On a fear and worry scale from one to ten, a knocking sound in the engine would be a nine……if not a ten.

A sound that sends shivers down a driver’s spine as it usually means a lot of spent money, time, and nerves on repairs.

But as you’ll see, sometimes it’s not all gloom and doom and the problem may not be that serious.

Whatever the case, this article should help you get a good idea of what the cause of the knocking sound may be. Once you know the cause, the solution will certainly be much clearer.

If you have the time click here to read a great article on Wikipedia on why engine knocking appears.






Proper fuel is essential for the good functioning and longevity of the engine. Namely in terms of quality and proper octane rating.

Although many drivers are not aware of this, every manufacturer has tuned the performance of its engines to certain fuel quality and octane ratings.

The engine, namely the combustion chamber, spark plugs, and else are simply designed upon this fact.

Let’s take for instance the petrol engine: yes, the engine will start and work on fuel with higher or lower quality and different octane ratings, but you’ll encounter several problems:

  • Lower octane fuel means less performance, mostly due to improper combustion
  • Higher octane fuel may mean better performance but puts higher stress on the engine which can eventually lead to a major breakdown
  • Poor quality fuel, especially in terms of purity, will give bad performance all over and can cause major problems with the engine, fuel intake, and else

Of course, there’s the matter of fuel consumption which will, in all of these cases, be improper and not as predicted by the manufacturer.

All of these problems will cause a knocking sound in the engine.

The appearance of water in fuel is rare and is mostly reserved for third world countries. But if it is present it will also cause a knocking sound. It’s usually accompanied by a radically irregular engine function (like strong misfires).


Try changing your gas station, especially if the knocking sound in the engine persists. This is a cheap method and if the fuel is the problem, you’ll notice a change in a matter of minutes once you’ve poured other fuel.

Also, if possible, use fuel with the proper octane rating provided by the manufacturer. Yes, sometimes it may be more expensive but in the long run, it pays off through less repair and better engine condition.



The air-fuel mixture is the substance injected into the combustion chamber. Its proper ratio is crucial for the proper functioning of the engine.

A knocking sound in the engine will appear if the mixture is to lean. This means that there is more air than fuel in the mixture leading to multiple detonations instead of a single, proper one.

Although all modern engines have the air-fuel mixture regulated by the ECU (the car’s computer) problems do happen. Mostly due to faulty sensors (like O2 sensor, airflow sensor, etc), fuel pump problems, fuel injectors, or else.

The fact is that the computer which regulates the air-fuel mixture is dependant on an array of sensors and electronics that are susceptible to malfunctions.

Getting the check engine light is one of the first symptoms once this happens.


The first step is a good diagnostic.  Many factors can go wrong with this problem, so pinpointing the exact cause will save both time and money.

In most cases like sensors, filters, or fuel pumps the best solution is the complete replacement of the faulty part. Sometimes cleaning helps but it’s usually short-lived.

Again, the main and more difficult part of the problem might be finding what’s wrong rather than replacing the faulty part.




In the combustion cycle process, the spark plug has a crucial factor as it ignites the air-fuel mixture.

Besides the overall condition, the gap between the electrodes is very important.

Too wide or too narrow and the spark won’t have the necessary power to properly ignite the air-fuel mixture.

Thes then leads to interrupting the combustion cycle and to the possible appearance of a knocking sound in the engine.

Spark plug problems are mostly caused by:

  • Material fatigue: spark plug electrodes wear out over time (which is the main reason for replacement, by the way). The gap gets wider over time and the spark weakens. Other problems are cracked insulation or even the electrode breaking off (this is in extreme cases).
  • Improper maintenance: it’s a common maintenance procedure to take off the spark plugs, clean them, and check the gap between the electrodes. This lengthens their service life as well as increases performance.
  • Engine problems: spark plugs that are flooded from fuel or greasy from the engine oil can not give a good spark. This is a tell-tale sign that somethings wrong, either with the engine or the fuel intake.

