Need a part or service found for you?
Call our experts on 1800 595 454

What Causes Engine Knocking & How to Fix It

Technical  ·  February 10, 2023

What Causes Engine Knocking & How to Fix It

People rarely ask about engine knocking until they experience it first-hand, but it’s something you ought to know before it happens. It’s a serious issue that you want to resolve and stop from getting worse.

Engine knocking can be a fuel or combustion-related problem caused by using low-octane fuel, having dirty spark plugs, or burning a lean fuel/air mixture. Problematic internal components, like a worn bearing or damaged crankshaft, will also cause the knocking sound.

You may want to bookmark this page to refer to if ever you hear an engine knocking sound when accelerating. The sooner you fix the problem, the faster you’ll prevent more severe engine problems.

What Is Engine Knocking, and How Do I Fix It?

All car owners should have at least a basic understanding of what causes engine knocking. That’s because your engine is the heart of your vehicle, and those knocking noises are never a good sign. Worse, they can lead to more damage and more expensive repairs later.

Here are the possible reasons your engine is knocking and the things you can do to get it fixed ASAP:

1. Low-Quality Fuel

Filling your tank with low-quality fuel (i.e., fuel with a lower octane rating than your engine needs) is the most common cause of engine knocking.

Using low-octane fuel in high-compression engines will result in prematurely igniting the air/fuel mixture before the spark plug does its job. This causes the knocking sound you hear as your engine runs.

Since this kind of engine knocking is due to your fuel supply, you will hear the engine knocking sound at idle and when you’re accelerating and moving.

How to fix it: The most important thing you can do to fix this problem and prevent it from ever happening again is to only use the correct fuel for your vehicle model. 

You can find the information you need in the car owner’s manual, which will show you the manufacturer’s fuel recommendations.

Stick to using only fuel with the octane levels recommended by the auto manufacturers, and you’ll prevent engine knocking from happening again.

2. Spark Plug Problems

The engine knocking sound you hear can also be due to spark plug problems.

Spark plugs deliver a high-voltage spark that ignites the compressed fuel-and-air mixture in an engine, combusting the mix and causing the engine to produce power. The combustion process happens continuously while the engine runs.

However, things can also go wrong for spark plugs. When they’re worn-out or dirty, spark plugs fail at their job. Without this spark or ignition, the compressed fuel-air mixture detonates in an unstable way, causing the loud knocking sound you hear coming from under the hood.

How to fix it: Spark plugs are small components, but as you can see, they play a crucial role in the engine’s functioning. Thankfully, they’re also pretty easy to deal with when they become problematic.

If it’s been a while since you changed your spark plugs and you notice them starting to fail, replace them immediately. It doesn’t cost so much to replace them with brand-new spark plugs, and you quickly eliminate the knocking sound and related issues.

3. Lean Air/Fuel Mixture

Now, let's consider the air/fuel mixture itself. Modern vehicles can automatically calculate how much air and fuel to mix for optimal performance. Various sensors feed data to the car's onboard computer, which decides how to handle that mixture.

Despite the onboard computer producing an ideal mixture, problems elsewhere can cause the mixture to become too lean. That’s when there is too much air and not enough fuel due to an engine vacuum leak, dirty fuel injectors, or even low fuel pressure.

A lean mixture will not ignite fast enough, leading to multiple detonations you will hear as knocking sounds from the engine.

How to fix it: The solution to this problem will depend on the root cause. So, fixing it begins with a thorough engine inspection to determine what’s causing too much air in the fuel mixture.

For example, that might involve cleaning the fuel injectors or plugging any leaks.

Addressing the root cause will restore a correct fuel/air mixture and stop the engine knocking you’ve been experiencing.

4. Worn Bearings

So far, you've seen three reasons for engine knocking related to fuel and combustion, but that’s not all.

Worn-out and damaged internal components can also cause engine knocking. As you can imagine, the problem is incredibly severe if it’s caused by damaged parts.

The first likely reason is worn-out bearings. Bearings allow parts to turn smoothly and quietly, with as little friction as possible.

When engine bearings become worn out, they’ll cause pistons to rattle against the crankshaft. That rattling causes a knocking sound you’ll hear as the engine runs.

How to fix it: The only practical solution here is to replace the worn bearings in your engine. Unfortunately, given that they're internal components, you'll need to take the engine apart to get the job done.

Rebuilding your engine is costly, but not doing it will only cause more damage to the inside of your engine.

5. Damaged Crankshaft

Last but not least, a damaged crankshaft can also cause the engine knocking noise that you hear as you drive.

The crankshaft rarely suffers damage from excess wear because they’re designed to last an incredibly long time and tolerate heavy use.

A likely cause of damage to a crankshaft is mechanical overload. The thing is, finding out the root cause of that overload is another challenge altogether.

How to fix it: Like the last, this repair will require taking the engine apart. However, simply replacing the crankshaft isn't enough.

Before you do that, you must figure out what damaged it in the first place. That way, the problem won’t repeat itself with the new crankshaft.


Heavy repairs on an engine like the ones described above are better handled by professionals. You may want to consider hiring an engine rebuilding service. Check out the CarpartAU Directories to find the best automotive professionals in your area!

By Ray Hasbollah

More from the Blog

Explore the Opportunity of Owning!

For Sale: - Your Gateway to the Automotive World

Are you ready to own a premium domain in the automotive industry? is now available for purchase, offering you an exclusive chance to step into the driver’s seat of this established online platform.

What’s Included in the Package?

  • Domain Name: Secure the highly sought-after, a single-word domain that speaks volumes in the automotive realm.
  • Website: Acquire the entire website, featuring the single largest directory of automotive suppliers. Benefit from top-notch Google rankings, providing instant exposure to a vast audience.
  • Instagram Page: Take over the Instagram page (@carpartdotcom) with a whopping 240,000 real car enthusiasts following. Become an instant influencer and leverage years of engaging content.
  • Carpedia: Dive into a treasure trove of automotive information. includes Carpedia, a comprehensive resource that draws daily traffic seeking valuable insights into vehicles.


  • Instant Exposure: With thousands of top-ten Google rankings, you can own the next car parts marketplace and enjoy instant visibility in a competitive market.
  • Endless Possibilities: The opportunities are boundless with a domain like In a time where single-word domains are rare, secure your place as a key player in the automotive industry.

Price: $1.45 Million AUD

How to Inquire: For inquiries or to express your interest, call the number at the top of the page telling us your interested in the sale of the website. Leave a message with our office, and expect a prompt call back.