Why is There a Knocking Sound in My Engine & How Do I Fix It?

One of the more alarming noises that may develop in our cars is a knocking sound from the engine or engine compartment. In this article we are going to take a look at the possible reasons for getting a knocking sound in the engine and how you would go about getting these problems fixed.

What Causes Knocking in the Engine?

There can be a number of causes for a knocking sound coming from the engine some of which are not too alarming. Other causes can be more ominous and potentially lead to an expensive and complicated repair.

In this section we will look at some of the more common causes of engine knocking and also touch upon how you can diagnose and resolve the problem.

Low Octane or Incorrect Fuel

Yes, sudden knocking sounds from your engine can be the result of a simple error at the gas pump. If you develop a knocking sound soon after getting gas this may be the culprit. Under acceleration you may notice a knock on the engine and this could be because of bad fuel or because you may have used the wrong type of fuel.

If your car is supposed to use a higher octane fuel and you get a lower octane version you can cause an issue known as premature combustion. This will create a knocking sound in the engine and is not good for your cylinders.

The gas station may also have bad gasoline or you may not have driven your car for a while allowing the gas sitting in the tank to go bad. If the fuel is the issue you may be able to fix the problem by adding an octane booster to the gas you have.

If the octane booster does not work however you may need to have the tank drained and to refill with good suitable gasoline. This is not the type of situation where you try and make the best of it and just use up what you have. A misfiring engine is not good and it can quickly become damaged if you let it continue.

Spark Plug Issues

The internal combustion engine works by filling cylinders with a measured air and fuel mixture that is then pressurized and ignited using a spark. This spark is created by the spark plugs so any deficiency with these plugs can cause you a problem.

A weak spark for example may fail to ignite the fuel/air mixture leading to what is known as a misfire. A misfire means that a cylinder either did not ignite or did not do so fully. This can cause a knocking sound in your engine as well as vibrations especially while idling.

The causes of problem spark plugs could be using incorrect spark plugs, wearing or fouling. Over time like all parts spark plugs can wear out and need replacing. They can also collect soot or oil on them causing the ignition coil to become useless for creating the spark.

Sometimes we might get the wrong type of spark plugs fitted to the car. This may be because we were looking to save money or it is simply an error. The wrong plugs might not create enough of a spark which as mentioned can lead to misfires.

To avoid this issue make sure you are using a suitable spark plug for your make and model of car. Also check and clean spark plugs frequently. If you find a damaged spark plug then replace it straight away. These are not expensive parts and they are generally easy to replace.

Bad Knock Sensor

Problems with your knock sensor can be a pretty common source of knocking from the engine. This is somewhat ironic as this sensor is supposed to detect engine knock and help prevent it.

The knock sensor essentially works by monitoring for knocking sounds in the engine and adjusting ignition conditions in the cylinders to try and counteract this issue. It will try to adjust the timing of the cylinder firings to compensate for issues such as failing spark plugs or low octane fuel.

If the knock sensor fails it may send incorrect messages to the Electronic Control Module (ECM) which may order a change in engine timing. If there is not really a problem with the timing however this change can actually cause a knocking issue.

To diagnose this you may need to hook up an OBD2 scanner tool and read the trouble codes. If the codes you find correspond to the knock sensor then you will need to replace this part to get your engine back in time and get rid of that knock.

Lean Air/Fuel Mixture

A little known fact is that when fuel enters the combustion cylinders one of its first jobs is to cool it down from the previous ignition event. If you have an air/fuel mixture with not enough fuel this is called a lean mixture and it will not cool the cylinders enough.

The excess heat in the cylinders can make the next ignition event go off prematurely before the spark plug actually creates its spark. This is a misfire and it can create a knocking sound. It can also melt or damage your pistons if you allow this misfire to occur over and over again.

There can be a lot of causes for this problem coming from both the fuel and air intake systems. The fuel system may not be pumping the fuel efficiently or the air system may be allowing too much air into the engine.

You will again probably need to diagnose this with a scanner tool to find out what the trouble codes say. It may be a simple fix but it could be more complicated. Whatever the issue however you should get it fixed quickly to stop these misfires.

Loose or Broken Engine Mounts

This is a problem that is not particularly common but can occur so it is worth noting. The engine is bolted to the frame of your vehicle via several engine mounts. This is not just so it stays in place but also to dampen the movement of the engine itself.

Essentially our engines are trying to control a series of constant mini explosions so if it was not bolted in then it would jump wildly around. If one of the engine mounts breaks you might get engine movement which can cause a knocking sound.

You can usually see the engine mounts clearly so if you look under the hood with the engine running you should easily be able to see if the unit is no longer fully bolted in. Fix, replace or tighten any mounts that are not well connected.

Other Causes for Engine Bay Knocking Sounds

The knocking sound may not actually originate from the engine but instead from somewhere else under the hood. If you get the sound only when you accelerate it is likely engine based but if it is more constant there could be other causes.

These include:

  • Serpentine or timing belt tensioners/pulleys
  • Bad rod bearings
  • Faulty hydraulic lifters
  • Noisy fuel injectors

How To Fix Engine Knock

Add Octane Booster or Change Fuel

If the issue is the result of bad gasoline or fuel with too low octane levels then you have two options to deal with this problem. You can try and add an octane booster to the existing fuel which may improve the ignitions and get rid of the knocking sound.

If a booster does not help you should drain the bad fuel and replace it with something more suitable. Allowing this knocking to continue until it is time to refill your tank could lead to expensive damage so do not take this risk.

Check Your Spark Plugs

If you are getting a knocking sound it may be an idea to pull your spark plugs and inspect them for damage or signs of fouling. You may need to either replace them or clean them to get things back on track.

If you recently changed them before the sound started you may have used the wrong spark plugs so check your owner's manual to make sure the replacements are suitable for your car. If they are not you will need to switch them with better spark plugs.

Check the Trouble Codes

If the issue can't be explained by bad fuel or spark plugs you may need to use a diagnostic scanner tool to read the trouble codes. Any engine faults will be recorded in the car's ECM and using a scanner you can retrieve these codes.

Using your owner's manual you can compare the codes to their specific meaning and quickly discover when your issue might be. Once you know what is wrong you can choose to try and tackle the issue yourself or take it to a mechanic.

Conclusion

Engine knocks can be alarming and we will not tell you not to worry about it as the chances are the issue if not serious now can become so if ignored. Often the knock is related to a misfire in one or more engine cylinders.

This can be caused by a host of issues but should most definitely be fixed quickly as uncontrolled misfires can wreck your engine and cost you a lot of money in repairs further down the line.

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