What is Rod Knock & What Does it Sound Like?

In this article we will be looking at a very distinct sound and issue that you really want to get fixed quickly. This new sound may indicate an issue known as rod knock. The name may produce a giggle but this is no laughing matter as you will see if you read on.

What Does Rod Knock Sound Like?

We will start out by describing the sound you should be listening for if you suspect rod knock. What you are looking to hear are loud bangs coming from your engine when you rev it and then let off the gas. It may specifically happen directly after you let off the gas.

What Is Rod Knock?

So what exactly is rod knock? Well it is a deep rapping sound that emanates from within your engine. It is generally caused by rod bearings becoming worn or damaged. This might create excessive clearance for the connecting rod bearings which allows more movement than normal.

The noise is created when the pistons change direction and the overly mobile connecting rods end up hitting the internal surface of the engine. It is the sound of metal on metal impacts, creating what sounds like a knocking noise from deep in the engine. It will worsen the harder you rev your engine.

What Can Cause the Rod Knock Sound?

It should be noted that not all knocking noises from the engine are rod knocks so in this section we will look a little deeper at some of the possible causes for an internal engine knocking sound. If you are lucky the issue will not be rod knock but an easier to resolve issue so read on.

Worn Bearings

If the sound is rod knock then the cause can only be worn bearings, there is no other cause. The pistons move up and down in the engine rotating the crankshaft as they do so. This process transfers the engine power to the car's wheels and creates the forward momentum.

The bearings help the piston movement stay contained, smooth and controlled but as they wear out they can move out of position. This will affect the pistons as they are now no longer being restricted. They will start to rattle against the crankshaft creating the knocking sound.

Low Octane Fuel

When is a rod knock not a rod knock? Possibly when it’s a detonation knock. The sound of a detonation knock resembles a rod knock so obviously this can be alarming.

The engine runs at its best when the fuel to air mixture is well balanced producing a single detonation with each engine cylinder to a preset timing. If the mixture is off it is possible that detonation can occur out of order and possible simultaneously in two cylinders at once. This will create a knocking noise in the engine.

This issue can be caused if your fuel has too low an octane level. There are several reasons this can happen from spoiled gasoline to using the wrong type of fuel. If for example you have a high performance car but use basic gasoline you might get a detonation knock.

If you do not drive your car for an extended period the gas in the tank may also degrade and lose some of its potency. The result would be the same, an octane level too low to run your engine efficiently. If octane is your issue, getting fresh fuel and the right type might stop the knocking noise.

Poor Timing

As mentioned, not only does the fuel to air ratio have to be right for the engine but the cylinders have to combust in the right order and at the right timing. This can also cause detonation knocks and it is caused because the spark plugs are not firing in the right sequence.

When the timing is off a spark plug may not do its job leaving fuel and air in a cylinder that may ignite when the next closest cylinder fires correctly making them happen at the same time. The result will be a detonation knock.

You will have to diagnose the cause of the timing issue which may be a work spark plug or an issue with the timing belt. Once fixed the timing will be back to normal and the knocking should cease.

Belt Tensioners/Pulleys

From inside the car's cabin it is hard to differentiate a knock from within the engine from a noise created outside it elsewhere under the hood. One such cause could be damaged tensioners and pulleys that are used to keep the belts tight.

The accessory belt for example needs the right amount of tension but if the tensioners or pulleys cause it to loosen you may hear a knocking noise. It is actually a slapping, rattling or clicking sound but it can sound like a knock as you drive along.

When the belt has the correct tension it will move smoothly and quietly so if your belts are loose it may be a tensioner or pulley issue. You would have to replace the offending part which might be the belt itself if it has become worn or stretched out.

Bad Knock Sensor

There is a part in the engine known as the knock sensor and its job is to listen for knocking sounds in the engine. When it detects such a sound it alerts the car's electronic control unit (ECU) which will attempt corrective action to stop the sound. This might be altering fuel mixes or some similar alteration.

If the knock sensor does not report a knocking sound then it may have gone bad and need replacing. Without the input from this sensor the ECU does not know to fix the knocking sound so it will persist and may lead to engine damage.

Issues with Fuel Mixture

We have mentioned the fuel mixture already as a possible cause of engine knock but not specifically the reasons the mix may be off. The knock happens with a lean fuel mixture meaning there is too little fuel in the chambers.

The reasons that there may not be enough fuel could be related to a defective O2 sensor, bad fuel injectors, broken fuel pump or an issue with the mass airflow (MAF) sensor. This means that it can be one of several issues but once you fix the issue the knock should cease.

Are There Other Symptoms to Rod Knock?

So far you are probably thinking that all you have to go on when diagnosing actual rod knock is the sound itself. This is obviously worrying because as we have pointed out several other things can cause a similar sound.

The problem we face is the issue causing rod knock, is happening deep in the engine so we can’t see the parts that may be worn without opening it up. There is however one other indication of rod knock that is worth noting.

Aside from the knocking sound that we have already described you would also see low oil pressure. It is most noticeable when you first start the engine and it may even give you a check engine oil light. If the light stays on for a few minutes but then turns off this can be an indication that the knocking sound is most likely rod knock.

How Much Does Rod Knock Cost to Fix?

We’ll start out by saying that the other causes of an engine knocking sound are going to be cheaper to resolve than rod knock. So you will want to explore all possibilities just to make sure that you have the right problem.

Anything relating to the piston rods is going to be expensive simply because of the labor involved in even accessing these parts so deep in your engine. Roughly speaking you will get no change back from spending $2500 if the issue is rod knock and you are likely to pay way above that.

The prices can vary based on the type of car you have and the extent of the damage. The longer you ignore rod knock the higher your repair bill will be. It may even reach a point where the damage is so bad buying a new engine may be your only option. As this is very expensive you might even just scrap the car and get a new one.

Can You Drive With Rod Knock?

Knocking in your engine bay can be a sign of a number of issues including rod knock and almost all of them are serious if not dealt with quickly. The engine may run and the car might keep going but you are living as the saying goes on borrowed time.

If you get a knocking sound in your engine you should start looking for the cause straight away. If you are lucky perhaps it was just cheap gas and you can use an octane booster to fix the problem. If something is wrong with the engine you must fix this.

Over time poor ignitions in the cylinders can cause damage and if piston bearings go bad serious damage can happen inside your engine. The moral of the story is to make your next drive to a mechanic to get the issue resolved.


Rod knock is a major issue in your engine that must be fixed quickly. There are other things that can mimic this fault which are less ominous but if you truly suspect rod knock you should not delay in acting upon the issue.

Bad Piston bearings will only get worse and if the pistons are shaking loose you can be on your way to a catastrophic engine failure. It won’t be a cheap fix and you may even opt to get a new car rather than throwing money at an already old vehicle.

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