Rod Knock: What It Is, What Causes It, and How to Fix It

Rod knock is a much-dreaded problem among car owners. One day, you’re driving around perfectly content with your car, and the next, you hear a banging noise coming from your engine. Immediately, you start to worry, thinking about why this is happening and how much it’s going to cost you.

Rod knock occurs when one or more rods get loose and start hitting other surfaces. This happens because of worn bearings. Drivers typically hear a clunking noise coming from the engine, especially when driving fast. The rods have to be replaced, and in some cases, you’ll have to get a new engine.

In this article, I’ll go over what rod knock is, why it happens in the first place, and what you can do to attenuate and fix the problem. Hopefully, this guide will save you from a headache and some shockingly high repair expenses.

What Is Rod Knock?

Rod knock is a noise that occurs when a bearing gets worn, causing a rod to hit the crankshaft as it moves. This produces a characteristic banging noise coming from the engine. There are other engine problems that might cause similar issues.

The connecting rod, or simply the rod, connects a piston to the crank, so the piston can move up and down as the crank turns. There is also a bearing between the crank and the connecting rod, with a tiny gap between the two. The gap is about 0.001 inches (25.4 microns) wide.

This gap is lubricated by a thin layer of oil, which allows the proper movement of the crank and rod. When everything is going well, these components should work smoothly, creating no noise. However, when the bearing gets worn out, the problems start.

As the bearing wears out, the gap between it and the rod increases, which allows the rod to become loose. When it becomes loose, it starts hitting the crank as it moves. This produces the noise that we know as rod knock.

What Causes Rod Knock?

A damaged or worn-out bearing causes rod knock. However, even though this is the most common cause, it isn’t the only one. Other possible causes include poor-quality oil and excessive engine speed. Both lead to damage to the bearings, causing rod knock.

Let’s examine these in more detail.

Poor Oil Quality

Using pure, high-quality oil is crucial when it comes to the proper functioning of your engine. Using oil that is unclean and not viscous enough, especially if the oil pressure is low and not enough of it gets to the bearing, is going to cause problems.

This is often the reason why you experience rod knock. Poor oil, or oil in low quantities, won’t lubricate your bearings well enough, and the rods may slowly damage them, increasing the gap and thus creating the dreadful noise.

Using unclean oil that contains particles that shouldn’t be there can also add to the damage and cause further problems. That’s why it’s essential to always use high-quality oil.

Excessive Engine Speed

Excessive engine speed and increased pressure during combustion can also lead to rod knock. The high engine speed will increase the speed of the pistons, which will, in turn, increase the wear and tear of the bearings.

Increased engine speed will also increase the heat produced. Heat can contribute to this problem because it may damage the material the bearings are made of. Of course, this doesn’t happen all the time, as the phenomenon requires excessive heat, but it’s a possibility and can contribute to the issue.

Symptoms of Rod Knock

Rod knock manifests itself as banging and clunking noises. The noises are particularly loud when you release the gas. Apart from that, it leads to low oil pressure, especially when starting up the car.

The most obvious symptom of rod knock is the noise that it produces. This banging is especially audible when you start the car and when you accelerate and then release the gas pedal. At that moment, the engine isn’t burning any gas, so there’s no noise from that, letting you clearly hear the noise produced by the rod.

Another symptom of rod knock is low oil pressure. This problem mostly occurs when you start the car, and the Check Engine Oil light often turns on. If it turns on and then off, it indicates that you’re suffering from rod knock-related problems. You’ll need to get everything checked out before you carry on driving.

What Does Rod Knock Sound Like?

Rod knock sounds like banging and knocking coming from the engine. Accelerating typically causes the noise to get louder. If the noise disappears when the engine heats up, the noise is probably caused by something else.

Is There a Quick Fix for Rod Knock?

There is no quick fix. You can change the oil to temporarily decrease the severity of the noise, and you can also use thicker oil, but there is no permanent quick fix. You’ll have to take your car to the mechanic.

