What Does a Bad Spark Plug Look Like?

When a car has problems starting, many people assume it’s the battery’s fault, but that might not be the case. Old, dirty, and damaged spark plugs are also a leading cause of cars’ starting problems. But how do you identify a dirty or damaged spark plug?

Bad spark plugs may be dirty or damaged. Dirty spark plugs often have black or oily residue, sometimes even chunks, on the ends and in the gap at the end. Damaged spark plugs may have chipped or cracked ceramic, burned electrodes, or broken internal components.

In this article, I’ll be detailing how to tell if a spark plug is damaged or just dirty, how long spark plugs typically last, and other relevant information about spark plugs you may find handy.

Used spark plugs.

What a Bad Spark Plug Looks Like

Bad spark plugs are usually easy to tell apart from clean and functional spark plugs. There may be various types of residue fouling the plug, or it might be damaged and/or worn.

Fouled vs. Damaged Spark Plugs

There’s a significant difference between fouled spark plugs and spark plugs that are worn, damaged, or old. The former may possibly be salvaged for further use, while the latter need to be replaced.

How to Tell if a Spark Plug Is Fouled

  • There’s a black oily residue on the sparking end; most common in 2-stroke engines that use gas/oil mixtures.
  • The spark plug has chunky carbon residue buildup between the sparking end and the hooked side.
  • Dark carbon-like residue has built up on the ends of the plug.

On the other hand, damaged spark plugs can look very different or may not have any apparent signs of wear and tear at all.

How to Tell if a Spark Plug Is Worn or Damaged

  • The electrode side appears white or ‘blistered.’ This is a sign of excessive heat warping the plug.
  • The ceramic insulator looks cracked or otherwise damaged. This can happen if someone tightened the plug too much.
  • There might be no apparent damage to the exterior, but it may still be internal damage. You can use a spark plug checker to test this.

How to Fix Bad Spark Plugs

Fixing bad spark plugs depends on if it’s just dirty or if it’s functionally damaged. In the former case, it may be possible to restore the spark plug with a bit of elbow grease. Unfortunately, a damaged spark plug will have to be outright replaced.

How To Clean Dirty Spark Plugs

Cleaning dirty spark plugs won’t make them operate as new spark plugs would, but you can probably squeeze a few hundred or thousand extra miles out of them with some diligent cleaning.

  1. Soak the dirty spark plugs with WD-40. This will displace any moisture on the plugs and, more importantly, remove carbon residue/build-up.
  2. Wait 5 minutes for the WD-40 to soak into and dissolve the residue.
  3. Use a small wire brush to carefully scrape off the black residue, being careful to get any in the gap.
  4. When scraping, be very careful not to damage the threads or external components.
  5. When the entire spark plug appears to be shiny, and like new, you’re finished.
  6. Let the newly clean spark plug(s) dry before reinstalling into your engine.

Damaged Spark Plugs

If you suspect a spark plug is damaged, but it appears fine, the electrical parts inside may be damaged. You can check if the spark plug is good or not with a spark tester, like the Ram-Pro Inline Spark Tester (Amazon). Spark testers make checking spark plugs a fast and easy process, which saves you valuable time and frustration.

Signs That Spark Plugs Might Be Bad

Other than visibly checking them, there are many symptoms of bad spark plugs that can affect the day-to-day functioning of your car. Some of these are noticeable, but a few are more subtle. If you notice any of these, you should check your spark plugs for damage or grime.

You’re Filling Up More Often

Misfiring spark plugs significantly reduce gas mileage. If you’re driving the same distance as you always do but you notice yourself filling up more often, that could be because of bad spark plugs. When spark plugs misfire, it causes incomplete fuel combustion—this leads to less fuel efficiency and more exhaust.

Your Car Feels Sluggish

Incomplete fuel combustion because of misfiring spark plugs means when you press the gas pedal, your car is using less gas. Less gas means less power, making your car slow and unresponsive. I’m sure you know your car’s capabilities, so you’d notice it feeling sluggish.

Knocking, Rattling, or Pinging Noises

If you listen to loud music when you drive, it’s time to turn it down and just listen to your car’s natural noise.

If there’s a noticeable knocking or rattling sound, this could be because of a faulty spark plug. When a spark plug fails to ignite all the fuel it’s supposed to, erratic combustion causes this type of noise.

This mainly occurs when you’re idling. A rough or jerky idle can send vibrations through your car and cause damage to sensitive components.

Car Won’t Start

If your car fails to start and you start eyeing the battery, consider whether it could be a bad spark plug. When spark plugs fail to ignite fuel, your car can’t combust the fuel it needs to run. Other similar issues are sputtering and engine misfiring.

How Long Do Spark Plugs Last?

Spark plugs usually last for 80,000 to 100,000 miles (128,747.52 to 160,934.4 km) on most modern car models. Some luxury or sports models may wear out spark plugs a little faster because they’re usually less fuel-efficient than common consumer cars.

Taking Care of Your Spark Plugs

While they’re considered one of the simpler parts of an engine, spark plugs have an important role to play. There are some simple steps you can take to ensure the integrity and longevity of your spark plugs.

Clean the Area Before Removing Old Plugs

Debris and other gunk can build up around cylinders, so consider using canned air and a damp rag to clean around spark plugs before removing them from the cylinder. This debris can get into the cylinder and damage the inside, causing a whole host of other issues.

Spray New Spark Plugs

Before installing new spark plugs, spray the center cavity with carburetor cleaner. This will make any loose debris, such as ceramic dust, fall out. Such debris can get into the engine and cause misfires, not to mention cause friction damage to cylinder walls and rings.

Make Sure You Get the Right Spark Plugs

Every car is designed to operate best with certain types of spark plugs, whether or not copper or iridium spark plugs are ‘better.’ If you have to use excessive force or do extensive tweaking to get a spark plug to fit, stop and take a step back. Then, look up what spark plugs are designed to fit with your car’s engine. Chances are it’s not the one you’ve been trying to make fit.

Final Thoughts

Spark plugs can be damaged externally or internally, the former of which is easily visible. Grime and residue on spark plugs may be cleaned to get a little extra use out of them, but they won’t be as good as new ones.



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