5 Signs of Bad Spark Plugs (2023)

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Small but mighty, spark plugs pack far more of a punch than what their size leads you to believe they could. Hard to believe when looking at them, spark plugs are one of the most necessary components when it comes to a healthy, running car.

That being said, knowing the signs of bad spark plugs is one of the most important tools any car owner can have in their back pocket.

Use our handy guide to increase your knowledge about spark plugs, the source of combustion in your engine, and learn about how to diagnose spark plugs that may need some attention.

What is a Spark Plug?

Spark plugs are small modern marvels when it comes to the internal combustion engine that powers your car. They’re small, often less than six inches in length, and are made of durable material like ceramic capable of withstanding many small explosions over the course of their life. But how exactly do they work?

When installing spark plugs, the small ceramic end with the gap (the electrode) is installed into the engine, with the larger ceramic end (the insulator) facing outwards.

The electrode is a conduit for electricity. In the gap, small jolts of electricity are fired as fuel is fed into the engine, and the electricity causes the fuel to combust, creating the energy that moves the parts of your engine.

(Video) 5 Signs It's Time To Replace Your Spark Plugs

The insulator is capped with a highly insulated wire that is ultimately connected to the battery. This is where the spark plug gets its electricity from. A device controlling when the spark plugs fire, causing the combustion to happen, determines when the plugs are given spark and in what order they spark.

Without spark plugs, our engines won’t work. This is why knowing how to identify signs of bad spark plugs is incredibly important.


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Identifying Bad Spark Plugs

We’ve gathered information on the most common ways to identify the signs of bad spark plugs below.

1. Trouble Starting

It may come as no surprise that without combustion, the internal combustion engine won’t work. When spark plugs go bad, the vehicle that they’re in tends to become harder to start.

If the spark plug can’t create a spark, then the fuel being fed to the engine can’t be ignited, meaning that it won’t run. If your car won’t start and your battery is healthy, chances are you need new spark plugs.


2. Misfiring

Misfiring can sometimes be hard to identify in vehicles that are particularly noisy, but if you’ve got a quiet car you’ll know exactly when it’s misfiring. Misfiring occurs when fuel that’s fed to the engine isn’t combusted entirely, or at all.

Combustion happens due to the spark plug igniting the fuel, so if you have misfiring, it’s sure to be an issue with your plugs. Misfiring can cause jerking and vibrating, both uncomfortable side effects of bad spark plugs.

3. Rough Idle

When your spark plugs are starting to go bad, the idle in your vehicle becomes rough, similar to the misfiring identified above.

A rough idle tends to mean that the fuel is being combusted, but that it isn’t being done completely, and that your plugs are on their way out.

4. Lack of Acceleration

If your vehicle seems to have lost power and has trouble getting up and going, there’s a chance that your spark plugs are beginning to have issues. Low power means that the plugs aren’t firing at the rate that they should be, or with the power that they should be.

5. Higher Fuel Use

If spark plugs aren’t igniting the fuel properly and fully, then the vehicle’s on board computer will compensate for this lack of power by sending more fuel to the engine. This happens due to a number of things, but when you boil it down, the vehicle’s computer realizes through sensors that the fuel isn’t being used efficiently, and tries to send as much as needed to the plugs.

However, the plugs aren’t combusting the fuel properly, leading to wasted fuel (and possibly flooding)

(Video) 10 symptoms of bad spark plugs

As you can see from the list above, the signs of bad spark plugs all tend to lend themselves to a loss of power or an inability to keep the engine running. Spark plugs are some of the most important parts of your vehicle, even though they’re small and tucked away, out of sight.


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How to Tell If Your Plugs Need to Be Changed

Spark plugs are a major part of routine maintenance, and in addition to noticing the signs of bad spark plugs, knowing how to prevent them is just as important.

Here are a few ways to stay ahead of the curve and make sure you keep good plugs in your vehicle.

Stay on Schedule

As a general rule of thumb, spark plugs are normally designed to last 100,000 miles. Make sure that if your vehicle has surpassed that mileage that you pay attention to your spark plugs.

Check Your Plugs

In the event that anything else goes wrong with your vehicle, checking your plugs for issues is important. Spark plugs can be affected by a number of things. When looking at the electrode on your spark plugs, keep in mind the following:

(Video) 5 Signs You Need New Spark Plugs and how to change them #hipafixeasy

  • Oil leaks can cause plugs to misfire, not allowing the electrode to create the needed spark.
  • Ash deposits on plugs indicate that oil or fuel additives are burning in the engine, and causing excess wear on the plugs.
  • A fractured insulator tip may mean that the entirety of the ceramic insulator is cracked, and is causing detonation. This can lead to further damage if not replaced.


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The Spark of Life in Your Vehicle

Without exaggeration, it can be said that the health of your spark plugs affects the overall health of your vehicle. If you don’t pay attention to your vehicle, and ultimately can’t identify the signs of bad spark plugs, you may cause more damage to your engine than what a simple spark plug change can prevent.

It’s a relatively cheap job, and can often be done by the owner of the vehicle. Pay attention to your plugs, you’ll need them to keep going!

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(Video) How to Identify a Fouled Spark Plug - NGK


What happens when a spark plug goes bad? ›

If the spark plug fires at the wrong time, this could lead to constant rattling, pinging, or knocking sounds. Hard vehicle start. If your car is having trouble starting or just feels disjointed and jerky, your spark plugs may not be working right, and this can lead to misfires and erratic performance.

What does bad spark plugs sound like? ›

Sometimes, especially while accelerating, you will hear your engine making a distinct knocking sound. That sound is caused by your spark plugs not detonating properly and igniting all the fuel. The fuel and vapor that did not ignite eventually will catch fire and detonate.

How often should you replace spark plugs? ›

Spark plugs are designed for long-term durability, meaning that they only need to be replaced between every 80,000 and 100,000 miles. However, they can become damaged earlier than expected and require replacements to avoid pricey engine repairs.


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