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Why Your Hedge Trimmer is Smoking (with Fixes)

Why Your Hedge Trimmer is Smoking (with Fixes)

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Paul Brown

Powered yard tools like hedge trimmers are powerful but they come with one big caveat: There’s always something in need of maintenance or adjustment. (This is especially true of gas-powered models). In any case, a common issue many people face with hedge trimmers is unexplained smoke dispersing from the main body.

Why is your hedge trimmer smoking? It could be one of several reasons:

  • The motor is overheating and burning internal brushes due to wear and tear or buildup or debris or dirt. 
  • Its battery connections have degraded and need to be replaced
  • An improper mix of oil and gas.
  • Oil leaking.

Once you know how to determine which of these issues you’re dealing with when your hedge trimmer starts smoking, you’ll be able to decide which repairs or part replacements you need to make to get your hedge trimmer back to work. 

First things first though. Is your hedge trimmer, gas-powered, outlet-powered or battery-powered?

Different Types of Hedge Trimmers

Hedge trimmers can come with one of two different power supplies: electric or gas-powered. Within the electric category, there are two types, corded and cordless (battery-powered).

Whichever power source your hedge trimmer uses, the energy is used to run an internal motor that allows the trimmer’s blades to oscillate when the power tool is in use.

Electric hedge trimmers, which tend to be light and less expensive than gas-powered ones, are generally better used for less extensive trimming work.

Gas-powered hedge trimmers, which are needed if you want to do the most heavy-duty cutting work for your bushes, trees, or hedges, rely on gasoline to run the engine and power the oscillating movement of the blades.

Understanding the similarities and differences between these types of hedge trimmers will help you narrow down what kind of problem you’re dealing with if your hedge trimmer starting emitting smoke.

One of the biggest clues you’ll have is the color of the smoke itself, which is usually either white or black.

Read sure to read Gas vs Electric: 6 Reasons To Use Battery-Powered Yard Tools

White vs Black Smoke

In general, the smoke you’re observing from your hedge trimmer will either be:

  • White
  • Grey or Black (We’ll call it black for this assessment)

The biggest difference between these two types of smoke is that black smoke will pretty much only be coming from a gas-powered engine, while white smoke can be from either.

The three most common causes of black smoke in a gas-powered hedge trimmer are:

  1. Leaking oil that reaches the engine and is being burned off on contact 
  2. Excessive combustion in the engine, which can be caused by several factors, or
  3. Contamination within the fuel system that is reaching the combustion chamber

You’ll need to examine your hedge trimmer first to identify the issue and determine how you’ll fix it. No matter what type of hedge trimmer you’re using or what type of smoke you see, the first thing you should do when yours start to smoke is to turn it off right away.

Fixes for Black Smoke in Gas-Powered Hedge Trimmers

Gas-powered hedge trimmers work similarly to other power tools that use gasoline as fuel (source). When the power tool is turned on, fuel is routed through the carburetor and then into the engine, where firing pistons help draw the fuel into the combustion chamber.

For combustion to occur at the correct rate and efficiently power the hedge trimmer’s motor and oscillating blades, there needs to be a specific ratio of air-to-gasoline once the fuel mixture enters the engine.

When the fuel is burned in the combustion chamber, gaseous fumes are released and pushed back out of the engine.

This process helps keep the engine from getting too hot, but if there are any chemical changes to the fuel mixture burned in the combustion chamber, the fumes that are produced and expelled from the engine will be affected.

Identifying and Fixing the Underlying Cause of Black Smoke

The most dangerous issue that could cause black smoke is leaking oil or fuel from the fuel lines (source). These tubes carry your fuel mixture from the fuel tank to the carburetor. So after you turn off the hedge trimmer and allow it to cool down, let check your fuel lines to make sure there aren’t any leaks.

