Rod Knock Symptoms – How To Diagnose A Rod Knock?

Rod Knock Symptoms – How To Diagnose A Rod Knock?

August 19, 2021 0 By CarGuy

So, have you been experiencing some strange noises like something is knocking under the hood of your engine lately? Knocking sounds can be alarming, to be honest, they are a dead giveaway of a rod knock. Even though this does not mean that the engine is knocking, but it is a good idea to learn the rod knock symptoms.

The rod knock is something that no driver wants in his life. If you have a rod knock it means that your engine is nearing the end of its lifetime and it’s time to either retire the engine or repair it.

The knocking sounds are created by the loose rods that are rubbing with the camshaft. This is the case because the rod bearings are gone on some of your pistons. And this is not good.

The best thing to do in this situation is to learn the rod knock symptoms that we are going to share with you. And see if some of these symptoms resemble your problem.

In this article, we are going to learn what is a rod knock and what causes the rods to knock. Then we are going to cover the main rod knock symptoms that you will experience when you are having a rod knock. And then we are going to give some hints on what should be your next step in this process. And finally, we are going to cover the costs of this problem. So, let’s begin with the article.

What Is A Rod Knock?

As its name implies, a rod knock is when the rods in the engine are starting to knocking into the crankshaft.

This rod knock is a consequence of worn-out rod bearings. These rod bearings are known to suffer a lot in engines that were poorly maintained.

Basically, the material from with the bearings are made starts to wear and the rod bearing surface is diminishing as time progress.

When the rod bearings reach the final stadium, they tend to lose their balance and cause rod knock symptoms. These rod knock symptoms are characteristic of this problem and we are going to cover them later.

Other damage that can appear from this problem are the scarred cylinder walls and the damaged crankshaft. In the worst-case scenario, this problem can develop some permanent damage to the engine and the engine will not be repairable or rebuildable.

The best thing when you are facing a rod knock issue is to learn the rod knock symptoms and diagnose the problem. The second thing to do is to stop driving your car. If you keep driving the car, it means that the engine damage will be getting worse and worse. The more the engine runs like this the more damage to it is made.

In the end, if this problem is not resolved is going to result in catastrophic engine failure. Either one of the cylinders will blow the engine block or the engine will simply seize and stop working.

What Causes The Rod Knock To Appear?

This problem usually starts to appear on higher mileage vehicles. Especially vehicles that were poorly maintained by the owners.

By poor maintenance, I mean less frequent oil changes. For example, if you are changing the oil every 15,000+ thousand miles you are risking a rod knock symptoms start to appear.

These symptoms at the start are going to be mild but as time progress. The symptoms are going to be heavier and heavier.

The rod knock will soon gonna start to damage the oil rings on the cylinder, also the cylinder walls, and the crankshaft.

These components are crucial in your combustion process and without them properly working your engine will not going to have compression.

No compression means no engine movement or poor engine work. You can’t expect much from this engine and it has to be repaired if possible before the problem becomes bigger and scrap the whole engine. So, in order to be sure that you have a rod knock issue you have to learn all the rod knock symptoms that we are going to discuss next.

Rod Knock Symptoms

Like with every other engine component similarly the bearings when they go bad, they develop rod knock symptoms. Some of these symptoms are a dead giveaway, while some are more silent and the issue has to be further investigated.

The important thing when you notice a rod knock symptom is to not wait. Instantly after you notice a symptom you have to diagnose the issue. Diagnosing will going to save you a lot of money in the long run and that’s something that we are going to cover in the next chapters. But first, let’s discuss the rod knock symptoms.

  1. Rod Knock Sound
  2. Poor Engine Work
  3. Loss Of Compression
  4. Oil Consumption
  5. Carbon Buildup In Engine
  6. Bent Rods

1. Rod Knock Sound

The rod knock sound is very characteristic of this problem. If a rod is knocking, you are going to hear a knocking sound coming from the bottom of the engine.

This sound appears because the bearings that are between the piston rod and the crankshaft have lost their ability to keep them separated.

This causes friction between the crankshaft and the rod, and when there is friction the rod knock sound appears. And this symptom is one of the most distinguishable from all the rod knock symptoms.

If there is a knock coming from the bottom of the engine, you probably have a rod knock problem.

2. Poor Engine Work

If you are having rod knock you are probably experiencing poor engine work as well. This poor work is caused by the loss of balance inside of the engine and the damaged components.

The O-rings probably don’t seal very well as well as the pistons which are rubbing the cylinder walls.

This power loss is more notable as time progress and more and more issues start to develop. If it’s one cylinder that is affected you are probably going to not have a big problem with the performance of the engine if it’s a V8. But if you are having a smaller displacement engine. This loss of performance is going to be more and more notable and you are probably going to be frustrated because of this.

That’s why these problems require addressing on time before the engine develops some permanent damage. It is important to recognize the rod knock symptoms and act as quickly as possible.

This work is going to be labor intensive as well as expensive. But if you want to keep your car on the road it is necessary to perform. We are going to cover later what should be your next steps in this process in detail a bit later in the article.

3. Loss Of Compression

One of the rod knock symptoms is loss of compression. The loss of compression is caused by the O-rings that are not sealing properly.

The O-rings or also know as oil rings are rings mounted on the pistons. These rings are making sure that the engine has compression. Remember that the compression makes the engine turn. And if there is a lack of it, the engine will eventually stop working.

Compression can be measured with a compression tool. This tool is very simple to use and it gives you the proper readings. We wrote a whole article on loss of compression and how to perform DIY compression test. So, if you think that your engine suffers from a lack of compression this article is worth checking.

