How to troubleshoot a car engine knocking sound when driving & idle

We all use our car daily to get around town, whether to get to our jobs or go to our daughter’s friend’s birthday party.

But what happens when there’s trouble under the hood that needs mending?

Article summary

Uneven combustion of fuel air and fuel is the most common reason for an engine knocking sound when driving or idle. Many other things can also cause an engine knocking sound, including the following:

  • Carbon accumulation in the combustion chamber
  • Carbon accumulation on cylinder walls
  • The quality of the fuel
  • Bad timing of the sparks firing
  • A leak in the vacuum
  • Knock sensors deterioration
  • Overheating of the engine

Continue reading

car engine

It starts small. At first, it comes knocking on your hood and tries to tell you something is wrong. All you have to do in those moments is listen and repair.

Table of Contents

Reasons for a car engine knocking sound when driving & idle

We can solve all problems over time and with little extra care. Therefore, it is essential not to ignore them and hope they’ll go away independently. By the end of this article, I will cover all the causes so that you may understand them better and come up with a solution for all of your issues.

Let’s get started then and look at the reasons for a car engine knocking sound when driving & idle.

How to troubleshoot a car engine knocking sound when driving & idle

  1. Ignition Knock

    First of all, the most common engine knocking sound is the Ignition Knock. It’s the most important thing to understand the issue is to know its normal flow. 

    When the engine behaves normally, the fuel mixture that ignites burns in a single, administered explosion in each cylinder. The ignition Knock is the noise that will appear when the explosions happen multiple times in each cylinder.

    Due to the fuel mixture and air intake in the engine can explode earlier than it’s supposed to. It causes a cascade effect of explosions that run into each other, causing the Ignition Knock.

    If the car has a manufactured engine appraised for high-octane fuels, using low-octane rating fuel will damage all the parts. It causes buildup over time that leads to the slow deterioration of the engine and its slow demise.

    Using a higher-octane fuel will benefit the explosions in the cylinders, leading to consistent burning of the air/fuel mixture. 

    Using a lower-octane fuel for a demanding car will entirely damage the components of your engine. It is ultimately the source of your engine knocking sound.

    Always check the manufacturer’s manual to find the optimal octane rating designed for your car.

  2. Timing Knock

    Most of the newer brand cars have timing built into the motor’s computer. It determines the moment at which a spark should fire.

    Unfortunately, the computer can cause the spark to ignite at the wrong time at any point due to technical errors. It leads to a multitude of explosions in the cylinder. You guessed it. It creates an engine knock.

  3. Fuel/Air Mixture

    Explosions happen in the cylinders through a perfectly calculated mixture of air and fuel injected into the chamber. It then creates synced chain explosions.

    Any problem with your fuel injector/pump or any air sensors (oxygen sensor; airflow sensor) can create a surplus of the component needed for the explosion.

    If there is a surplus of the mixture in the cylinder, the fuel existing will not have enough time to ignite fast enough. It leads to multiple detonations.

  4. Malfunctioning Knock Sensor

    Fortunately, manufacturers have already thought about the existing problems regarding engine knocking. Modern cars have a designed, built-in computer. It controls the air intake ratios, fuel injection, and the mixture’s timing.

    A Knock Sensor is already built-in. It sends signals to the Engine Control Unit so that it can solve the problem on its own, without even beginning to notice the issue in the first place.

    Having a malfunctioning Knock Sensor will allow the engine to knock. Checking the sensor first place is an essential part of troubleshooting and repairing.

  5. Weary Bearings

    Although it is not the most common, it is crucial for your motor system. For example, the rod knock is frequently miss-checked and poorly treated because of the repair cost involved once it occurs.  

    What is its meaning?

    Its role is to transfer the power received from the motor to the wheels through the crankshaft’s support. The rod bearings can deteriorate over time by shifting position.

    Because of its importance in transferring the power from pistons, the wearing will start to be noticed over time.

    As they wear out, the pistons will begin to clatter against the crankshaft. In turn, it will cause the engine to knock.

