Inside Your Car 6 September 2022

Spark plugs in a car: What you need to know

Spark plugs are an essential part of your vehicle that when properly maintained, help it run smoothly by igniting the fuel mixture that powers your engine. If they aren’t working properly, you can run into all kinds of performance issues. They’re also one of the easiest parts to replace in your engine, so it’s a good first port of call if things aren’t quite working right.

If you want your car to perform at its best without fouled spark plugs putting a spanner in the works, read on and we’ll tell you all you need to know!

What do spark plugs do?

Spark plugs are found at the top of the cylinder head, and their role is to produce the spark that ignites the fuel and air mixture that powers your engine. This spark is created at the tip of the plug, between two electrodes, from power received through the ignition coil.

It’s important for them to work properly so that they don’t cause other problems with your engine; such as poor fuel efficiency and loss of power.

Bad spark plug symptoms

There are key indicators when it comes to bad spark plugs. Some of these can also be symptoms of other issues, but if you notice a few of them happening together, it’s a good time to check your spark plugs.

Check engine light is on. If your spark plugs are going bad it will prompt your check engine light to turn on.
Engine misfire. If you hear sputtering sounds coming from your engine, it can mean that your spark plugs aren’t igniting all the fuel in your engine.
Rough idle. If your car shakes when it’s at a standstill and the RPMs fluctuate noticeably, this is a common sign that your spark plugs need replacing.
Bad fuel consumption. If you notice that you’re using more petrol than usual, it can be that your spark plugs are burning fuel inefficiently and need to be replaced.
Lack of power. This can be due to the spark plugs not combusting enough fuel to power the engine.
Car cranks but doesn’t start. Whilst possible, this is very rare and this would usually be accompanied by another primary failure of a major ignition component such as the ignition coil, or a fuel injector.
Foul spark plugs. If your spark plug tip gets coated in oil, fuel, or carbon, it’s considered fouled, and it won’t spark properly as the charge will run down the body of the spark plug, and not between the electrodes where it needs to fire. This makes the spark plugs black instead of their usual light grey/brown colour.

How to check spark plugs

If you’re comfortable navigating your way through the engine of a car, then checking your spark plugs is fairly straightforward, so it’s good to learn how to ‘read’ them. You’ll not only be able to find out whether the actual plugs are working, but whether your engine’s healthy too.

To get a look at them, first you need to disconnect the wires from the spark plugs. Then, remove the plug carefully with a socket wrench - preferably one with a rubber spark plug insert as this will make the job much easier.

Things to check:

The centre electrode: This is the electrode coming out of the tip of the spark plug. It should be flat, not rounded—a rounded electrode indicates significant wear. In the case of a premium iridium tipped plug, this should be pointed like a small triangle or pyramid shape.
The gap: The space between the centre and side electrodes needs to be a particular width in order to fire correctly. For a few dollars, you can buy a feeler gauge that’s specially designed for this purpose.
The gasket: This seals the plug to the head, so you need to check that this is sitting flush and isn’t distorted or an oval shape.
The insulator: This is the plastic covering over the electrode, and it shouldn’t have any cracks – at all!
The tip: If you see black oil or black sooty residue, this indicates a fouled spark plug that will need to be cleaned. With oil, this indicates a leak, so it’s important to find out where that’s coming from too. With soot, this is due to carbon buildup, and could be from a dirty air filter, among other things.

When reinstalling each plug, don’t use lubricant as it may cause you to over-tighten it. This is true for all high-quality plugs with a zinc coating.

Also, make sure to use a torque wrench so you can be sure you don’t over-tighten. The spark plug box should have the correct specs, or check your owner’s manual if you want to be sure. Or of course, bring your car down to one of our local workshops and we can take a look for you.

Why cleaning spark plugs can help

Cleaning can be a good way to remove carbon or oil on spark plugs, when the plugs themselves still have a lot of life in them. It’s fairly straightforward, and all you need is some carburettor cleaner, a wire brush, and a little bit of elbow grease. Just spray on the carb cleaner and brush the tips and thread until there are no deposits left. It also helps to anchor the plugs in a vice so that it’s less fiddly.

When you’re finished, wipe off the plugs with a clean rag to make sure they’re dry.

When replacing spark plugs is necessary

It’s a good idea to change them every as per the manufacturer’s specifications, as they gradually wear out and become less effective. So even if they aren’t dead, they could put additional stress on the ignition system potentially cause an ignition coil failure by increasing the voltage required to bridge the gap, thus burning out the coil by overheating it. It’s cheaper and easier to replace them before this happens.

And if you’ve noticed the centre electrode is worn, or the gap between the electrodes is incorrect, go ahead and have them changed, as they won’t work properly in this condition.

Hopefully this has given you good insight into how your spark plugs work, and how to look after them. If your car needs a tune-up, bring it into a mycar service centre and one of our experienced auto electricians will get it sorted in no time.