Unfortunately, when the bearings are damaged and worn out, there is practically nothing you can do but replace them if your goal is to get rid of the problem permanently. Simply put, if a big piece of metal is worn out, it’s impossible to make it any less worn out, especially quickly.

This doesn’t mean that it’s impossible to reduce the noise quickly, but this is only going to buy you a bit of time before you go to the mechanic. You can either change your oil or use a thicker variety. Both fixes can reduce the noise, but they won’t eliminate the root cause.

Changing the oil may reduce the noise, especially if the oil you’ve already got is dirty or low-quality. It’s definitely going to help your engine run more smoothly. Using thicker oil can also mask the symptoms of the problem, so you might hear less or none of the noise temporarily, but you’ll have to use increasingly thicker oil to keep masking the problem, and eventually, you won’t be able to mask it any longer.

Therefore, it’s best to go directly to your mechanic as soon as the problem starts. Taking care of the root of the problem will probably save you money in the future because it will prevent worse damage to your engine. Quick fixes can mask the problem, but they might lead to more serious damage in the future.

It’s also important to note that rod knock won’t go away on its own. It will persist until the root of the problem is fixed, so ignoring it and waiting is not an option. In fact, it’s going to make things much worse for you.

How Much Does It Cost To Fix Rod Knock?

Repairing rod knock can be a very tricky process. Not every mechanic knows how to do this, and finding replacement parts for your car might be difficult. Additionally, the engine needs to be taken out and disassembled, which is a labor-intensive process, and, depending on the severity of the damage, you’ll have to replace parts such as:

  • Pistons
  • Gaskets
  • Bearings
  • Connecting rods
  • The whole crankshaft

Therefore, the costs can pile up quite quickly, and the repairs usually range between $2,500 and $5,000, and in some extreme cases, the price can get to $10,000. Therefore, replacing or rebuilding the engine might be cheaper, especially if you can find an affordable but high-quality one. In some cases, people even opt for buying a new car, as it’s much simpler and, in some cases, even cheaper than repairing the broken one.

Connection Rod Lifespan

Since a problem with a connection rod can quickly amount to a massive expense, you might panic and wonder how long one of these components can last so you can start saving up money for the repairs right now. Thankfully, if taken good care of, connection rods can last forever.

You need to change the oil regularly to get the longest possible run out of your connection rods. Of course, it must be high-quality oil compatible with your engine. If you fail to change the engine oil routinely or use the wrong kind, the rods can fail pretty quickly.

Preventing Rod Knock

Apart from changing engine oil regularly, there are a few other tips for preventing rod knock and increasing the lifespan of the connection rods and bearings.

  • Buy gas from trusted suppliers
  • Use high-quality, high-octane fuel
  • Change your spark plugs regularly
  • Use fuel additives to remove and prevent the accumulation of carbon deposits
  • When you hear a banging sound, reduce the throttle

Rod Knock vs. Lifter Tick

A similar problem that you might experience is lifter tick. It has some similarities to rod knock, but it is a distinct problem, happening in a different part of the engine; namely, in the lifters. There are a few characteristics that distinguish it from rod knock:

  • It usually disappears when the engine is warm
  • The noise is twice as slow as rod knock since the crank moves twice as fast as the camshaft
  • The sound is less heavy; it’s not banging, but rather softer ticking

Just because it seems like you’ve avoided rod knock, doesn’t mean that you can take the problem lightly. First, get an oil additive and see if it helps. If the thicking sound persists, get your mechanic to check the engine.

Other Similar Noises

Unfortunately, this is not the only engine problem that causes these types of noises. There are more than a few with similar symptoms that you need to distinguish from rod knock. Let’s see what they are, so you can learn to identify them.

Low-Octane Fuel

If your car has a demanding engine, then it needs higher-octane fuel than most cars to function properly. High-octane fuel burns uniformly, which prevents engine problems, including knocking sound.

