5 Causes of Car Engine Knocking Noise (and How to Fix It)

Have you been hearing some strange knocking noises or unusual vibrations from the engine bay? There could be an underlying problem that needs to be addressed. 

A fully functional internal combustion engine should not make any unusual noises, particularly if the noise is a loud ticking, knocking or clacking sound. If you do start to hear strange noises that you don’t normally hear, there could be a lot of different potential causes.

The exact type of sound that you hear might help narrow down the list of potential causes and help you determine the problem. Most people will hear a knocking sound when there is a mechanical problem somewhere in their engine. Some people call it a pinging sound which does not go away. It may even get worse as you step on the gas pedal.

What is Engine Knock?

four stroke gasoline engine

Engine knock (also commonly referred to as ping, preignition or predetonation) is an uncontrolled combustion event inside the combustion chamber. This often occurs due to extreme temperatures or pressures inside the combustion chamber.

As the piston compresses the air fuel mixture, that mixture heats up. Poor fuel quality, incorrect octane, or aggressive timing could cause more pressure than the fuel can handle and cause it to ignite before the spark plug fires.

This commonly results in a pining sound and additional vibration. Many people describe the sound as “marbles in a can.”

Top 5 Reasons Your Car is Making a Knocking Sound

noise while driving

People who hear strange noises coming from their engine may disregard them as being nothing. They often wait until the functionality of their vehicle is compromised before they do anything about it.

Do not put yourself in this situation because many vital components of your engine could be damaged if you do. It is best to investigate the cause of the knocking noises as soon as you hear them. Then you can take your vehicle to an auto repair shop and get the necessary repairs or replacements done.

Here are five of the most common causes of car engine knocking noises.

1) Imbalanced Air-to-Fuel Mixture

bad maf sensor

If the internal combustion chamber is not getting the proper balance of air and fuel mixing together for combustion, then it will cause the fuel to unevenly burn. If this continues on a regular basis, the cylinder wall and piston will be damaged.

This will cause those strange knocking noises to be made from the cylinder wall and piston. Replace these components of the engine immediately before further damage is caused.

2) Octane Rating is Low

low octane fuel

The fuel at the gas station should have an octane rating indicated on the pump. This rating refers to how susceptible the fuel is to engine knocking while the internal combustion chamber is compressing it.

If you are driving a high-performance vehicle that needs a highly compressed fuel and air mixture, then you will need fuel with a high octane rating of 91 or above. But if the octane rating is too low, the air and fuel mixture will ignite before the spark is generated by the spark plug. This will cause those knocking sounds to be heard.

Related: Best Octane Boosters for Better Performance

3) Incompatible Spark Plugs

best spark plugs

If you are using the wrong type of spark plugs in your vehicle, then this could easily cause knocking noises from your engine. Basically, spark plugs have different levels of resistance to hotter temperatures from the engine.

If you use the wrong spark plug which cannot tolerate your engine’s heat, then it will not work correctly. This will result in a knocking noise in most cases.

4) Carbon Deposits

head gasket leak
The carbon deposit on the wall of cylinder #4 (right) is a textbook sign of a head gasket leak.

Gasoline needs to have a carbon cleaning additive to prevent carbon deposits from building up on the spark plugs, valves, and various other parts which assist in the internal combustion process.

Once the air and fuel mixture is ignited, carbon molecules can easily form on these parts if there is no cleaning detergent to wipe them away. If you regularly bring your car in for maintenance, you can have your mechanic add this cleaning agent for you.

Read also: 10 Causes of Car Engine Hesitation

5) Cylinders Not Lubricated Enough

engine oil type

It is important for the upper area of the cylinder head to receive oil for lubrication purposes. If the cylinder head is not adequately lubricated in this area, then you can expect knocking noises from the metal on metal contact.

This is usually the result of loose lifters or valves which may not be getting the proper amount of oil they need. In any event, check to ensure that your oil is flowing properly to the cylinder head area. Either you have an oil leak or perhaps your oil is too old and needs to be replaced.

Related: Best Oil Additives to Stop Engine Knocking

How to Fix Engine Knocking (Especially at Idle)

best oil additive for engine knock

If you start up your car and hear knocking noises, this usually means the fuel of the car was not ignited properly. This could be due to low-quality fuel, bad spark plugs, or another reason above. If the problem is not fixed fast enough then it could damage the components of the engine, which would mean a hefty repair bill.

Instead of spending thousands of dollars on replacing the engine, it would be much cheaper to just repair the cause of engine knock before it does damage.

Before taking your car to an automotive repair shop for an expensive diagnosis, there are a few steps you can take to try and get the knocking to go away.

The first step is to use better quality fuel. Look by your gas cap or in the owner’s manual of your car and see what the octane rating of your fuel is supposed to be. Then make sure you pump fuel into your car which has, at minimum, the same octane rating. For most, standard 87 octane fuel is fine. But for many high compression or high performance vehicles, 91 or even 93 octane fuel is necessary.

Next, you’ll want to flush out your cylinders in case they are dirty. This dirt is actually carbon residue that comes from the carbon components of gasoline. As you use up your gasoline, small amounts of carbon deposits find their way onto your cylinders and will build up over time. That is why you need to periodically flush them out before the buildup is too severe. 

To do this, add a cleaning additive or detergent to your gasoline. The most common one is a fuel injector cleaner which cleans the residue from the combustion chambers and injectors. Seafoam is another common method used to blow carbon out of an engine. 

If you still hear knocking noises from your engine, then check to make sure you are using the correct spark plugs. To find this out, you’ll have to remove the spark plugs first and then write down the model numbers that are stamped into them.

Compare these model numbers to the numbers listed in your owner’s manual. If the numbers match, then you should be fine unless they look very dirty. If the spark plug model number don’t match, it’s best to replace them with the correct ones.

A lot of the tasks above require knowledge of cars and the use of mechanical tools. If you are a novice then you should have a professional mechanic do these tasks for you or else you could risk further damage to your vehicle.

Can You Drive a Car With a Knocking Engine?

oil pressure check engine light

While it’s hard to answer this question without knowing exactly what’s wrong with the car, the safe answer is no; you should not drive your car with a knocking engine. 


14 thoughts on “5 Causes of Car Engine Knocking Noise (and How to Fix It)”

  1. my 06 dodge charger has knocking noise when idling i’ve had spark plugs replaced oil changed added fuel injector cleaner and still its making knocking nosie only when idling

  2. My allion car makes noise when i start it but eventually dies down..but when l reev it that knock sound is heard…what should l do

  3. my 2008 ford sport trac with a 4.6 lit starts and runs great but after i drive it and then idle up my street it makes a knocking sound for about 20 seconds and then clears up only does it at a low rpm and not all the time

  4. When I start my car from cold there is a slight knocking sound which disappears after a couple of seconds. Then the car is fine all day until the next start from cold next day.
    I was thinking of puting an oil addative in the oil. Something like extralube zx1.
    Do you think that would fix it??
    I guess the only answer is to try it and see, but just wanted your opinion if it might work.


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