Bad Spark Plug Symptoms: How to Tell If a Spark Plug Is Bad

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All homesteaders should know how to tell if a spark plug is bad. Learning is easy – and we’re about to brainstorm some of the worst bad spark plug symptoms you may encounter. Because a bad spark plug can strike any time – and it’s always enough to ruin your day!

Perhaps your truck has lost power, or your lawnmower won’t start. Maybe your chainsaw splutters and shudders? The question is – when should you single the spark plug as the culprit? And then, where do you begin diagnosing these performance issues in an ailing gasoline engine?

We asked our in-house mechanic, Dan Meager, for more tips and advice.


Then let’s begin!

A Failing Spark Plug Could Be the Culprit!

Spark Plugs! Yes, those simple-looking ceramic things screwed into the head of an engine are an essential part of its ignition and power-delivery systems. The fact is that spark plugs will make or break a gas engine’s ability to power on and function. 

Luckily – spark plugs are easy to remove and give insight into the health of a gasoline engine. Spark plugs are also easy to replace in most engines. And we can show you how to troubleshoot them, as we have tons of experience troubleshooting them for many gasoline engines.

So, let’s grab that plug socket and troubleshoot these low-tech but vital hotheads! 

cleaning a spark plug with a wire brush
How to tell if you have bad plugs? The first step is to understand the reasons your spark plug could fail in the first place. Carbon fouling is one of the most common reasons. Carbon fouling is easy to fix using a wire shop brush – wipe the gunk off your spark plug! A broken insulator, oil deposits, and warping electrodes are other common signs of a bad spark plug. It’s possible to troubleshoot a failing spark plug for hours. Or days! However, we often find it easier to replace the spark plug outright to see if that fixes the engine trouble. (We advise homesteaders to have the proper tools to remove and replace spark plugs. Knowing how to change your spark plug is a wise way to avoid costly repair expenses – and it can potentially spare you endless hours of frustrating engine troubleshooting.)

How Do I tell if a Spark Plug is Bad? Learn the Top Bad Spark Plug Symptoms! 

A collection of used and fouled up spark plugs.jpeg

Common symptoms of a bad spark plug include engine misfiring, reduced power, fuel economy, failure to start, and increased emissions. To tell if a spark plug is bad, remove it from the cylinder head and check the electrodes for carbon buildup, rounded edges, wetness, and an increase in the electrode gap.   

To perform at its best, a spark plug should:

  1. Have the reach and heat specifications of the OEM original.
  2. Be set to the correct gap using a feeler gauge.
  3. Burn clean with a light-brown color when cool.
  4. Be free of any cracks.

Always use a professional spark plug wrench to prevent stripping the threads in the cylinder head!

How Do You Know If You Have a Bad Spark Plug? 

If your engine misfires, runs roughly, or suffers power loss under load, it’s probably a spark plug problem. These issues may arise due to one or more faulty spark plugs in the engine. Faulty or fouled spark plugs will negatively affect throttle response, fuel consumption, and exhaust emissions. 

To remove carbon buildup from a spark plug, use a pneumatic sandblaster kit or a fine wire brush.  

  • The easiest way to test if a spark plug is faulty is to put it into another cylinder (swap it with its neighbor, so to speak). If the problem persists, the spark plug needs replacing. 
  • Closely examine the tops of the electrodes. They should be flat. If they appear rounded, replace the spark plug. 
mechanic replacing an old worn spark plug
Are you trying to figure out if your spark plug is bad? Then check out this spark plug condition chart! It’s our favorite way to check common signs or defective spark plugs. Without second-guessing your spark plug scoring! The spark plug chart shows perfect examples of worn-out spark plugs and spark plug faults that may lead to loss of power, poor fuel economy, a jittery sound, or costly engine repairs. The chart is perfect for printing and hanging in your garage. (Your combustion chamber will thank you later – especially if you need your engine to work in harsh weather conditions!)

What Does It Sound Like If You Have a Bad Spark Plug? 

Telltale sounds of a faulty spark plug include inconsistent engine noise due to misfiring cylinders, backfiring, and sudden drops in engine noise when the throttle is applied. 

Engine spluttering may occur due to a bad spark plug or faulty fuel injection and ignition timing. 

How Can You Tell If an Ignition Coil Is Bad?

The easiest way to check an ignition coil is to disconnect the spark plug lead from the plug, insert a screwdriver in the socket holding the plastic handle, and crank the engine. A spark should jump from the screwdriver to the engine block covering a distance of half an inch. If it doesn’t, the coil is weak.

Testing the ignition coil with a screwdriver
  • You can also use a spark plug tester to determine the strength of the coil charge.
old used and damaged spark plug unfit for use
Whenever our friends ask about the importance of spark plugs, we remind them of all the gas-powered farm tools that need a spark. In other words – all gasoline engines need a clean, functional spark plug! That means your car, chainsaw, lawnmower, power generator, and tractor need trustworthy and clean spark plug wires and a routine maintenance schedule. Because dirty (and worn) spark plugs cause several problems – from a faulty combustion process, lousy gas mileage, failing engine power, a flashing check engine light, or expensive repairs at your service center.

