Here Are the Most Common Symptoms of Defective Spark Plugs

Spark plugs are critical components of any gasoline engine, but since they ignite the compressed mixture of fuel and oxygen millions of times during their lifespan, they eventually cease to fire and need to be replaced.
Spark plugs 6 photos
Photo: Drazen Bajer from Pixabay
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Just as the name suggests, they create a spark in the combustion chamber that ignites the air-fuel mixture, setting the piston in motion.

Each time they do this, molecules from the electrodes are evaporated, and eventually, the spark gap widens and the mixture does not ignite properly. There are also several engine problems that can contaminate the plugs and prevent them from working properly.

Compared to other engine parts, they are cheap and easy to replace, but how can you tell if they need to be inspected?

Spark Plugs
Photo: Michael Schwarzenberger from Pixabay
Check engine light comes on

The most obvious sign the car will give you is lighting up the dreaded check engine warning on the instrument panel. It can be connected to a wide range of engine problems, but if it’s associated with the following symptoms, it might hint that you should get the ignition system checked.

The car has trouble starting

If your car has trouble starting, it’s likely that a worn-out spark plug is the problem since it has a harder time creating the actual spark. If the engine does not start at all, no matter how many times you try, then you should check the battery.

The engine misfires

Cylinder misfire typically occurs when there is a problem with the ignition system. In modern vehicles, the most common cause is a sensor malfunction, but it can also occur due to worn spark plugs, the connecting wires, or an ignition coil.

Rough engine idle

If you notice that the engine is significantly louder than usual when idling and the car vibrates, it is most likely that the spark plugs are not working properly. This is basically a cylinder misfire that only occurs at idle.

The car will not accelerate properly

You are probably accustomed to how your car behaves, so if you notice that it isn’t as responsive as it normally is when trying to overtake, it means that the spark plugs can’t handle the increased demand generated by sudden acceleration.

Fuel economy drops

Increased fuel consumption is another symptom. If one or more plugs do not work correctly, the engine control unit (ECU) will try to compensate for the poor combustion by injecting more fuel into the chamber. So, if you are keeping an eye on fuel economy and notice that it suddenly goes up, you should check the spark plugs.

Remember that they have a limited lifespan that varies due to multiple factors. They generally last from 30,000 to 120,000 miles (48,000 km to 193,000 km). If you have a new car and are approaching that interval, it might be a good idea to have the ignition system checked before these symptoms occur.

NGK Spark Plug
Photo: Ronald Plett from Pixabay
As these are not expensive parts, make sure to buy high-quality OEM parts. Manufacturers that make excellent spark plugs are NGK, Bosch, Denso, Beru, or ACDelco.

Finally, unless you have basic mechanical skills, do not attempt to change them yourself, especially if you have an older car.

While working for a dealership in the past, I saw countless owners who broke the old plugs while trying to remove them. In some cases, the entire engine had to be taken out and stripped apart, which ended up costing 100 times more than having a professional replace them.
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About the author: Vlad Radu
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Vlad's first car was custom coach built: an exotic he made out of wood, cardboard and a borrowed steering wheel at the age of five. Combining his previous experience in writing and car dealership years, his articles focus in depth on special cars of past and present times.
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