What Does A Bad Spark Plug Look Like? 5 Simple Ways to Tell

Spark plugs are often an afterthought when it comes to your vehicle’s engine. After all, they usually last anywhere from 20,000 miles to over 100,000 miles depending on which materials they were made of. So it could take years until they go bad making them very easy to forget about.


It is very important though to make sure to replace them before they become bad spark plugs, (also called foul spark plugs). If you don’t replace them at the right time you are not only wasting gas, but you also risk causing serious engine damage as well as an expensive trip to your mechanic.

What does a bad spark plug look like? Look at the image below. The one on the left is bad, or also known as a “fouled spark plug”. The one on the right is a new spark plug.

Via howrah.org

Typically bad spark plugs will have black or heavy ash deposits around or on top of the center electrode, which is the little pen-tip like object found in the center of the plug. This makes it harder for a strong spark to occur to fully ignite the gas-air mixture in your cylinder.

Bad spark plugs can have many negative symptoms on your vehicles, including…

  • Bad gas mileage.
  • A sluggish engine that doesn’t perform like it used to
  • Or trouble starting your car or a rough idle and engine vibrations

If you experience any of these issues, you may be in need of a new set of spark plugs.

In this article I will explain to you the 5 most common symptoms and tell tale signs of having bad spark plugs. I will also list which steps you need to take to solve this problem.

This way you won’t have to spend your time and effort pulling your spark plugs out if there wasn’t a problem in the first place. Let’s get started…

1. Bad Gas Mileage

If you are noticing that you are visiting the gas station more frequently than usual, this could be a tell tale sign of a bad spark plug.

You see, if a spark plug is not performing at it’s best capability, it may not have enough spark to ignite all of the fuel-air mixture inside of your cylinder. This is also called incomplete combustion.

This causes only some of your gas to ignite, while the rest is being wasted… being blown right out of your exhaust pipe!

2. Loss of Engine Power

If only some of your gas is being ignited because of a bad spark plug, that means your engine will be generating much less power. You may notice this in a few different ways including:

  • Slow or sluggish acceleration - If you are pushing down hard on the gas, but your car doesn’t seem to be accelerating like it used to.
  • A misfiring engine - If you notice while your foot is on the gas pedal, that your engine frequently ‘skips a beat’, or makes random loud ‘bang’ or ‘pop’ sounds, it is almost guaranteed your engine has misfired.
  • Rough sounding, or loud vibrations while idling - Right after you start your car, or if you are sitting at a traffic light you notice your engine is not idling smoothly.

If your vehicle is experiencing any of these symptoms, there is a very good chance you have a bad spark plug, and they should be fixed or replaced right away to avoid expensive engine damage in the future.

If at this point you are already realizing the signs of having bad spark plugs, consider buying a spark tester from Amazon. The video below will describe how to use one.

3. Blue Smoke Coming Out Of Your Exhaust Pipe

Via 2carpros.com

Blue smoke coming out of your exhaust pipe is not necessarily a sign that your spark plugs have gone bad yet, but it can signal that they may need to be replaced soon (as well as other parts of your engines).

The cause of blue smoke is from excess oil in the engine’s cylinder. This excess oil can start to build up small deposits onto your spark plug making them give off a weaker and weaker spark.

However, replacing just your spark plugs is not enough if you encounter this problem. Excess oil in your cylinder is actually caused by worn down piston rings or your valve guides. Once that issue is solved, then it would be a good time to check your spark plugs.

If you are wondering how to remove your spark plugs, I have found this great article on Axle Addict that explains the whole process. You will just need a Spark Plug Wrench and a Spark Plug Gap Gauge.

4. Failed Emissions Tests

Do you live in a city where you need to have your car regularly pass emission standards and your car been letting out more emissions than in the past, possibly failing these tests? Bad spark plugs could be the cause.

As I explained above, if your spark plugs aren’t working optimally this can cause ‘incomplete combustion’, which sends raw fuel right out of your exhaust pipe.

If this is the case it will cause your car to emit higher Hydrocarbon (HC) Emissions, which could very likely be the cause for a failed emissions test.

5. Engine Surging or Car Starting Problems

If you are having troubles starting your car you may want to check your spark plugs. If your spark plugs are not producing enough spark it won’t cause proper combustion, which is needed to get an engine running.

Engine Surging is probably the most dangerous problem that bad spark plugs can cause. If you have bad spark plugs this could cause your engine to be very unpredictable causing sudden jerking followed by slowing down.

Your engine could also start and stop continuously which is very dangerous, especially in heavy traffic.

In Conclusion…

If your spark plugs still don’t work properly after cleaning them you may have to buy a new set of spark plugs for your vehicle’s engine. You should go to your local dealer with your car model to figure out which type of spark plug you will need.

P.S. It is better to buy spark plugs sooner rather than later. Because if you allow problems to persist from a bad spark plug you risk causing much more damage to your engine if you leave the problem go unchecked.

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