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Is it safe to use mismatched spark plugs?

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Updated 4 Oct 2019

Joel Ilton

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The spark plugs are what provides the ‘spark’ to ignite the air and fuel mixture inside the engine.

They are crucial in the running of your engine and any issues with the spark plugs can lead to poor running, or even to being stranded on the side of the road waiting for roadside assistance to come and help (near Ship Creek perhaps?) But if there is a problem with only one spark plug, is it safe to just replace that one and leave the others?

Firstly, we should look at how the spark plug operates and the different materials from which they are made.

The basic operation of the spark plug is to transmit the voltage provided from the ignition coil and distributor into the cylinder to ignite the air and fuel mixture.

This operation is precisely controlled via the engine control unit (ECU), which utilises a number of sensors to accurately ignite the mixture at just the right time.

The manufacturers use spark plugs specifically designed to fit perfectly inside the cylinder and that have the correct heat rating to ensure the spark plug can handle the extreme conditions inside the cylinder without melting.

This brings us to the different materials used to make spark plugs.

On older vehicles, spark plugs were mainly made of copper with a ceramic insulator and utilised a nickel electrode to ignite the air and fuel mixture.

The downside to these spark plugs was their relatively short service life of 20,000km - 40,000km and their susceptibility to the phenomenon known as quenching (say that five times fast!).

Quenching happens when the ‘spark’ loses its energy while trying to ignite the air-fuel mixture, increasing fuel consumption and harmful emissions.

This effect has been heavily reduced by using precious metal spark plugs - ones manufactured with platinum or iridium electrodes - as the electrode surface area is much smaller in these types, providing less surface area for spark loss.

Precious metal spark plugs also have a much longer service life (100,000km) and are much more heat resistant than nickel spark plugs, so can be used in higher compression engines where cylinder temperatures are much higher during operation.

Now that’s out of the way, we can dive deeper into the question of mismatched spark plugs.

If your vehicle is fitted with precious metal spark plugs and you decide to replace one with a standard nickel electrode, this can cause all sorts of issues.

Misfiring due to excessive heat, quenching, increased fuel consumption and even engine failure can occur from the use of incorrect or mismatched spark plugs, so it’s best to replace all of your spark plugs at the same time, and with the correct type of spark plug, to ensure smooth and consistent operation.

Even if your vehicle runs nickel electrode spark plugs, replacing one or two at a time will cause potential running issues, so replacing all when there is an issue with one is the best course of action.

As the saying goes, ‘Do it once, do it right’.

Want to dive into some more spark plug info? Check out these useful articles.

What happens if my spark plugs fail?
What causes spark plugs to fail?
Can spark plugs increase horsepower

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Written By

Joel Ilton

Finding a passion for cars from a young age, Joel carried out work experience as a mechanic whilst at school before starting an apprenticeship after finishing year 12.

After almost 10 years on the tools and in customer service, he moved into the IT realm as a Data Analyst and In-House mechanic at AutoGuru.