Rod Knock is What? The Causes and Symptoms of Rod Knock (& the Repair Cost).

You might hear rod knock, which is an engine noise that occurs when the car idles. You might hear a banging sound, which can be enough to cause panic and make you think about expensive engine repairs. Not all engine knocks can be referred to as rod knocks.

So what exactly is rod knock? What causes it, and what are the signs of an engine problem?

What's Rod Knock?

The engine's rod knock sounds like a heavy rapping noise. This is usually caused by damage or wear to rod bearings. The vehicle’s connecting rod bearings have excessive clearance, causing additional movement. When the piston is moved in one direction or another, metal will start hitting metal which causes the noise of knocking. 

As the speed and load increase, this noise will also rise. 

Is Rod Knock a sound you hear?

If you have ever heard rod knock, it is easy to notice. You will hear loud bangs from the engine as you rev your engine up and release the gas. It's most noticeable right after the engine has been turned off.

This video shows how it sounds.

Rod Knock: What Causes It?

Wearing connection rod bearings is the leading cause of rod-knock. Although rod knock can be caused only by one cause, there are many other possible causes that may lead to similar symptoms to rod knock.

These are the possible causes of rod knock, or similar symptoms to rod knock.

1. Worn Bearings

Wearing bearings are the only reason for rod knock. The pistons in your car's engine move upward and downward, rotating the crankshaft. This rotates the crankshaft which sends power to the wheels. Bearings ensure smooth piston movement.

These bearings can wear down over time, and they may be displaced. Wearing bearings causes piston rods to rattling against crankshafts, producing a distinct knocking sound. 

It is possible to fix this by changing the bearings which are found deep in the engine. 

2. Low Octane

Detonation sounds similar to rod knocks. When the engine is running as normal, it means that the mixture of fuel and air has only one detonation in each cylinder. Detonation knock is when the mixture explodes in more than one location at once, resulting in a loud knocking sound.

Octane too low to the engine is one reason for this knock. A performance engine will require a higher octane rating that most cars. A high octane rating burns evenly, which prevents knocking. 

It is easy to fix this problem by simply using a higher-octane gasoline the next time you fill your car.

Related: 6 Reasons Your Car's Engine is Knocking Or Pinging

3. Poor Timing

Bad timing can also cause a detonation sound. Timing refers to when spark plugs start firing. Computers control this timing.

When the timing is off, the spark won’t fire when it should, leading to multiple detonations within the cylinders. That’s what causes the detonation knock. This must be resolved by adjusting the timing. 

4. Fuel/Lean Air Mixture

A lean fuel/air mixture is another reason that can cause detonation knock. Defective oxygen sensors, malfunctioning fuel injectors or a defective mass airflow sensor can all cause this problem. 

The lean mixture occurs when there isn’t enough fuel and too much air. Without the right amount of fuel, the mixture can’t burn fast enough, which causes multiple detonations. 

5. Bad Knock Sensor

Also, a defective knock sensor can cause detonation. Thankfully, this doesn’t occur too often on newer cars because the air/fuel ratio, timing and fuel injectors are controlled by the computer. 

Any situation that could cause the knock sound is detected by the knock sensor and the ECU immediately alerts it. Corrective actions can then be taken. So, if the car has a bad knock sensor, the computer won’t know to correct issues and might allow the engine to knock. 

6. Bad Belt Tensioners/Pulleys

The final reason that might you hear an engine knock is because of something that isn’t coming from the engine at all. When the accessory belt doesn’t have the right amount of tension, it can create a similar noise.

The engine's rotation causes the belt to turn. It’s connected to numerous pulleys in the engine bay and must be pulled just right to work quietly and smoothly. If the belt gets loose, it means the tensioner isn’t working right. If one of the pulleys is damaged, it can cause problems. 

The condition can cause clicking, clicking and rattling sounds that may be confused for an engine knock. You can fix it by replacing your belt, tensioner, pulley, etc. 

Rod Knock: Symptoms

If the vehicle truly has a worn-out rod bearing and it isn’t knocking because of one of the other reasons, there will be two common symptoms.

1. The Knocking Sound

A rod knock sounds like a banging sound. When you start your car for the first time, this will be a loud banging sound.

The acceleration will increase if you push down the accelerator or load your car with more. 

2. Low oil pressure

A bearing that has been damaged or is about to be replaced may cause oil pressure to drop below normal. It’s most evident when the vehicle first starts up. 

Your dash might also have a Check Engine Oil indicator that lets you know the level of pressure. The light will go out and the pressure goes back to normal within a couple of minutes. This is an indication that your bearings have failed. 

Rod Knock Repair Cost

Repairs for rod knocks will run $2,500 and up. For some vehicles like the Subaru Forester and other models, it can cost as much as $5,000 to repair the connecting rod using parts and labor. 

The task of replacing a rod bearing isn't easy. It is important to locate the problem component deep inside the engine. Not only will you need to replace the bearings on your connecting rod, but also your cylinder head bolts as well as new gaskets and seals. The engine, cooler lines and pistons all must be flushed. Sometimes, it is necessary to replace connecting rods or pistons as well as timing chains and crankshaft bearings. It might make more sense to replace your engine with all the extra components.