Is It the Spark Plug or Coil?

To check the status of an ignition coil, detach the spark plug lead and insert a screwdriver into the plug socket. A spark should travel 0.5 inches (one-half inch) from the screwdriver to the engine block. Otherwise, the coil is faulty and needs replacing. 

To determine if the spark plug is the problem, remove the spark plug for examination. If the electrodes appear worn, fouled, or too far apart, the plug should get replaced, cleaned, or reset.

What Will Bad Ignition Coils Do?

Common ignition coil problems include:

  • Failure to generate a spark.
  • Failure to start the engine.
  • Failure to maintain a spark once the engine heats up. 
  • Failure to maintain a spark of sufficient strength to power the engine under load. 

What Does a Bad Coil Sound Like?

If the engine doesn’t fire when the ignition switch or flywheel is activated and only the starter motor turns over, the ignition coil is not generating a spark and is faulty. Aging spark plug coils often work intermittently. But they can weaken as they heat up, causing the engine to run rough. 

faulty spark plug preventing the trimmer from running well
There are a few ways to tell if your spark plug is bad. Without needing to examine the spark plug itself! Remember that spark plugs are vital during combustion and help ignite the engine’s fuel-and-air mixture. Therefore, the most critical sign of a spark plug failure is if a gasoline engine won’t start – or has trouble starting. Rough idling, misfiring engines, power loss, and increased emissions are other signs that your spark plug is failing.

What Problems Do Bad Spark Plugs Cause?

Typical problems arising in a gasoline engine due to bad spark plugs include:

  • Hard starting.
  • Breaking down under load. (Running rough with a motor load.)
  • Misfiring cylinders.

Spark plugs can also cause a host of other engine problems.

Consider the following ideas.

Can Bad Spark Plugs Cause Fuel Problems?

Spark plugs do not directly cause fuel problems. But a faulty fuel injection system or carburetor will foul up the spark plugs, negatively affecting their ability to spark at the specified rate. 

  • A carbon-clogged wet set of electrodes on a spark plug indicates a too-rich air/fuel ratio, which can lead to increased fuel consumption and poor engine performance.

Can Bad Spark Plugs Affect Acceleration? 

Bad spark plugs do negatively affect acceleration. A fouled or worn spark plug cannot generate a sufficient spark to ignite the air/fuel mixture in the cylinder when the engine throttle is applied. 

  • As engine revs increase, so does the demand for sparking, invariably at a rate and strength a bad spark plug cannot deliver. 

Read More!

Can Bad Spark Plugs Cause Bogging?

Bad spark plugs often cannot supply the necessary sparking power to the engine to ensure optimum combustion under acceleration, aka under load, which will result in bogging. (A drop in power generated by the engine.)

When the vehicle encounters obstacles such as inclines, mud, or long grass, and the throttle is applied to overcome these obstacles, the failure of the spark plugs to ignite the air/fuel mixture in the cylinders will result in a drop in engine revs, aka bogging. 

Does Changing Spark Plugs Save Gas?

A new set of spark plugs will deliver the best possible fuel economy from an otherwise healthy engine. 

  • The extra engine throttling required to compensate for faulty, failing, or bad spark plugs reduces the motor’s fuel efficiency. 
  • Faulty spark plugs allow unburned fuel to escape the exhaust pipe, wasting money and posing an environmental hazard.
holding and examining a wet sparkplug from a flooded chainsaw
Here you see a wet spark plug. Wet spark plugs usually mean the spark is failing and the engine is flooding. The first thing I would try when attempting to fix this wet plug is to clean it. Then dry it. Then try starting the chainsaw engine again. If the engine still fails, no worries. I always keep a spare spark plug with me – especially when using the chainsaw. (I’ve noticed that chainsaws have more wet spark plug issues than other tools I use around the farm.) Also – remember that your chainsaw might start roughly due to other problems like a mucky carburetor, dirty fuel lines, old gas, overheating, filthy fuel filters, faulty ignition coil, clogged air filters, et cetera.

In Summation

Get to know your spark plugs! They’re stethoscopes’ for auto doctors – excellent diagnostic tools to help you troubleshoot all your gasoline engines, from weed whackers to 4x4s to ATVs.   

Plus, they’re one of the cheapest components in a gas engine.

Follow this guide to get acquainted with your new workshop wingmen!

And in the meantime, feel free to ask us if you have further questions.

We have experience troubleshooting more fussy spark plugs than we care to count.

And – we are happy to help.

Thanks again for reading.

Have a great day!

Bad Spark References, Guides, and Works Cited

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