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Spark Plugs 101: Everything You Need to Know

August 20, 2020

For such a small part, a worn spark plug can cause some pretty big problems. Poor fuel consumption, misfiring, hard starting, and engine knocking – the humble spark plug can be the cause of all these problems and much more.

What is the purpose of a spark plug? What are the symptoms of bad spark plugs? How often do you need to change spark plugs? By understanding the role of your spark plugs and recognizing the signs of worn spark plugs, you’ll be rewarded with a boost in fuel economy and a smooth ride.

What Do Spark Plugs Do?

Before we delve into what spark plugs do, we need to briefly explain how an engine works, so you can understand the important role your spark plugs play.

An engine’s purpose is to convert gasoline into motion. This happens largely due to a process that goes on inside the engine called internal combustion.

Inside the engine, valves fill the cylinders with an air/fuel mixture which is highly explosive when exposed to a source of ignition. The pistons compress the air/fuel mixture, increasing the amount of potential energy. At the peak of compression, a spark plug creates an arc of electricity lasting approximately 1/1000 of a second, igniting the air/fuel mixture inside the combustion chamber. This creates an explosion that forces the piston back down to its starting position. The crankshaft turns this energy into rotational motion, and your car moves forward.

Spark plugs are one part of your vehicle’s ignition system, which also consists of electrical and timing equipment. They are made from material durable enough to ignite millions of times before wearing out. The voltage at the end of the spark plug can be anywhere from 20,000 to more than 100,000 volts.

We should note that you won’t find spark plugs in a diesel engine as they use higher compression ratios to create combustion instead of relying on ignition from a spark plug.

Spark Plug Characteristics

Inside the spark plug is a central inner electrode that is insulated from heat by its white porcelain shell. It gets electricity to perform a spark via the central electrode which is connected to your vehicle’s ignition coil by a heavily insulated wire.

The bottom part of the plug is threaded to allow it to be screwed into your engine’s cylinder head. The bottom tip extends further into the combustion chamber where ignition of the air/fuel mixture takes place.

Heat Range Explained

Besides acting as a source of ignition, the other important role a spark plug plays is to transfer heat from the combustion chamber into the cooling system. This ability to dissipate heat is defined by the spark plug’s heat range. The heat range must be matched to your application or pre-ignition and fouling (electrical leakage that causes a misfire) of the spark plug can occur.

You may hear spark plugs being referred to as ‘cold’ or ‘hot’. This is referring to your spark plugs heat range. Changing to a hotter or colder spark plug should be done with extreme caution as running a spark plug that is too hot for your needs can damage your engine.

What Causes Spark Plugs To Wear Out?

Every time a spark plug creates an arc of electricity, it very minimally shortens the electrodes. Over time, the gap between the electrodes grows larger. As this gap increases, more electricity is needed to spark ignition of the air/fuel mixture. As this happens, the ignition performance deteriorates leading to unstable ignition. Eventually, the gap will grow so large that the spark plug won’t arc at all.

Other causes of spark plug wear include:

These will all cause premature wear, but by design, spark plugs are designed to wear out. Continuing to drive with worn spark plugs will shorten the life of your vehicle’s ignition coil.

What Are Symptoms Of Worn Spark Plugs?

Once a spark plug becomes sufficiently worn, it will affect the performance of your engine. Replacing your plugs at the recommended intervals written in your vehicle owner’s manual schedule will virtually eliminate these problems, but a fouled or faulty plug can cause issues.

These are the signs of worn spark plugs:

If you spot any of these signs, it’s best to ask a professional to check the spark plugs to see if they need to be replaced. You should do this as soon as possible to prevent further damage to your vehicle.

What Is The Spark Plug Gap?

All spark plugs must have the proper ‘gap’ to operate optimally. The gap is the distance between the center and ground electrodes. It must be set precisely to ensure the spark plug arcs at the correct voltage.

How to Check the Spark Plug Gap

Modern spark plugs are sold pre-gapped. Nonetheless, the gap is always checked before a new set of spark plugs are installed as an incorrect gap will cause engine issues such as loss of power, misfires, and poor gas mileage.

Your owner’s manual will suggest the proper gap for the set of spark plugs that are recommended for use with your vehicle and your mechanic will check it before installing a new set of spark plugs.

You will know if the spark plug gap is set correctly by inspecting them. An incorrectly gapped plug may show burnt or dirty electrodes, along with the familiar signs of worn spark plugs such as an engine that is missing or hesitating, or exhibits a knocking or pinging noise.

You can also check the spark plug gap using a gap gauge or feeler gauge tool. Run either one of these tools through the spark plug’s electrodes to measure the gap. Then, compare this measurement to the recommended gap measurement in your owner’s manual. If the measurements are not equal, the spark plug is not properly gapped.