Regular maintenance and using quality spark plugs are key here. Modern spark plugs from quality manufacturers are made from highly durable material and without the need for checking the electrode gap during the regular service interval.

So using quality spark plugs and changing them on time should solve this problem.

If it does happen, then best see a mechanic (or do it yourself if you have the expertise), take out the spark plugs, check and set the gaps, clean and return them.

A complete replacement would perhaps be better in this case both in terms of cost efficiency and longevity of the repair.




Fuel injectors have the task of injecting the fuel into the combustion chamber.

They do this under a tremendous amount of pressure and tension. Although they are made from very durable metal materials, over time these factors take their toll mostly through material fatigue.

Injectors are meant to deliver fuel into the combustion chamber in the form of a fine mist. Instead, if they’re faulty, fuel enters in the form of a liquid-stream (in a ˝spitting˝ manner so to say).

Once this happens, the combustion cycle is interrupted causing the appearance of a knocking sound in the engine.

One more problem may be that the injectors get clogged. This mostly happens if the fuel is filthy or the filters haven’t been changed on time.


Regarding a faulty fuel injector, the best solution is a complete replacement. Since this is a pricey repair for almost all cars out there, people tend to replace only the faulty one.

This is completely OK under the circumstance that the other injectors are taken off and checked. Otherwise, you may end up repeating the same pricey job soon.

One more good option is refurbishing the old injector(s). Certain parts can be replaced and the old injector returned with a successful outcome.

When the fuel injector(s) are clogged adding various fuel additives may help solve the problem. If so, the results will be seen within the consumption of one full gas tank, additives added.




Most modern engines have knock sensors installed on them.

The purpose of this sensor is to monitor the engine and send electric impulses to the ECU if a knocking sound in the engine appears. It registers the knocks as vibrations which are then turned in to electric signals.

Simply put, it warns the engine’s computer through registering the knocking sound that somethings wrong.

The ECU filters and processes this data and then adjusts the ignition signal to prevent the knocking sound from appearing.

Like all sensors, this one also has its service life. If it gets faulty, it may cause a disturbance in the combustion cycle by disturbing the ignition signal.


Good diagnostics and a sensor replacement. If the knock sensor is faulty the diagnostics should read a P0325 code making the problem much easier to find.

Replacement is not that complicated under the circumstance that the sensor is accessible.





A natural result of combustion is the appearance of a carbon deposit and soot on the piston and cylinder.

Although most of today’s fuels have a certain percentage of substances in them to prevent this, the appearance of carbon deposits is unavoidable over time. This goes double for higher mileage engines.

When the carbon deposits build up inside the combustion chamber (mostly on the piston head) the space for compression gets smaller.

This then leads to an uneven and improper combustion cycle which can cause a knocking sound in the engine.


The best thing you can do is prevention. Proper maintenance (like changing filters on time) and making sure to use the right fuel in terms of octane rating and quality is the best you can do.

The second solution is to use fuel additives that help remove or reduce carbon deposits. These additives help burn up the deposits during the combustion cycle and get them out through the exhaust.

If none of these solutions help and the carbon deposits are causing bigger problems (which is likely in higher mileage cars) then the only solution is to dismantle the engine (namely take off the cylinder head) and clean everything up.




At the center of almost every pulley and tensioner is a bearing. As with all bearings, they simply wear out over time mostly due to constant tension and material fatigue.

Once this happens, they can produce sounds among which are grinding, squeaking but a knocking sound also.

Typical pulleys and tensioners that may produce this sound would be from the alternator, water pump, or AC compressor.

Sometimes they produce such a sound that it can be mistaken for a more serious problem like when the piston or crankshaft bearings are faulty.


Complete replacement of the part. The bigger task is to find the exact pulley or tensioner that’s bad.

Most modern engines have lots of them so pinpointing the correct tensioner or pulley may prove to be a daunting task. The old-fashioned listening is a good method. Also taking off the serpentine belt and manually checking the pullies and tensioners is a good option.

Prevention through regular maintenance on this part should ensure that you don’t have problems like these.




