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Here’s How to Know if a Lawn Mower Spark Plug is Bad


Identify a bad lawnmower spark plug

If you’ve got a lawn mower that isn’t working properly, there are ways you can tell if it’s your spark plug or another engine part that is the problem.  When you’re ready to mow your lawn on a Saturday morning and you’ve got all the gear out and ready to go, the last thing you want is for your lawn mower to malfunction, depriving you of the satisfaction of your grass looking fresh and groomed to perfection.  

The spark plug in a lawn mower could be bad if:

  • You can’t get the engine to fire up at all
  • You have to tug extra hard for longer than usual on the rewind
  • Your lawn mower loses power while moving
  • The gas runs out quicker than it used to

Sometimes identifying if it actually is the spark plug that is the problem can be a challenge.  Read on to see what makes a spark plug go bad, how to tell if yours has reached the end of its life, how to reinstall one, and when it needs replacing.

Signs that the Spark Plug is the Problem

Lawnmowers can have a variety of problems, and it can sometimes be difficult to diagnose which part is to blame.  There are some key things to watch for that will let you know that the spark plug is the culprit.

If the engine is turning over but won’t start, the spark plug is often to blame.  An overly worn or dirty spark plug will not spark as quickly and can take more work to get it functioning.

Secondly, if you’ve gotten your mower to start and then, in the middle of mowing your lawn, it dies while moving, this could be a sign that the spark plug needs some attention.  Spark plugs may be inconsistent or unreliable if they are damaged or worn.

This YouTube video from Sears PartsDirect covers the basics. Although they are demonstrating on a push mower, the symptoms and solutions are the same for a riding mower:

Lawn Mower Won't Start: Spark Plug and Ignition Problems

You can also tell that you have faulty spark plugs if your fuel tank is running out of gas much faster than usual, and you have to refill it too often.  The spark plug may not be burning all of the fuel if its spark is weak, allowing too much fuel to move through the chamber without being burned and utilized, thus causing you to refill it sooner.

Source: Champion Auto PartsOpens in a new tab.

How to Clean and Reinstall a Spark Plug

If you discover that your spark plug is not functioning correctly, there are a few things you can do to move forward. If your spark plug is damaged or too worn down for continued use, the best option is just to get a new one.  Spark plugs can cost anywhere from $2-$10, depending on where you get them and the brand or type you prefer.  

If your spark plug is not damaged or overly worn down, then you could consider cleaning and reinstalling it.  This is an excellent option for spark plugs that just have a little gasoline, oil, or carbon buildup on them but have the porcelain sheath intact and the electrodes still in good shape.

To start, make sure you’re working in a clean area to avoid any debris or dirt getting into the combustion chamber.  

You’ll need to use a spark plug socket to remove the spark plug.  This is a tool designed specifically for spark plug maintenance.  It will allow you to unscrew the spark plug safely after you’ve removed the spark plug lead. 

Next, you can use a soft-wire brush or a spark plug spray cleaner to gently remove anything coating the spark plug.  Using a soft-wire brush is essential because any damage to the spark plug can render it unusable, so being as gentle as possible will save you time and money in the long run. 

Once the spark plug has been cleaned, it can be reinstalled in the mower, reversing the steps taken to remove it.  Use the spark plug socket as needed and be sure not to overtighten the spark plug.  

Sources: infobaseOpens in a new tab., Dummies.comOpens in a new tab.

How Long Does a Lawn Mower Spark Plug Last?

Spark plugs are an item that needs regular replacing, no matter how well-maintained they are by the user.  How often they need to be replaced depends on how often the lawn mower is used each season.

Typically, the consensus among the mechanical engineer community is that a spark plug for a lawn mower will last up to 25 hours of use.  

Some users simply prefer to replace it every mowing season as a part of their yearly lawn mower maintenance.  If you use it for the full 25 hours each season, this is a great option to ensure you don’t run into spark plug problems. 

If you aren’t sure that your season requires 25 hours of mowing, then perhaps tracking the hours you use the lawn mower could help you determine how often to replace the spark plug.  

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Source: besthomegear.comOpens in a new tab.

What Makes a Spark Plug Go Bad?

There are many parts of a small engine that are made to last through the lawn mower’s lifetime.  They are hardy and can withstand a lot of wear and tear.  Spark plugs are not one of those parts, unfortunately.  They are not made to last through long term use.

All spark plugs eventually wear out and need replacing. It’s the nature of the item, and it’s unavoidable.  

Spark plugs go bad because the electrodes at the bottom wear down with each use, making them shorter and shorter until they are no longer long or sharp enough to create the necessary spark to get the engine running.

If the spark plug is not worn down, though, and is still malfunctioning, then it could be due to other factors, such as:

  • Overheating: When a spark plug overheats, it can cause the ignition spark to be weaker by widening the space between the electrodes and the center of the plug.
  • Damage to the Electrodes or the Porcelain Sheath: The structural integrity of a spark plug is essential to its functionality, and if any part of that is scratched, cracked, chipped, or worn too much, then the spark plug will not function properly.
  • Buildup of Carbon around the Plug: When there is too much debris getting into the area where the spark plug is igniting, it can cause buildup around the plug and make it less effective.
  • Oil Fouling the Tip of the Plug: This is something that can happen if the spark plug has been worn and overused.  Oil can get into the combustion chamber and can prevent the spark plug from working properly.  

If any of these issues are present, they can cause a spark plug to be less effective or completely useless.  

Sources: Manitoba UniversityOpens in a new tab., Champoinautoparts.comOpens in a new tab.,  

Conclusion

A bad spark plug is a simple fix on a lawnmower. You can often clean the plug and get it back into running shape but they are relatively inexpensive so if there is any doubt, just replace it and move on.

Did that fix your problem? If not, look through this troubleshooting guide that I put together when my John Deere lawn mower wouldn’t keep running. The troubleshooting steps are pretty much the same for any mower.

Paul Brown

Paul has a two-acre yard on red clay soil in Southeast Texas. He knows exactly what the challenges are to nurturing a thriving yard in difficult soil. He takes a practical approach to yard improvement and enjoys putting best practices and “golden rules of lawn care” to the test. Click here for Paul’s author page

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