Spark plug guide: How to clean, test & change spark plugs

Spark plug guide: How to clean, test & change spark plugs

Spark plugs are a critical part of your internal combustion engine which means that mechanics can charge a pretty penny for the removal and refitting of these. But, the good news is that you don't have to shell out on pricey repairs if yours become worn or break: changing spark plugs is a fairly straightforward task that most people with a bit of mechanical knowledge can do at home.

To prolong your spark plugs lifespan as much as you can, it's important that you know how to thoroughly clean them and how often you'll need to test them. Similarly, you'll need to be clued up on signs of spark plug failure, so you know when to change the spark plugs in your car.

If you've never done this yourself before, don't worry! Our handy spark plug guide will ensure you at least know the basics of changing, cleaning and testing your spark plugs. We'll cover:

What are spark plugs and what do they do?

While spark plugs may appear simple in design, these parts are integral to a handful of varied and critical jobs within your car's motor and are vital to its overall efficiency. But, what exactly does a spark plug do? They are responsible for delivering an electric current from your ignition to the combustion chamber. This in turn ignites the mixture of compressed fuel and air via the electric spark while also containing combustion pressure inside of your engine. In simpler terms, they generate tiny bolts of lightning which deliver electricity to your engine, helping it to run.

There should be a spark plug for each cylinder, although in certain engines — including the Alfa T-Spark — there are two per cylinder. The spark plugs resemble metal shells and can be found screwed into your engine's cylinder head. They are electrically grounded, meaning they will protect you from electric shocks and provide a return path for the electrical current.

You should be aware that in newer car models where the engine compartments that have lots of emissions tubing and computerised sensors, it may be better to get a professional's help to ensure you don't damage any engine parts while trying to access your spark plugs.

What are the signs of a bad spark plug?

Before you spend time removing, cleaning and refitting your spark plugs, it's important to inspect their condition and function. There are a couple of signs that suggest you may have a bad, or failing, spark plug which will need to be replaced. This could include:

Slow acceleration

Poor acceleration is commonly associated with a problem within the ignition system, most likely a worn-out spark plug. Spark plugs are made from materials that work together to produce a spark that's a high enough temperature to ignite the air-fuel mixture. And, when these materials wear out, the effectiveness of the spark plug is reduced, meaning the acceleration of your car can significantly slow down.

So, pay attention to the movement of your car, particularly if it feels like it's struggling or doesn't accelerate as fast as it used to. These can be symptoms of a range of other problems with your vehicle including faulty or blocked fuel filter or oxygen filter issues, so we'd advise consulting a mechanic before you begin any work on your spark plug.

Poor fuel economy

A spark plug that's working correctly will help to burn fuel efficiently through the combustion cycle, helping your car to achieve better fuel economy. However, when the plug isn't functioning properly, it's often because the gap between the spark plug electrodes is too close or far apart. This is when you will need to adjust the gap and return them to the distance that your manufacturer's manual recommends. Similarly, if your car is struggling to use the fuel in in an economical way, it could be that your spark plugs are worn out and need completely replacing.

Misfiring engines

A misfiring engine is usually to do with an issue in the ignition system, or down to a fault with the computer sensors in newer models. But, it can also be caused when a spark plug wire or tip of the spark plug that connects to the wire is damaged.

An engine that is misfiring can be associated with intermittent spluttering sounds and will cause your engine light to illuminate on the dashboard. And, if you leave it misfiring, polluting exhaust emissions will increase, while acceleration and fuel economy will significantly drop.

A misfiring engine can be a serious problem so you mustn't try and fix this yourself. Instead, you'll need to get a professional to take a look and advise you.

Problems starting the engine

Having trouble starting your vehicle can be a sign that your spark plugs are worn out and will need replacing. But, as with any of the above, an engine that won't start should be initially looked at by a professional before you begin replacing the spark plugs.

Rough idling

Rough idling is characterised by a rough and bouncy feeling when the engine of a vehicle is running. This is usually caused by a problem in the ignition system of a vehicle and can be commonly attributed to faulty spark plugs and their wires. As well as the vehicle feeling shaking, you might also experience odd RPM counts — the RPM of a car is usually a smooth, consistent 1,000. But, anything higher or that fluctuates is a problem.

How to test a spark plug

Knowing how to test a spark plug is crucial for ensuring it is in good working order, which contributes to better fuel efficiency, engine health and safety. While there is no set amount of time that needs to elapse before you check these, it's best to take the guidance from your manufacturer's manual. Although it won't give you exact times for when to test your spark plugs, it'll provide a guideline distance for how often to change your spark plugs, and therefore when they'll need replacing. But, be aware that different spark plugs will last for various timeframes. For example, this could be between 30,000–60,000 miles, so a good time to test your spark plugs would be at the halfway mark.

To do this, you will need to earth your spark plugs. To do this:

  1. Remove the spark plug and reconnect the spark plug wires (also known as high tension or HT leads), holding the boot of the wires with a pair of insulated pliers.
  2. Hold the spark plug in a position that almost touches the engine block.
  3. Remove the fuel pump fuse. This will stop fuel from being injecting in the cylinder and out through the spark plug, which could potentially be very dangerous. It will also prevent the car from firing up the other cylinders.
  4. Get somebody to attempt to start the engine.