  6. Worn Belt Tensioner

    Another typical knocking sound is a worn belt tensioner, although not so much in newer model cars. Although it is similar to engine knocking, it is not coming from the engine itself but the accessory belt

    Multiple tensioners on the accessory belt are constantly running attached to pulleys all over the engine bay. The moment the belt becomes out of synchronization and the tensioners extend, the pulleys become misshapen

    That is also when you hear sounds like the engine knocking, such as clattering, bangs, clangs, and jingles. 

    A simple replacement of the accessory belt or adjustments to the tensioners will eliminate the noisy sounds. 

  7. Poor Oil Quality

    The most common issue when encountering knocking sounds is a poor oil quality choice. Although we invest a lot of money in other parts, not choosing a good quality oil or even premium can lead to the downfall of our engine. 

    Oil is integral to lubricating and cooling the operating engine components that play an essential role. 

    The camshaft, valve stems, and the gaps between the parts are all lubricated with oil. These components will manifest knocking sounds when those gaps enlarge and are tattered due to poor oil quality. 

    As we know it, the cold start makes the engine knocking sound most noticeable. When it starts, oil doesn’t have enough time to circulate through all components and gaps entirely. 

    High engine temperatures can also play an essential role in the slowly decaying oil quality, interfering with its development. Therefore, always use the car’s manufacturer’s recommended oil for your engine to avoid further lubrication problems.

    Some other relevant causes:

  8. Overheated Engine

    An overheated engine traces back to cooling system problems. Take a quick look at your coolant levels and complete with the recommended quantities, if any are needed. A glance at your water pump could also do the trick. 

    Higher temperatures in the ignition chamber may cause pre-explosions. It results in knocking sounds since the fuel-air mixture ignites earlier.

  9. Low engine revving

    Knocking sound noticeable at low engine rev is often caused by a low octane level and stale fuel. It can be at revs at 1000-2000 RPM.

Can you fix it?

car bearing

I’m sure we’ve talked a lot about all the problems we and those with this passion encounter.

The next question is… can we fix it ourselves? Or do we trust specialized ones to do their jobs on this matter? The answer is straightforward!

It depends on the gravity of the problem that we encounter. For example, we can solve most issues in minutes or hours. 

Does that mean that for every problem that we have, we should try to handle it ourselves first?

The answer is no. Doing that without first having basic knowledge of the matter at hand will be making things worse. The bill will only start growing, making matters worse.

But that’s why you’re here after all. Right? 

You are coming to the right place for the correct information so that we may give you the basics and more.

Below you will find a list of valuable things to watch out for so that you may take good care of your engine.

Regular oil and filters change.

overheated engine

Oil is the first line of defense for preserving the engine components. Its lubrication properties will disperse the heat and withdraw any debris and tiny metals existent in its components. 

Regular oil changeNot changing the oil regularly
Reduced risk of engine damageIncreased risk of engine damage
Improved engine performanceDecreased engine performance
Improved fuel economyReduced fuel economy
Reduced maintenance costsIncreased maintenance costs

Although it comes with a great list of benefits, ignoring any check-ups and revisions will make it your greatest enemy instead of your best friend. 

Because of its cleaning properties, not changing the oil in time will create a particle buildup. It eventually results in sludge that will recirculate into the components again and again. At a point, you will pay a visit to the mechanic sooner than you thought.

Usage of higher octane fuel

belt tensioner

Low octane fuel types can negatively impact your motor’s performance and durability. As we mentioned before, it will only cost you considerably to repair the damage done to your motor. 

Take a look at the differences between high and low octane fuel.

High Octane FuelLow Octane Fuel
Contains more hydrocarbonsContains fewer hydrocarbons
Higher octane ratingLower octane rating
More expensiveLess expensive
Better engine protectionLower engine protection

Rarely adding octane boosters will improve its rating, potentially making the noises disappear.

Scrub the combustion chamber

You can quickly do it with the help of some great cleaning products. They will remove the excess carbon deposited onto the chamber’s walls. Unfortunately, the buildup often gets in the way of the combustion process. It lowers any power the wheels can receive.

Don’t stress yourself if you find it difficult to follow the procedures online. That’s why you can leave it to your mechanic to do it for you, as it’s neither hard to do nor expensive.

While you’re at it, make sure your car doesn’t smell of smoke or have dents. All these points apply to travel trailers as well.

When I start my car

The first thing we do when we start our cars is to listen to the sound of our motor roaring. When you press the Start button, all the sounds from the motor should be in rhythm. If you start to hear engine knocking noises, the sparks did not ignite the engine correctly.