This is perhaps the most common reason for a knocking sound in the engine.

Lack of engine oil means loss of oil pressure and improper lubrication. This of course leads to major engine damage and consequently to an engine overhaul or complete replacement of the engine.

Problems like these mostly happen to drivers with older, high mileage cars that burn more or less engine oil.

Simply, they may change the engine oil but are not aware of the fact that some oil has to be added from time to time, all depending on the engine condition.

Another common cause may be oil leaks due to things like faulty gaskets, cracked oil lines, a damaged oil sump, or else.


Check the engine oil regularly. If you don’t know-how, there’s a separate article on that topic which you can read by clicking here.

By doing this you’ll know on time if the oil level is low and if you have to add some. You’ll also know if the engine is burning oil so you can pay more attention to adding some on time.

Also, a regular check around the car is good for spotting oil leaks.

Remember that even if your engine is in the poorest condition, if it has the proper oil level it will probably continue to run.

Never wait for the oil warning light to come up on the dashboard as then it’s usually too late.




Without proper oil flow, the engine is doomed, for sure.

The main role in a proper engine oil flow is played by the oil pump. It drives oil through various parts of the engine ensuring proper lubrication and oil pressure.

Also, good clearance of all oil canals in the engine block is very important. Clogs caused by gunk are a common problem, especially in high-mileage cars.

Either of these may cause a knocking sound in the engine due to improper lubrication or oil pressure.

Oil pumps usually have a long service life but at some point have to be replaced.

Clogs are usually caused by not regularly changing the oil and oil filter. This leads to forming deposits and gunk within the engine block preventing the oil from reaching all parts of the engine.

For instance, a knocking sound in the engine may appear if the oil cannot reach the cylinder head and properly lubricate it. Then a combination of knocking and rattling may appear.


To prevent clogs, use quality engine oil, and change it on time. Besides that, change the oil filter regularly. If possible, every time the oil is changed.

Regarding the oil pump, pay attention to the regular service interval and there shouldn’t be any problems.




For the end of this list, a cause that’s the most severe and expensive to fix.

Once the engine is started and the air-fuel mixture detonates, the piston is pushed down and the crankshaft is turned producing engine power for the vehicle.

The rod has bearings on the lower side making the rotation of the piston smoother and easier.

A similar case is with the crankshaft bearings. These allow the crankshaft to rotate smoothly within the engine block.

Over time, especially with high mileage engines, these bearings may wear out creating a small clearance between the piston rod and crankshaft or the crankshaft and engine block. Once it appears, it can cause knocking sounds in the engine.

This sound is usually accompanied by weak engine performance, engine overheating, and sometimes burning engine oil.


Partial or complete engine rebuild. Replacing these bearings means opening up the entire engine. Usually, when this is done, other parts are also replaced to achieve the quality and longevity of the repair.

A huge, timely, and very expensive repair. Sometimes, depending on the engine type and construction, replacing the complete engine is a better solution.

Once more, a worst-case scenario when mentioning knocking sounds in the engine.

For more information on engine rebuilds, click here.



A knocking sound in the engine does not have to mean Armageddon for your car.

First, inspect everything thoroughly and make a good diagnosis of the problem. Otherwise, you may end up changing a lot of parts for a lot of money only for the sound to re-appear.

What’s also important is to react on time. the moment you hear the sound.

If it happens during driving, stop the car as soon as possible and check it out. For instance, in case the oil level is low, the reaction on time may mean the difference between some simple adding of oil and an expensive engine overhaul.

If you can’t solve the problem yourself, be sure to visit a good and trustworthy mechanic as soon as possible.

Also, have in mind that other parts located in the engine bay can mimic a knocking sound. Loose plastic panels and coverings, various loose metal holders, and else can confuse. Especially when the engine is running and vibrations are present.

Above all of this, the best advice is to do regular and quality maintenance on your car. If you do, there’s a good chance that you’ll never encounter a knocking sound coming from the engine.


Written by: Sibin Spasojevic


Former car technician, life-long car and DIY enthusiast, author for