What Does Engine Knock Mean and How to Fix It

What Does Engine Knock Mean and How to Fix It

Last Updated January 3, 2024 | Nate Moonis

Engine knock is a well-known boogeyman in the automotive community. Severe cases can spell the death of your engine. If you hear a knocking sound coming from your engine bay while your car is running, it's essential to diagnose and fix the issue as soon as possible. However, the cause of engine knock can be vague and hard to pinpoint. In this article, we're going to go over a few of the most common causes of engine knock and how to fix them.

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What is Engine Knock?

Before we can talk about what causes engine knock, it's important to understand what engine knock is. Engine knock is an umbrella term for any type of knocking sound coming from your engine. It is commonly caused by the air/fuel mixture in your engine's cylinders burning unevenly. If everything is working properly in your engine, the air/fuel mixture burns in a controlled manner and creates little "pockets". These pockets burn and create shock waves that ignite other pockets in a controlled and progressive manner.

A mechanic listening to a running engine

If this isn't happening in a controlled, progressive manner, this causes engine knock. It also causes that "pinging" sound commonly heard in engines affected by knock. When your engine starts to knock, the cylinder pressure in each affected cylinder drops considerably. This dramatically reduces performance and can permanently damage your engine if not fixed quickly.

Engine knock isn't the only type of knocking you can hear in an engine. Rod knock is another type of knocking that is terminal for your engine in most cases. This is caused by your rod or rod bearings going bad. As they wear down to the point of failure, they cause the pistons to bang against the cylinder walls and damage them.

What Causes Engine Knock?

Engine knock has a few different causes. They range from easy fixes to time-consuming and expensive repairs that can spell the end of your ride's engine.

Low-Octane Fuel

When you go to fill up your car, you've probably noticed the numbers associated with each type of gas you can buy. The lowest is usually around 86-87 and it can often go up into the mid to high 90's. These are called octane numbers. They measure how resistant the fuel is to detonation. Usually high-performance engines and engines with higher compression ratios require higher levels of octane. If your car requires high-octane or premium fuel, putting in regular 87-octane is going to mess with your engine's ability to detonate properly. This results in your fuel detonating prematurely.

A close up of a gas pump

Luckily, this issue has a simple fix. All you need to do is put higher octane fuel in your car the next time you go and fill it up. If you're unable to find the proper octane level fuel in your area, you can use octane booster to raise the octane level artificially. Octane booster is a premix that you add to your tank before filling up. Pour it in your tank before you refuel and you're good to go.

Worn Out or Broken Spark Plugs

Bad spark plugs can cause engine knock, but luckily this is another relatively simple fix. Your spark plugs provide your engine with the spark necessary to ignite the air/fuel mixture in your cylinders. As with any part in a car, they wear down over time. This can cause them to misfire, fire out of time, or stop working entirely.

Mechanic wiping a spark plug free of debris and other build-up

It's a good idea to change your spark plugs if you haven't replaced them in awhile. The replacement interval for spark plugs vary by manufacturers, but anywhere from 30,000 to 50,000 miles is a good bet for your standard copper or nickel spark plug. If you have platinum or iridium spark plugs, they can be changed around every 100,000 miles. Check out these articles to learn how to replace your spark plugs on your Ford Focus ST or SN95 Mustang.

If you're looking for more information about spark plugs, we have an article about what spark plugs do and when to replace them.


Bad Engine Timing

Your engine's timing is essential to ensuring everything happens when it needs to during operation. Timing is controlled by a computer called an engine control unit (ECU) in modern engines. If that computer is having issues or there are any faulty readings due to broken components, sparks can occur out of time. If you have sparks firing before or after the required time window, it can cause damage and en gine knock. Engine knock, in this case, is usually caused by sparks occurring after the optimal window.

There are a few ways to fix this issue, but they're more intensive than previous fixes. The first thing to do is to check your knock sensor. This sensor detects when your engine is knocking and tells the ECU. The ECU then fixes this issue automatically. While it's not always due to timing, it's a potential issue that can cause your car's engine to knock.

A mechanic tuning a car on the dyno

Another fix is to try and manually adjust your car's ignition timing. In most modern engines this requires you to tune your car. ECU tuning is a time intensive job that should be left to professionals if you don't know exactly what you're doing. If you have an older car without an ECU, you adjust the timing by adjusting your distributor. While this is easier than ECU tuning, it should still probably be left to professionals unless you know what you're doing.

Carbon Deposit Buildup

Carbon deposits in your engine can lead to engine knock. As the miles pile up it's not uncommon to see excess carbon from the combustion reaction in your engine start to build up and obstruct airflow. This can impact the flow of air into your cylinders which alters the air/fuel ratio during combustion. This leads to premature or late detonation and engine knock.

An open engine block with carbon buildup on the cylinder heads

Carbon buildup can be fixed by thoroughly cleaning your engine. This is another job that should probably be left to professionals unless you feel comfortable with taking apart your engine to access the cylinder heads. You can use special sprays to remove some of the deposits, but they rarely solve the problem completely. If you want to be sure your engine is free of carbon buildup, having a professional do it is your best bet.

Stop Your Car From Knocking Today

Engine knock is a scary phenomenon and can cause catastrophic damage if left alone. Stop driving your car immediately and address the issue if you hear your engine knocking.

This article was researched, written, edited, and reviewed following the steps outlined in our editorial process. Learn more about CJ's editorial standards and guidelines.