Poor ignition contact is a consequence of low-quality fuel, malfunctioning sparks, or any of the reasons above. If you mistreat your car’s problem, the motor could be harmed. It only means a faster visit to your nearest repair shop.

Another cause might include a malfunctioning knock sensor. Take a look at the following signs.

Sign #1Poor acceleration
Sign #2Decreased fuel efficiency
Sign #3Abnormal engine noise
Sign #4Engine hesitation
Sign #5Increased exhaust emissions

Avoid spending a fortune on parts replacements. You could prevent all of the causes before doing any harm by using the right fuel octane and suitable motor components. Also, pick up on good driving habits. 

There are a few ways to determine what unfortunate event has descended upon your motor.

What causes the noise?

You should never stop looking for issues to fix if there are any when it comes to hearing engine knocking sounds. 

As we have stated in all of the problems listed above, many things can cause trouble and stop functioning correctly. 

When it comes to the knocking sounds that the motor is making, the first time you hear it is also the first time you should take action.

It is never great to wonder about the meaning of the sounds but to search and eliminate the problem as fast as possible. 

Although you would believe that only the performance would be affected for some time, I suppose you might be wrong. In the long run, taking care of the components will only prolong your engine’s life and enhance its performance and viability.

It includes the:

  • the valves
  • cylinders
  • pistons
  • rods
  • and rings

The consequences are far more significant as you may end up a pedestrian. It could be worse. You could be a pedestrian forced to pay the bills of a no longer usable vehicle.

When accelerating

The most common engine knocking sounds are present when accelerating. If that’s the case for you, it’s your lucky day. 

When accelerating, the presence of the knocking sounds will be the cheapest option you will have to pay for out of all possible reasons that could cause them.

Most causes include:

  • using low octane fuel
  • a faulty injector/injection system
  • defective sparks
  • or even old engine parts.

Upon hearing the ragged sounds, the first thing is to start wondering when your last oil revision was. If you haven’t changed it in a while, I strongly recommend doing so.

There’s an easy way to determine if the oil is the problem, and that’s the cold start. Whenever you perform a cold start, consider this. If engine knocking is among the first things you hear, you’ve also found your solution. 

Always pay now so that you may pay less later!

How to fix engine knocking in a car

car oil

Please take my advice, and always diagnose and repair with your trusted mechanic. 

Although it’s not a bad habit to diagnose the problem yourself, it’s always best to not spare the dollars if you have any possibility to do so.

Most problems can be fixed early on by ourselves without causing much damage or taking a heavy toll on your engine. However, doing so requires certainty and knowledge.

I strongly recommend that the first times you visit your mechanic, always ask questions about the work done. It includes any other curiosities that may help you further, in the unfortunate case that you may reencounter the issue.

Stop thinking you will waste their time with questions, as you are paying a good deal for their advice and services. 

It will serve you well to know more about your engine parts’ meaning and how to treat them well.

car engine

Why you should trust us?

Every piece of knowledge here is not personal advice but rather tested research. We have tested and checked the reasons for this malfunction and only given you the results of our deep research. That being said, several experts (both online and offline) were also considered to confirm our findings.

Factors to consider

Take a look at what you should know before you head to the repair shop:

The age of the car

Is your car coming of age? Were you going to sell it off and buy a new one anyway? Then why waste money on costly repairs. Go ahead and sell it off with minimal damage and buy the dream car you’ve always wanted.

The repair place

Where you get your repairs also matters. It’s not like every place you come across will be trustworthy and inexpensive. You’ll need to take a look at customer reviews online and do some research around the neighborhood to find the best place for your ride.

How much does the problem cost to troubleshoot & fix?

To fix this problem it could cost you anywhere between $2000 to $3000. It could be more if there are more underlying issues.


It is regrettable to experience technical issues with your car; yes, I know.

Every driver that has ever had a car experienced problems with their engine of any kind at least once. 

It’s normal to own a car we are proud of, and the only thing we can do as drivers is to keep taking care of them. 

Always keep in mind the first 3 rules when encountering a knocking noise:

  1. Diagnose
  2. Repair
  3. Enjoy

We will be happy to hear your thoughts

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