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Why is there a knocking sound coming from my engine?

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Updated 10 Oct 2019

Fionna Blackburne

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Knock, knock.

Who’s there?

Your engine!

Our cars make some weird and wonderful noises, but a knocking sound is no joke.

What can cause engine knocking?

Spark Plugs

Using the incorrect or worn spark plugs can cause a knocking sound.

Your spark plugs literally create an artificial bolt of lightning within the cylinder head of the engine.

It transmits an extremely high voltage in order to light the fire within the cylinder head - also known as the combustion chamber - and it is always a good idea to use spark plugs that are recommended by the manufacturer.

Low-octane fuel

Using a lower octane fuel in your car than that recommended by the manufacturer can cause a knocking sound.

High-octane fuel is more expensive, but it is worth the extra dollars in the long run.

Long-term use of the incorrect fuel in your car could damage your engine and decrease your fuel economy.

The octane ratings:

  • Standard unleaded petrol is 91
  • Premium unleaded is both 95 and 98
  • The ethanol-blended E10 (a mixture of up to 10% ethanol in petrol) is a substitute for 91 in most cars.

The best thing to do is refer to your owner’s manual and use the correct octane rating required for your vehicle by the manufacturer.

Carbon deposits

Carbon Deposits are a natural by-product of the process of burning fuel.

Fuel is required to include carbon cleaning detergents to help prevent carbon deposits from forming, however, some deposits will still form.

When theses deposits form, there is less room for the fuel and air to reside, leading to increased compression.

Changes in the compression results in decreased performance, lower kilometres, overheating and a nasty knocking noise.

Generally speaking, carrying out regular fuel injector and carbon cleans can assist with resolving this issue.

Always best to get your car to your mechanic to give it the once over and rectify the knocking noise.

Let AutoGuru quickly and easily find you a local mechanic to diagnose any engine knocking noise problems you may have.

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Written By

Fionna Blackburne

Fionna is the owner of Southeast Auto Mechanical in Beenleigh, Queensland. A former lawyer, Fionna decided on a radical career change, qualified as a light vehicle mechanic and took over Southeast Auto in 2016.

As well as delivering quality service to their customers, Fionna and her team run an award-winning, free car maintenance workshop program for women.