What Causes Engine Knocking When Accelerating?

Do you hear some metal clucking sounds from the engine while accelerating? This is a common experience for most drivers, and it is known as engine knocking (detonation). Engine knocking happens when some fuel in the cylinder detonates before the other fuel. During normal operation, the spark plug starts the combustion process and burns all fuel in the cylinder, thus controlling the explosion.

Spontaneous ignition of fuel and air in the cylinders results in engine knocking. In this article, we shall discuss in detail engine knocking while accelerating. We shall also get to know the reasons behind it and how to prevent the engine knocking from happening, and how dangerous it is to the engine; keep reading if you want to know more about engine knocking sounds.

What Causes Engine Knocking When Accelerating?

Before we learn what causes engine knocking, let us first know how ignition happens in normal combustion. The engine burns the air/fuel mixture consecutively. At the same time, the combustion is started by the spark plug some 10 to 40 degrees prior to TDC (top dead center). The Spark across the spark plug’s electrodes produces a kernel of flame. This kernel grows in size and temperature.

There are various causes of engine knocking sound. You may experience a knocking noise when accelerating from a stop or a knocking sound under a car when braking. Let us discuss things that may cause engine knocking sound:

how to fix engine knocking

Too Low Octane:

Every engine is designed to work on a certain number of octanes. This octane number determines the tolerance of fuel to pressure rise. The higher the octane number, the more the piston moves toward the spark plug, producing more power output. When the octane number is low, make several pockets of exploded air/fuel mixture before the piston reaches the maximum level of travel. This, therefore, results in producing a knocking sound and also reduces vehicle performance.

Bad Timing:

To make the spark fire in an organized manner requires the computer distribution of the electricity from the ignition coil. If this fails to happen, it leads to detonation, which results in the engine knocking when accelerating and the speed rising. In this case, the car starts but stays not running.

Lean Air/fuel Mixture:

If you have a lean air/fuel mixture, it means that the percentage of the fuel is too low compared to the percentage of the air in the AFM. This happens when there are faults in engine management sensors such as oxygen sensors, spark plugs, spark plug wires, fuel injectors, and mass airflow sensors of the fuel pump. Without enough fuel in each cylinder, the mixture burning time will be incorrect, causing the mixture to explode at the wrong time. This finally results in producing some detonations causing the engine to knock.

Bad Knock Sensor:

Modern cars equipped with an engine knock sensor detect some faults and send a message to the ECU to solve the problem. If there is a problem with the knock sensor, it will not detect the knocking, and the ECU will not know that there is an error to work. This eventually causes the engine to knock.

Faulty Spark Plugs:

As we all know, spark plugs are essential for burning air/fuel mixture to provide the necessary power for the engine to run. Spark plugs age or gets damaged in time, causing them to produce weaker Spark. At times the spark plugs may delay the sparks making the engine produce a knocking sound when accelerating.

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Carbon Deposits:

All the fuel used in cars has carbon cleaners. They help to prevent carbon deposits from buildup and clogging the cylinders. When carbon clogs or buildup in the cylinder, there is less space for air/fuel to reside, resulting in engine knocking. We recommend using the FTI Carbon Cleaner Washing Kit; you Can Easily do the job.

Low Oil Level:

Another reason may be due to the engine knocking low oil. In case the level of oil is below a certain level, the rod bearing between the crankshaft and the piston rod begins to develop a gap. This causes a rattling noise while going up and down.

How to Fix Engine Knocking

Now that we know the sources for car knocking, we can now learn how to fix engine knocking. These techniques will help stop the rattling sound from your car.

High up your octane rating fuel to get a higher quality and more accurate combustion. This is one way to make the knocking sound vanish. Increasing the level of octane used by your vehicle enables the ignition to be performed at the right time. You can use the octane booster to raise the number of octanes if it is below the desired level.

Using a certified car scanner, take the readings of all sensors in real-time in different loads, accelerations, and speeds. Compare these readings with the ones recommended by the manufacturer. Monitor your vehicle’s timing and take the shop manual as a guide in fine-tuning the timing of the engine.

