piston rings, so you may as well put it back together with new parts. This will ensure that your mower runs well for a long time.

 

7. Air/Fuel Mixture

Lawn mowers are carbureted. A carburetor is responsible for making sure that the proper air fuel ratio is achieved for proper combustion. When the air/fuel mixture is off, there can be white smoke.

How to Fix

If you are mechanically inclined, you can get a carburetor rebuild kit. If you are not so inclined, you can get a new carb online pretty cheap. Before you do that though, check where the carb meets the engine. If they gasket is bad, this will cause a vacuum leak and white smoke.

 

8. Blown Head Gasket

Your mower’s head gasket provides a seal between the engine block and the head. When it fails, the oil that would normally be going to the head can make its way into the combustion chamber and burn out with the fuel. This causes your mower to billow a lot of white smoke.

How to Fix

The only fix for this situation is to replace the head gasket. It’s a relatively simple job to do on a mower, but it can be intimidating if you are not mechanically inclined.

 

Conclusion

Many of the reasons that a mower will billow white smoke are oil related. Now oil, when it burns, can appear to be white. It can also look a little blue. It will certainly stink like burning oil.

It’s not the sharp white color of condensation burning up in the combustion chamber, or the wrong air/fuel mixture.