An engine that has low compression has to be opened and the issue needs to be investigated. It could be something simple like a bent valve or something bigger like a rod knock. That’s why the engine has to be disassembled and inspected.

4. Oil Consumption

If the O-rings fail, there will be oil consumption as well. The O-rings are basically holding the oil separated from the combustion process.

So, if the rings fail, there will be oil spillage inside of the combustion process. This oil spillage will increase the consumption of oil in the motor. More oil burnt means less oil in the oil pan to lube the engine.

This will result in even more engine damage. Not to mention the other consequences to the environment. An oil burning engine will cause a lot of emissions and black smoke from the exhaust.

If your engine consumes oil and you have rod knock symptoms, you have to address this problem as soon as possible in order to prevent more damage to it.

The engine has to be opened and disassembled in pieces and then further inspected. The O-rings have to be replaced and the camshaft and the cylinder bores have to be honed in order to return it to factory spec. This is a labor-intensive process, but it’s worth it. In the end, you are going to have a perfectly running engine that runs great and doesn’t consume oil.

5. Carbon Buildup In Engine

When the engine is consuming a lot of oil it is normal to have a lot of carbon buildup in the engine as well. This carbon buildup is mostly located on top of the pistons and in the valves. More precisely on the exhaust valve and the exhaust port.

This carbon buildup when it becomes excessive it becomes very difficult for the engine to work properly. It can cause the engine to misfire and lose compression.

These symptoms are very pronounced in engines that have not been well cared for during their life. This oil consumption will basically ruin the heads and everything has to be machined and resurfaced.

In the long run, this can cost you a lot of money. If the cost to repair is more than the cost for a good used engine then the best option is to go for a used engine and make sure that you find one that had been taken care of.

6. Bent Rods

Bent rods are also one of the rod knock symptoms. The bent rods usually occur when there is lost balance on the piston, caused by the worn bearings.

The piston starts to lose its balance and moves in different directions. Making damage to the rod and also on the O-rings and the cylinder wall.

This unnatural movement can cause the rod to bend and even more to ruing the balance in the engine. If the bent rod cracks and destroys itself it’s going to cause catastrophic engine damage and your engine will be irreparable.

Bent rods can be caused as well by other culprits. Namely high loads on the engine. This is because the rods are the parts that get the biggest load inside of the engine. And when they are overloaded, they tend to bend or break.

In order to prevent this issue, you should not overload your engine. If the engine was not built for racing and does not have racing rods it is very likely that some of your rods will like to escape from the engine.

In this case, you are going to need a completely new engine and a ton of money spent on this engine swap.

How To Diagnose A Rod Knock?

An experienced mechanic can diagnose a rod knock only by hearing the engine. The rod knock symptoms are a dead giveaway. He probably heard thousands of engines that produce this sound.

If you are a beginner, you can look online for some videos that show how rod knock sound and they can help you immensely.

Another way to diagnose a rod knock if you are 99% sure that your engine is knocking is to open the engine. Tear the engine apart on a table and inspect the parts.

This way you are going to have a clear idea of what should be your next steps. When you know what components are damaged it is easier to replace them later.

For this specific problem, you should look at the cylinder or cylinders that are affected. Inspect the pistons and see if they have any scratches and damage. Also, see if the o rings are in place and see if there is damage to the cylinder wall.

Also, inspect the crankshaft and see if there are any scratches as well. Not to forget the bearings which are the most important piece of the puzzle. Usually, the bearings will tell you the whole story of your engine and its maintenance.

If oil changes were frequent, these bearings should be in mint condition. But if the oil changes were rare, you may find something that doesn’t even look like a rod bearing.

What Should Be Your Next Step?

If you have learned the rod knock symptoms and you have diagnosed a rod knock on your vehicle the next step you should make should involve finding a solution for your problem.

This solution will probably gonna be finding a good mechanic that has specialized in rebuilding engines. And all that in case if the engine is savable at all.

If the knock is too loud and the mechanic tells you that it’s better to do an engine swap. Maybe that is the route to go. A good used engine is a hundred times better than a rebuilt engine. With a rebuilt engine you never know how long it’s going to last because nothing is good as the way it came from the factory.

Rod Knock Cost To Repair

To be honest, this job ain’t cheap. If you have a rod knock you are going to probably pay more than $1000. And that is only if you replace the bearings and the affected piston.

If you have to do the whole engine the price can go even higher than that. $1,500 to $3,000 is probably going to cost to rebuild the engine.

That’s why replacing it makes sense. If the engine does not have any sentimental value, the best option is to replace it altogether. With this route, you can be sure that your engine will be in good working order and is going to last for a long time.

How To Prevent A Rod Knock To Appear In The First Place?

How you can prevent rod knock symptoms to start to appear is pretty straightforward. Regular maintenance. Make sure that you change the oil every 5,000 to 7,000 miles and you can be sure that your engine is going to last long. Also, use good quality oil, a good quality oil is a guarantee for success.


In this article, we have learned everything when it comes to rod knock and rod knock symptoms. We learned what is a rod knock and what are the common rod knock symptoms. They were the knocking sound, the low compression, oil consumption.

Then we learned what should be your next steps. The next step after diagnosing will probably gonna be tearing the engine apart and see the damaged parts. If the components are too damaged then considering a new engine will be a good idea. Sometimes it is a lot simpler and cheaper to get a new engine than repair an old beat-up engine that is no good anymore.

If you want to read more about engine related issues you can check our articles on metal shavings in oil and white sludge on oil cap. Or if you want to read something about electrical problems you can check our articles on spark plug fouling and bad starter symptoms.