Check on the knock sensor regularly with the car scanner to make sure it is working properly.

Ensure that you replace the spark plugs as soon as they get damaged or old to avoid vehicle engine knocking. The spark plug will wear out due to age or mileage, thus delaying the sparks, which later causes the knocking sound.

Always use the proper type of oil compatible with your engine type, age, and mileage. This prevents the damage of any internal parts using the wrong oil type or viscosity. Keep engine oil changes in a predefined schedule.

Avoid carbon buildups by using engine cleaner, throttle body cleaner, and injector cleaner. Use the cleaners to clean oil passages and injectors (to prevent no injector pulse) and clean the throttle body. Also, use the cleaners to clean the gas tank. The products dissolve carbon buildups which can cause blockage in fuel and spark the workspace.

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Frequently Asked Questions (FAQs)

Q. Can You Drive a Car With a Knocking Engine?

Yes, you can drive your car with a knocking engine, but you have to be gentle while driving. Try to ease up the throttle and check the problem in your vehicle once you get to your destination.

Q. What Does a Knocking Engine Sound Like?

It is like a rapid pinging and a metal clucking while the vehicle is accelerating. At first, you will only hear it while accelerating. As soon as you start hearing the knocking sound, look for the problem, and solve it before it damages your engine.

Q. How Much Does It Cost to Fix a Knocking Engine?

Fixing a knocking engine does not cost a lot of money. At times you will be required to replace the spark plugs, thus adding extra costs. In most cases, it costs approximately $200. If the knocking sound continues even after changing the spark plugs, the cost will increase since you will be required to disassemble the engine and inspect its internal parts.

Q. Will an Oil Change Stop Engine Knocking?

Ok, you need to differentiate between the cluttering noise from the engine due to too low oil level or pressure and the knocking sound that comes from the combustion chambers. The knocking sound is a result of the problems with Spark and fuel. Oil change, therefore, will stop the cluttering sound from the valves in the cylinder head and not the knocking sound.

Q. Can Thin Oil Cause Knocking?

Old engines tend to have worn-out engine bearings between the crankshaft and piston rods. When the engine bearings wear out, they make a rattling sound every time the piston moves up or down. This is called crankshaft knock. Therefore, using thick oil will help to block these gaps and stop the sounds.

Knocking Noise In Engine – Simple Fix YouTube

Final Words

An engine knocking while accelerating is a problem faced by many people in the course of driving. This sound is not only disturbing to the ears but also has a destructive effect on the engine and its internal parts. The causes of engine knocking sounds may range from bad fuel, faulty spark plugs, or even a dirty combustion chamber.

The most important action to take has a good diagnosis to ease the repairing process. The prudent thing to do when you hear an engine knocking sound at idle or accelerating is to seek professional advice immediately. Dealing with the problem early enough protects your engine from future damage.


Hi there, I am R. Hasan Tito, a mechanic, and owner of this website. My friend and I created this website to share our knowledge, expertise, and experience with our fellow mechanics' community and car users. I am a specialist and certified automotive mechanic (Both Heavy Commercial and Private Cars). I worked as a Mechanic and Mechanic Supervisor for over fifteen years at Global Rebound Automotive companies - Toyota, TATA, BMW, Nissan, TVs, and Others. Now, I enjoy my new role of leading a team of automotive experts (in their respective fields) and publish new content on a regular basis on my website and social media.

4 thoughts on “What Causes Engine Knocking When Accelerating?

  1. Thanks for your concern and information given…my Peugeot 807 ….2003 model gives me knocking sound every time in the morning ( 1st starting for about 10 minutes and then gives normal sound and throughout the rest of the day, it will give normal sound…why my Engr….? It carries ew 12 engine, Thanks once again, I need your help here….

  2. Hey Roy, glad it works for you. But mind what you do next time to avoid creating more problems.

  3. Yes it just what my 2004 Chevy V-8 4-8L Silverado sounds just like I did put low gas in it and let it set about one day, and when I did started it as the first time I ever heard that sound.

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