The knocking sound you might get is often called detonation knock. In this case, it happens when the mixture of air and fuel detonates at different places at the same time, creating the noise.

High-octane fuel ensures that this problem doesn’t occur because it matches the needs of your engine. High-end performance engines typically require better fuel to work properly, and without it, you might run into all kinds of problems.

Lean Mixture of Air and Fuel

If there’s a lean air/fuel mixture, it means that there is too much air and not enough fuel, which doesn’t allow the latter to burn fast enough and causes multiple detonations. This is another common cause of detonation knock.

This problem may be caused by several factors:

  • Bad fuel injectors
  • Bad oxygen sensors
  • Faulty fuel pump
  • Faulty mass airflow sensor

This is a problem that usually occurs when you start the car, which is how you can know it’s not rod knock. However, it’s not an issue that you can just brush aside. You should get your car checked by a professional because running lean can cause several problems, such as:

  • Stalling
  • Poor engine performance
  • Trouble starting
  • Worn-out spark plugs

Therefore, it still requires a trip to your mechanic if you wish to prevent complications in the future.

Bad Timing

In this context, bad timing refers to when the sparks are supposed to fire. If the timing is not right, the sparks won’t fire when necessary, leading to multiple issues, and again to detonation knock.

This problem can often be fixed by using a higher-octane fuel, especially if you’ve got a heavy-duty engine. If this doesn’t fix the problem, then you might want to seek professional help. There might be something wrong with the computer that controls the timing, and that’s probably not something you want to temper with on your own.

Malfunctioning Knock Sensor

The knock sensor detects problems that might lead to noise and informs the ECU. The computer can then take action against this and fix the problem. However, if there’s something wrong with the sensor itself, it might not detect noises, and thus they won’t be fixed on time, which may produce symptoms similar to rod knock.

Thankfully, this doesn’t happen very often to modern cars, so you’re not very likely to encounter this issue. However, it’s good to know that it may still happen, just in case your engine starts getting noisy and you don’t know what to do.

Problems With Belt Tensioners and Pulleys

Problems with the accessory belt can create noises that are very similar to the ones produced by rod knock. The belt, connected to multiple pulleys, turns as the engine rotates, and it has to be under the right amount of tension so it doesn’t get loose.

If it gets loose, it starts to produce slapping and rattling noises that you might take for rod knock. A belt getting loose usually indicates that the tensioners are not working right or that one of the pulleys is bent. If that’s the case, you’ll need to replace one or more of those components.

Is It Safe To Drive a Car With Rod Knock?

It is safe to drive the car, but it’s a bad idea. Doing so will damage the engine components and lead to more expensive repairs. It’s best to get this problem fixed right away.

Driving a car that’s experiencing rod knock is technically safe. Your car won’t blow up suddenly and you will get to your desired destination without problems. However, just because it’s technically safe, doesn’t mean it is a particularly good idea.

While this won’t total your car, it will lead to additional damage, and additional damage leads to additional repair costs. Since the bearings and rods are probably already worn out and damaged, driving will wear them out even further, potentially causing damage and raking up repair costs.

In extreme cases, the bearings will overheat and the rods will break, which can cause serious issues, including punching holes in the engine block. This will render your car useless and turn it into a heap of scrap metal, which is the last thing you want.

Therefore, if you notice weird noises coming from your engine, take the car to your mechanic as soon as possible. While this can be inconvenient, it may potentially save you a lot of money down the line, so do your best to not waste any time.

Final Thoughts

Rod knock is a serious issue that points to malfunctions in the engine. It usually indicates a problem with connection rods and bearings, which has to be addressed immediately. Ignoring the problem will lead to complications and, eventually, to tremendous expenses.

Rod knock has to be fixed by a professional, as multiple parts of the engine will have to be replaced. In some cases, you may need a whole new engine. There are no quick fixes you can resort to; you can only mask the symptoms, which might make them worse eventually.


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