How to Know What Your Spark Plugs are Telling You

Reading your damaged spark plugs gives you an invaluable insight into your engine’s condition. When you understand what the state of your spark plugs means, you’ll be able to diagnose spark plug faults easily.

Spark plugs ignite compressed petrol from an electric spark generated by the plug. During this process, slight carbon deposits of a brown, grey or tan colour are normal. But carbon deposits, oil deposits, wet, burned, scratched, chipped or otherwise damaged spark plugs are not.

Understanding Your Damaged Spark Plugs

Oil build up?

If your spark plugs are particularly oily, an excessive amount of oil is entering the combustion chamber. It’s likely that oil is being sucked into your combustion chamber because of leaky valve cover gaskets, crankshaft end seals or oil pan gaskets. This is often the result of a worn piston ring, cylinder walls, valve guides, guide seals, piston rings or piston.

Rounded electrodes?

Spark plugs with rounded electrodes have usually been damaged by simple wear and tear. This means that your spark plugs are probably due for a replacement after years of use. You might also have noticed loss of power and stunted vehicle performance.

Scratches and missing chunks?

Scratched and broken spark plugs usually indicate that there is a foreign object in your engine. Meanwhile, split or chipped spark plug insulators are a sign of pre-detonation. Check for ignition timing faults, engine overheating, a faulty EGR valve or a clogged combustion chamber.

Blistered, white spark plugs?

A white coating on overly dry spark plugs typically indicates that your engine is overheating and that there is too much air in the air-fuel mixture. There may also be an issue with the engine’s cooling system or ignition timing. It’s important to seek mechanical advice and replace the spark plugs as soon as possible so that they don’t burn through the top of a piston.

Black carbon deposit?

Black spark plugs are a sign of a weak spark or too much fuel in the air-fuel mixture. Carbon deposit can build as a result of numerous faults: a stuck choke, a misadjusted carburetor float, a damaged carburetor needle valve, low coil output, high resistance in the spark plug wires, a leaky injector or damaged oxygen or coolant sensors.

Wet spark plugs?

The three most common causes of wet spark plugs are: a flooded engine, a cracked ceramic insulator, or a faulty ignition cable.

Smashed or flat spark plugs?

You may have the wrong spark plugs for your engine. Spark plugs that are too long can protrude into the combustion chamber, colliding with the piston or a valve. This can damage the piston and even shatter it. Meanwhile, spark plugs that are too short can cause the engine to buck – the spark plugs struggle to ignite the fuel, reducing MPG.

Eroded or corroded spark plugs?

Pitted spark plugs with rough electrodes are often the result of fuel reacting with the metals in the electrodes. Corroded spark plugs can increase spark plug gap, leading to poor performance.

When Your Spark Plugs are Damaged

Spark plugs are amongst the most cost-effective car parts to replace. But the damage that they can cause when they are worn is amongst the priciest. Therefore, it’s important to regularly service your spark plugs and understand what their condition means.

When replacing your damaged spark plugs, you can enter your number plate into our car lookup tool, and we’ll recommend spark plugs that are will fit your model.

Choose from a range of branded OEM and aftermarket options. Order before 1 pm and we’ll ship your replacement spark plugs to you on the same day.

All DIY car service parts are manufactured by trusted, high-quality brands. Here are just a few of the brands that we stock.