What is a spark plug?

Thursday September 09 2021

Bad spark plugs past their prime can lead to poor combustion. misfirings, and generally less-than-stellar performance PHOTOS/Promise Twinamukye

By Mustafa Ziraba

What is a spark plug?

“It is that thing that gives the engine power”, “It is the thing that helps the engine accelerate”, “I have heard the mechanic talk about it but never really bothered to find out what it does”, “I actually do not know”.

These are some of the answers I got to that question. Many drivers might know what a spark plug looks like but a whole lot less know what it actually does and how it works.

Spark plugs have been around as long as internal combustion engines have, and are often a misunderstood component.

Many times we are told to make sure that we check and change our spark plugs on a regular basis in order to preserve our fuel consumption.

When you have a perfectly running engine it needs two things to keep it going and that is fuel and air, in the correct ratio, and a spark to ignite the mixture.


Simply put, the primary function of the spark plug is to ignite the air/fuel mixture within the engine under any operating condition. The secondary and less known use of a spark plug is to remove the heat from the combustion chamber.

The spark plug is positioned at the top of the cylinder, where the air-fuel ratio is compressed.

The tip of the spark plug sits inside the engine, recessed into the side of the cylinder wall.

The other end of the spark plug, which remains outside the engine, is attached to a spark plug wire in much older cars and a coil pack in newer cars.

As the piston travels up the first time, compressing the air-fuel mixture inside the combustion chamber, the engine computer sends an energy surge and the plug fires, igniting the air-fuel mixture.

The explosion inside the chamber forces the piston down again, turning the engine further as the same process happens in all of the other cylinders, one, two or three at a time depending on the engine set up. As the piston comes up the second time, the exhaust valve opens, and the burned gases are released into the exhaust pipes.

As you can see, the spark plug is a very important part of the engine. Therefore, in order to maintain the highest performance a car can offer, care should be taken to perform the proper scheduled maintenance in a timely fashion.

Spark plugs are one of the few things that an amateur mechanic can repair without much trouble.

Spark plugs last longer in modern cars, but at some point they need to be replaced.

Bad spark plugs past their prime can lead to poor combustion, misfirings, and generally less-than-stellar performance. How do you know if your plugs need to be changed?

The surest sign is on your odometer.

Spark plugs on average need to be changed every 25,000 kilometres. Some special premium plugs can go as long as 100,000 kilometres before replacement. If you do not know when yours were last changed or if you have an engine that runs roughly or has recently exhibited a decrease in fuel economy, well, that could mean that your engine might benefit from some fresh, clean sparks.

As far as when to change them, if your car is fine, there is no reason to do it too often or wait too long. Every 30,000 kilometres is a good baseline for standard spark plugs, but keep in mind that all engines will have different requirements. Some newer cars will never require the average driver to even think about the spark plugs.

However, many mechanics do replace spark plugs every time they service a car. This is not necessary, unless of course one always uses the cheaper low quality plugs or other engine problems are ruining the good ones. Remember spark plugs are very robust components, and do not fail unless they are performing under severe conditions, for long periods of time 

The bottom line concerning spark plugs is that in order to keep the cost of fuel down and get better mileage you should make sure that you have original (there are authorised dealers for most brands) spark plugs in your car. There is no need to change your spark plugs every service because they do last a long time.

As well if you have to, the right spark plugs are designed specifically to work with the engine they’re being screwed into. While there are several generic plugs, it is always better to get the manufacturer recommended plugs.

There is a lot of information online for this, where it goes as far as showing the equivalents from different brands like NGK, Denso and Bosch for the same engine.

Signs of a faulty spark plug

There are some common signs that your car may be experiencing an issue with its spark plugs. If you start to notice that your vehicle has uneven idling and is starting to vibrate more than normal, this could be an indicator that you are need to check your spark plugs.

Another sign that is generally associated with a malfunctioning spark plug is that your car may start to have a tougher time cranking.

Since the spark plugs are the component that create the spark the allows for the energy to be produced in order for your vehicle to run, if you have trouble starting your car, one possible cause could be worn out spark plugs.

Your engine may start to stall or take longer to start, which will start to drain your car’s battery life. If you have begun noticing that your car is taking longer to start, it may be wise to get it checked out because if you drain the battery too much, or else you may have to replace it as well.

Finally, another common indicator of a spark plug failure is an increase in your car’s fuel consumption.

Spark plugs function by creating energy through combustion.

When the air-fuel mixture is ignited, it creates a small explosion that produces energy to move the cylinders. If you have faulty spark plugs, this process is not efficient and will require more fuel to produce the energy needed for your car to run.

You may notice a significant increase in fuel consumption, so it is wise to get that checked out immediately, so you are not spending more money on fuel.