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How Do You Know If Your Spark Plugs Need Changing?

Spark plugs are one of those seemingly insignificant and relatively easily replaceable parts that can cause some potentially serious issues if not cared for properly. They’re small devices that generate electric current and provide a spark that ignites the air/fuel mixture within the engine’s combustion chamber. In other words, they’re essential to the vehicle’s operation since one wouldn’t be able to start without a spark plug. Like most car parts, however, spark plugs also have an expiry date, and in this article, we’ll focus on how to know if spark plugs need changing.

Broken down or failing spark plugs can cause all kinds of engine problems and even lead to their early demise. Bad spark plugs will typically be unable to provide even electricity distribution and, in turn, engine combustion. Your car will have trouble running, exhibit lower fuel economy, and perform poorly in general.

If you’re wondering whether your car’s spark plugs are ripe for replacement, this article will provide the answers. We’ll go through typical problems caused by faulty or damaged spark plugs, which should tell you if your spark plugs need changing.

How do Spark Plugs Work?

old spark plugs

Before we move on to the symptoms of bad or failing spark plugs, let’s take a look at how the spark plugs work and what they consist of.

Spark plugs can only be found in petrol-burning cars as diesel-run vehicles rely on compression combustion instead of spark-induced combustion.

The main parts of a spark plug are:

  • Terminal – found at the top of the spark plug and used to connect the spark plug with the ignition system.
  • Insulator – made out of hard ceramic material such as sintered alumina, the insulator holds the central electrode and provides insulation.
  • Seal – seals the combustion chamber and makes sure there’s no leakage.
  • Jacket – a metal case or shell that dissipates the heat from the insulator and acts as the ground for the sparks passing between the central and side electrodes.
  • Threads – used to screw the spark plug inside the combustion chamber.
  • Central electrode – the hottest part of the spark plug designed to emit the electrons from the electrode tip.
  • Side electrode – also called the ground electrode and welded to the jacket.

In essence, the spark plug is there to provide the power spark at the right moment and combust the air and fuel mixture within the combustion chamber. The explosion created by combustion is used to push the piston within the cylinder, which creates a force that moves to the crankshaft via a connecting rod and, in turn, moves your car.

Types of Spark Plugs

spark plug
Photo by: Pixabay

There are spark plugs that differ in size, shape, and operating temperature, but the most common spark plug classification is by the material from which their central electrode is made.

Copper/nickel spark plugs

These spark plugs use a copper core coated with nickel alloy. They’re inexpensive, predominantly used in older vehicles, and last the shortest compared to other spark plugs.

Platinum spark plugs

Platinum spark plugs are long-lasting, more durable, and more expensive than their copper counterparts. They’re primarily used in modern applications and are the most common type of spark plugs nowadays.

Iridium spark plugs

Although the most expensive of all spark plug types, iridium plugs are also the longest-lasting by far. They’re six times harder and eight times stronger than platinum spark plugs and have a 700°C higher melting point. They’re becoming more common with each passing day, but they still have a long way to come before they replace platinum spark plugs as a predominant choice.

Silver spark plugs

Due to having the best performance of all spark plug types, silver spark plugs are typically favorites among performance car enthusiasts and pro mod racers. Although they are the best thermal and electrical conductors, they don’t last as long as their platinum or iridium counterparts.

How to Know if Spark Plugs Need Changing?

Finally, let’s take a look at the most common problems faced by vehicles with faulty or failing spark plugs which should provide the answer to the question: “How to know if spark plugs need changing?”

Problems starting the car

how to know if spark plugs need changing
Photo by: Envato Elements

Although there might be many possible culprits, problems with starting your car can typically be traced back to worn or failing spark plugs. At least, they’re one of the first and easiest things to check should such a symptom occur.

Worn-out spark plugs have a tougher time creating the spark needed to turn on your car. Even a damaged spark plug wire or connector can be the cause of car starting problems. Replacing the spark plugs is typically neither expensive nor labor-intensive, but some car models with unorthodox engine configurations might require more elbow grease than those with more conventional ones.