Diagnosing Worn Spark Plugs

Removing and inspecting a spark plug will give you an indication of how it’s performing and how your engine is running. If you do not have experience removing and installing spark plugs, we strongly advise booking an appointment to have a professional assist you.

The Different Types Of Spark Plugs Explained

Manufacturers like NGK, Bosch, and Denso give their spark plugs different characteristics by manufacturing them from different materials. Although they all perform the same function, they were designed with different applications and engines in mind. These are the most common types of spark plugs you’ll see in modern vehicles.

Copper

For a long time, copper spark plugs were the industry standard, and the most common and affordable type of spark plug used. They get their name from their copper, nickel-coated inner core. The soft nature of copper and nickel means copper spark plugs have a short life-span due and are unsuitable for vehicles that use high-energy distributor based ignition systems. Nowadays, they’re typically found in older engines.

Platinum

As their name suggests, platinum spark plugs have a platinum center electrode. Platinum is a harder metal than copper which gives the spark plug greater longevity. They can operate efficiently at a wider temperature range and generate more heat than copper spark plugs which reduces carbon build-up.

Double Platinum

Single platinum spark plugs have a platinum center electrode, but double platinum spark plugs also use platinum on the ground electrode. They are pricier, but offer another step up in both performance and longevity.

Iridium

Iridium spark plugs are touted as the best plugs on the market. The ‘fine wire’ center conducts electricity very efficiently, and the small center electrode means less voltage is required for ignition. Iridium spark plugs are commonly found in high-performance engines.

Silver

Silver spark plugs are pretty uncommon unless you own an older European car or a motorcycle. The electrodes are silver coated which gives the plug the characteristics of better thermal conductivity, but poor longevity.

Which Spark Plugs Are Best For My Car?

Installing the wrong spark plug for your car will cause poor engine performance and can seriously damage your engine. In almost all cases, it’s best to match your vehicle manufacturers’ recommendations as closely as possible.

Likely, you’ll never have to upgrade to a different type of spark plug unless you’ve modified your engine, fitted an aftermarket ignition system, or your current spark plugs are showing signs of abnormal wear. Your mechanic or local automotive parts store will be able to advise you on your best option should you be looking to change to a different type of spark plug.

How Frequently Should You Change Spark Plugs?

How long your spark plug will last before requiring replacement depends on the type of spark plug as different types of spark plugs wear at different rates. Your vehicle owner’s manual will list the recommended replacement frequency which, on average, is around 30,000 miles.

However, a number of factors could affect how often you need to change worn spark plugs, including:

These are some of the many factors that could affect the frequency at which your spark plugs need to be replaced. Because of this, it’s important to remember that you may need to check the spark plugs more often than every 30,000 miles.

Can I Change My Own Spark Plugs?

If you are mechanically inclined, you can certainly change your own spark plugs. Though, there are a few things to watch out for if you have never changed a set of plugs before. Making even a minor mistake while changing your spark plugs on your own could lead to serious engine damage.

Tips For Changing Spark Plugs

First, it’s important to ask yourself “What spark plugs do I need for my car?” Don’t simply choose the least expensive option. Check your owner’s manual to find out which type of spark plug to install.

Using the right tools can make the task of replacing spark plugs much easier. You may need a number of different tools to complete this job, including a ratchet, spark plug socket, ratchet extension, universal joint extension, and gapping tool. Keep these tools nearby so you can easily access them while replacing your spark plugs.

You must be cautious to not allow dirt or debris to fall into the spark plug well. If dirt or debris gets into the spark plug well, it could make its way into the engine, where it could cause serious damage.

You should inspect your old spark plugs for unusual wear, and be sure you are installing the right-sized spark plugs for your vehicle. If you install the wrong size, the tips of each spark plug could make contact with the pistons inside the engine, which can cause extensive damage.

Additionally, care must be taken not to cross-thread spark plugs, and a torque wrench should always be used, so your new set of spark plugs are torqued as instructed by your owner’s manual. 

Make sure the engine is cool before you start the process of removing and replacing the worn spark plugs. A warm or hot engine could damage the threads.

You will also need to check the spark plug gap if the spark plugs are not already pre-gapped. Use a gapping tool or feeler gauge to measure the spark plug gap and adjust it if needed.

Replacing spark plugs is a doable DIY project for many vehicles, but it’s easy to make a costly mistake if you aren’t 100% sure of what you’re doing. 

How Much Does It Cost to Replace Spark Plugs?

A lot can go wrong during a DIY spark plug replacement, which is why it’s recommended to let a professional replace your spark plugs instead of trying to handle the task on your own.

The cost of a spark plug replacement will vary depending on several factors, including the type of vehicle you drive. Some vehicles require more expensive spark plugs than others, and some vehicles might have spark plugs that are more difficult to access. These are some of the factors that can increase the overall cost of repairs.