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How do spark plugs become fouled?

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Updated 10 Oct 2019

Rowan Johnstone

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Spark plug fouling is essentially a spark plug becoming too worn, dirty or contaminated to work.

Things inside your engine, such as fuel and carbon, can layer onto your spark plugs in quantities that prevent them from firing or working properly.

Additionally, things that shouldn’t be touching your spark plugs, such as oil, can cause a spark plug to foul.

If this happens, you could be looking at more sinister problems happening to your engine, such as failing gaskets or O-rings.

How to bring the spark back into your life

Spark plugs are made to be self-cleaning.

The heat generated by high engine temperatures allows the spark plugs to burn off anything that may be covering them.

If you haven’t driven your car for a while, only drive it on very short journeys, let it idle for long periods of time or only travel at slow speeds, you may find that your spark plugs have been fouled because they haven’t been able to ‘clean’ themselves.

If this is the case, laying into the throttle and taking your car for a quick 20-minute trip on the motorway will do the trick in cleaning your fouled spark plugs.

If this doesn’t work, you’ll need to remove the spark plugs and inspect them to make sure they aren’t dirty or worn to the point of not being able to operate.

It’s recommended to get your car to a qualified mechanic on AutoGuru and let a professional take a look, as they’ll know exactly what to look for and how to clean and replace them correctly.

If your spark plugs cannot be cleaned or are too worn, you’ll be up for a replacement set which can cost between $120 and $400+, depending on the type of spark plugs required and the car you own.

Back to those sinister problems I mentioned earlier

If you replace your spark plugs with a new set and they continue to foul out, all signs point to a problem with your engine.

These problems can include;

- A rich fuel mixture leaving large black spots on the plugs

- A leaky head gasket, which allows coolant to contaminate the spark plugs

- Damaged piston rings, engine cylinders, valve guides or valve guide seals which allows oil to seep onto spark plugs

If this is the case, It’s best not to waste any more time and get your car booked in for an inspection with a local mechanic through AutoGuru!

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Written By

Rowan Johnstone

On weekdays Rowan can be found in the AutoGuru office, driving content and growth with the rest of the marketing team.

On weekends you’ll probably find him in the garage with his dad restoring a 1958 Ford Star Model Customline or enjoying a cruise through the Gold Coast hinterland on his Suzuki GSX-R600.

Despite his passion for being behind the wheel (or handlebars), he looks forward to the day when he can commute to work in his own driverless car.