Slow acceleration

mechanic with a spark plug
Photo by: Pixabay

Like the previous example, slow acceleration can stem from several different problems. The simplest to check and fix, though, is faulty spark plugs.

Spark plugs in a state of disrepair won’t be able to create optimal combustion, and you’ll feel that while driving your car, especially when trying to accelerate. If a spark plug replacement does the trick, you’re in the clear. Otherwise, the problem with slow acceleration could be slightly more complex as it could be down to fuel injectors or other parts of the fuel system, one of the numerous sensors, or even the head gasket.

Rough idle

Porsche engine
Photo by: Pixabay

Your car doesn’t have to move for a fouled spark plug to rear its ugly head. Even while idling, you might notice problems related to failing spark plugs, such as rough and jittery idle and vibrations that follow them.

Engine misfiring

engine with spark plugs
Photo by: Pixabay

Needless to say, faulty spark plugs will often lead to the misfiring of the engine. While the engine misfire can be attributed to other problems, one bad spark plug is enough to cause it.

You’ll notice your engine is misfiring by sputtering, faltering, and then catching up again once the healthy spark plug/cylinder takes its turn. Leaving the problem unresolved will lead to poor fuel economy and potentially more serious problems further down the line. Since a misfiring engine sends unburnt fuel into the exhaust, you might damage your catalytic converter.

Poor fuel economy

engine block
Photo by: Pixabay

Poor fuel economy is an early warning sign of failing spark plugs. Since spark plugs on their last legs can’t burn the fuel inside the combustion chamber in an efficient manner, your car will start using more fuel to compensate.

Sometimes, the spark plug doesn’t even have to go foul to cause a drop in fuel efficiency. It might only be the gap between the central electrode’s tip and the ground electrode. In such instances, it’s enough to simply clean them, adjust the gap and return it within its optimal range.

Final Words

Spark plugs might be inexpensive and easily replaceable, but as you can see, they’re essential to the engine’s optimal operation. Failure to keep them in optimal working order will result in a number of problems that could have potentially greater ramifications. That’s why every car owner should know the answers to the question of how to know if spark plugs need changing. Aside from saving you money on fuel, such knowledge might also save you some serious money on engine repairs further down the line.

iridium spark plug
Photo by: Pixabay


How long do spark plugs last?

The life of a spark plug depends on its type and quality. Copper/nickel spark plugs should last anywhere between 10,000 and 20,000 miles. Performance-oriented silver spark plugs typically last anywhere between 20,000 and 30,000 miles, while the most common platinum spark plugs can last between 60,000 and 100,000 miles, making them a sound investment. Finally, the iridium spark plugs last even longer than that, with an expected lifespan of up to 150,000 miles.

Why do spark plugs need to be replaced?

As mentioned before, spark plugs have a life expectancy, and after they fail, they need to be replaced. Replacing spark plugs won’t only improve your vehicle’s fuel economy but overall performance as well. Not to mention that it might save you some money on potential repairs caused by foul spark plugs.

How often do I need to replace my spark plugs?

Spark plug replacement intervals depend on your car manufacturer’s recommendation, the type of spark plugs your car is currently running on, and your driving habits to some extent. It’s a fine idea to replace the spark plugs preventively before their recommended life expectancy runs out.

How many spark plugs are there in a car?

There are as many spark plugs as your vehicle’s engine has cylinders. Four-cylinder vehicles come with four spark plugs, six-cylinder cars have six spark plugs, V-8 engines sport eight spark plugs, and so on.

Do all spark plugs need to be replaced at once?

No, they don’t. However, most mechanics will recommend replacing all spark plugs at once, even if only one of them needs replacing. Your spark plugs need to be on a similar level of performance to produce optimal combustion, and if only one or a minority of them are up to the task, you won’t see any benefits of spark plug replacement.

Nikola Potrebić

Nikola enjoys digging into history and finding everything he can about rare, limited-run, and special-edition models of the bygone muscle car era.

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