Leaking fuel on the ground can put you at risk for starting a fire, so you’ll want to fix this problem right away if its what’s causing your hedge trimmer to smoke. Fuel lines can decay and begin to leak, especially if you are using an ethanol-based gas.

If you don’t find any leaks, you can move on to the next cause, a fuel system that needs cleaning.

Since hedge trimmers often use a fuel mixture that contains oil, over time the combustion of carbon inside the fuel system will allow black soot to build up and turn the combustion fumes black.

Although the black smoke can look like a serious problem, if this is the cause, it’s a simple fix. Allow your hedge trimmer to cool down and take it apart to thoroughly clean the fuel system with a cleaning kit like this one (link to Amazon).

Then, you can try running your hedge trimmer again. If it runs without producing any more smoke, you’ll know that a dirty fuel system was the cause of the smoke.

Otherwise, the process of elimination will allow you to move on to the final potential cause, poorly mixed fuel.

When using a gas-powered hedge trimmer, you will have to use a mixture of oil and gasoline that is poured directly into the fuel tank.

What many people don’t realize is that each model has its optimal ratio of oil to fuel that should be used. Make sure you are not incorrectly mixing the gas and oil.

  • When too much oil ends up in the fuel mixture, the excess oil is burned off at a faster rate and produces a large amount of gaseous carbon that turns the fumes black.
  • Fix: Refer back to your hedge trimmers owner’s manual to find out what gas to oil ratio you should be using.
  • Also, make sure to get rid of any of the old fuel mixture, as it can’t be salvaged for further use.

Be sure to read Does Premix Fuel Go Bad? HomeMade Vs Commercial Gas-Oil Mix

Fixes for White Smoke in All Hedge Trimmers

White smoke can appear in both gas-powered and electric hedge trimmers, and that’s because a common underlying cause of this type of smoke is an issue with the motor which all hedge trimmers are prone to.

The hedge trimmer’s motor will produce white smoke from excess friction that causes the motor to overheat while in use. Although this may sound less risky than the combustion issues that cause black smoke exclusively in gas-powered hedge trimmers, that doesn’t mean you shouldn’t take this problem seriously.

If there is significant friction with your motor, continued use of your hedge trimmer can cause the motor to die and need to be replaced. However, if you address the problem early, you may need to only replace a smaller, less costly part.

The most common causes of excess friction are:

  • not enough lubricant on the blades and/or motor
  • wear and tear on the brushes on the motor.

In the first case, all you’ll need to do is lubricate the blades and/or motor with a compatible oil for your model hedge trimmer. Star Brite’s Black Ultimate Lubricating Fluid (link to Amazon) is a great all-around lubricant and the one I use for my power tools.

If the latter is a problem, depending on how degraded your motor’s brushes are, you might have to replace the brushes alone or the entire motor, which is why it’s a good idea to stop as soon as you notice the smoke rather than finishing your work before checking the damage.

For more information on the causes and issues of overheating with gas-powered models, see What Causes A 2-Stroke Engine To Overheat?

White Smoke in Cordless Hedge Trimmers

With cordless hedge trimmers, you’ll need to consider another potential cause of white smoke: an issue with the connection between the battery and the hedge trimmer’s motor.

If the connection of the battery power supply to the motor degrades, which can often occur from water damage to the wiring, your engine will short circuit. As a result, your battery will start sending excess electrical power directly into the motor or other parts of the engine.

That excess energy will start to burn the material that makes up your hedge trimmer’s casing, which usually includes some kind of plastic.

Those white or grayish fumes are dangerous to inhale.

If the damage has gotten to that point, you can potentially still fix or replace the connections, but given the relatively inexpensive cost of cordless electric hedge trimmers, it might make more sense to replace it entirely.

Our top recommended hedge clippers:

Gas-powered: Husqvarna 21.7cc 23.7-in Dual Action Hedge Trimmer (Link to Amazon)

Battery-powered: Greenworks Pro 80V 24-Inch Brushless Hedge Trimmer (Link to Amazon)