If your spark plugs are in good working order, you'll observe a strong blue spark go from the plug to the engine, whereas there will be a yellow spark or none at all if they aren't working properly. However, if you have trouble checking this yourself, you can get a spark plug tester which is simple to operate.

All you'll need to do is to remove the spark plug wire and plug the tester in, so that it connects to the end of the HT lead and base of the spark plug. Once you've set this up, turn on the engine and the engine will light up if it's receiving good electrical energy.

However you decide to test your spark plugs, if they are omitting a weak spark, it may be time to remove them and give them a good clean. We will be explaining how to do this in the next few sections.

Removing your spark plugs

When you reach for each of your spark plugs, you'll undoubtedly be greeted with a build-up of soot, which is completely normal considering they create thousands of explosions inside your engine each day. But, before you can begin cleaning these, you'll need to know how to remove your spark plugs in a safe and correct manner.

To do this, you'll need to follow these steps, disconnecting one spark plug at a time:

  1. Locate the wire connected to the spark plug or ignition wire, and then locate the spark plug from this. Be aware, that these could be hidden under a number of parts including the ignition coil, so look carefully.
  2. Using a ratchet, place it around your spark plug socket and pull on the boot of the spark plug (the connector at the end of the wire). Be careful not to pull on the wire itself as this could damage both the wire and the connector.
  3. Remove the spark plugs from the sockets using a torque wrench by turning it in an anticlockwise direction. If this is proving tough, don't force it as you may break the spark plug off. Instead, try a penetrating oil that will help loosen these.
  4. Inspect the old spark plug: it should only be a little dirty. If you spot any oiliness or white substances on it, it could mean there are other issues at hand which will need to be investigated by a professional. If not, then you can begin cleaning your spark plugs (see next section).
  5. Repeat for the other spark plug.

How to clean spark plugs

Your spark plugs require maintenance just like any other car part, and cleaning is particularly integral to prolonging their lifespan. Most notably, you'll need to pay attention to the gap between these and the ignition and combustion chamber, which can increase the length of time that your spark plugs last for.

More specifically, under moderate driving conditions, the gap in your engine will increase about 0.001 inches with each 2500 miles you travel. But, taking the time to adjust your spark plugs back to their correct position (as specified by the manufacturers) and clean between the gap can increase their lifespan by several thousand miles.

Cleaning your spark plugs is relatively simple to do, and all you'll need is a piece of sandpaper, an air hose and a little bit of elbow grease. For the best way to clean your spark plugs, follow these steps:

  1. Fold the sandpaper in half and slide it back and forth through the gap between the spark plug arm and electrode. If the gap isn't large enough for folded sandpaper, use it flat but remember to sand each side.
  2. Slide the sandpaper between the white insulator and the electrode to remove any built-up debris that may be hiding in the groove.
  3. Blow loose sanding debris away with an air hose.


The above method can be used for cleaning almost all types of spark plugs, however if you have platinum- or iridium-tipped spark plugs, your cleaning routine may need to be adapted slightly. Both of these spark plugs have either platinum or iridium welded onto the centre electrode and ground electrode and will need to be replaced every 100,000km–240,000km travelled, depending on manufacturer's instructions. But, they won't need the plug gap adjustment and in most cases won't need cleaning between replacements if the engine is running smoothly.

However, if you do have to clean platinum- or iridium-tipped electrodes, they should be cleaned for a few seconds — around 2 seconds — in a spark plug cleaner. Do not clean them how you would other types of spark plugs as this could damage the electrodes.

How to fit spark plugs

Once they're clean, you'll need to know how to fit your spark plugs again. You should fit one spark plug correctly before doing the other.

  1. Check the gap on the spark plugs are correct before installation. You can do this using a gapping tool and the guidance given in your manual. Once you've set these, be sure to double check they're right.
  2. Place the spark plug in the socket and turn it clockwise a couple of times by hand.
  3. Use a torque wrench and the spark plug socket to tighten it, using a clockwise motion.
  4. Replace the spark plug wire boot or ignition coil and wire connector. If you have a plastic boot, be sure to listen out for a clicking sound which means it's correctly secured.
  5. Repeat for the other spark plug.


If you'd like to watch the procedure being done before you give it a go, or you want something to follow as you are changing your spark plugs, this DIY tutorial by ChrisFix shows you just how it's done:

Spark plugs are important to your car's transmission and ignition systems, so repairs can be costly when you go to a garage. Hopefully this guide has given you a good understanding of how to detect bad spark plugs, as well as how to clean and change them. Here at GSF Car Parts, we stock a selection of replacement spark plugs and other key ignition system parts to help you keep your car running smoother for longer.

If you're wondering what spark plugs you need for your car, simply enter your registration number into the box provided and you'll find our best recommendations for your vehicle model and make. We offer free UK delivery on all orders over £25, as well as a Click and Collect service, so you can organise delivery to suit your schedule.

If you have any questions about our products, don't hesitate to call us on 0121 626 7971. And, remember to keep up to date with our knowledge hub for more useful information and advice.