What does a Rod Knock Mean? | What are the causes of Rod Knock?

A rod knock is a potentially catastrophic engine problem that occurs when one of the connecting rods which connect the piston head to the crankshaft — creates metal-on-metal contact. Rod knock is something you never want to hear coming from your engine. This article mainly explains the causes and symptoms of a rod knock and the process to fix it.

What is Rod Knock?

Rod knock means a noise or sound of one or more connecting rods hitting or rubbing against the crankshaft as it rotates in the cylinder bores.

The connecting rods are connected to pistons and crankshaft. There is a bushing (mainly known as bearing) between the connecting rod and the crankshaft which is made up of soft metal than the crankshaft and the con rod.

Rod Knock

The vehicle’s connecting rod bearings have excessive clearance, causing additional movement. As the piston changes direction, the metal starts hitting metal, which produces the knocking noise. 

Rod knock typically manifests as a low-pitched knocking sound that fluctuates depending on your vehicle’s speed. As you accelerate, the knocking sound becomes faster and louder. As you decelerate, on the other hand, it becomes slower and quieter. While some drivers simply assume a bad serpentine is causing the sound, this typically isn’t the case.

Symptoms of Rod Knock

Following are the symptoms of the rod knock:

Symptoms of rod knock

1) Knocking Sounds

One of the major symptoms of a rod knock is a knocking sound. You will likely hear this loud banging when you first start the car.

It will also increase as you up the load on your vehicle or push the accelerator down. 

2) Low Oil Pressure

When engine components like crankshaft, camshaft, and bearings start wearing out, the engine oil pressure will reduce. This is more noticeable when you first start up the car.

In some cases, it will trigger the check engine light and the oil light to display on your instrument cluster, indicating there’s an issue with the oil pressure. If these lights go off after a few minutes of idling or running the car and pressure returns, it indicates a rod knock.

Causes of Rod Knock

Rod knocks are usually caused by wearing or damage to the engine’s internal parts, such as connecting rod bearings.

The rod knock occurs due to one of the below-given causes:

  1. Low engine oil
  2. Bad belt tensioners or pulleys
  3. Bad knock sensor
  4. Lean air-fuel mixture
  5. Poor timing
  6. Low Octane

1) Low Engine Oil

It is generally caused by an irregular oil change and a low oil level. As the motor oil flows in the engine, over time, it loses its density, viscosity rating, and lubricating properties. All these lead to engine sludge formation and internal corrosion.

Low Engine Oil

Driving your car with a low oil level will result in low oil pressure and not circulate well where it should. This could result in excessive engine wear and damage. 

Read More: How to change engine oil?

2) Bad Belt Tensioners/Pulleys

Bad belt tensioners or pulleys may also lead to rod knock. When the accessory belt doesn’t have the right amount of tension, it can create a similar noise.

Bad Belt Tensioner

As the engine rotates, the belt turns. 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. However, you can also have problems if one of the pulleys gets bent. 

This condition leads to rattling, slapping, and clicking noises that could be mistaken as an engine knock. To fix this, you need to replace the belt, tensioner, or pulley.

Read More: Different types of Engines

3) Bad Knock Sensor

Another reason for engine knock is a malfunctioning knock sensor. This doesn’t occur too often on newer cars because the computer control unit controls the air/fuel ratio, timing, and fuel injectors.

Knock Sensor

The knock sensor detects any situation leading to the noise and alerts the ECU, where corrective action can be taken right away. So, if the car has a bad knock sensor, the computer won’t know to correct issues and might allow the engine to knock. 

4) Lean Air/Fuel Mixture

Another reason for detonation knock is a lean air-fuel mixture. Defective oxygen sensors can cause this problem, failing fuel injectors, a bad fuel pump, or malfunctioning mass airflow sensor.

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) Poor Timing

Bad engine timing is one of the most common causes of rod knock. The timing refers to the point when the spark plugs fire. This timing is controlled by the ECU.

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. To resolve this, the timing must be fixed. 

6) Low Octane

A detonation knock can sound similar to a rod knock. If the engine runs as it should, the air and fuel mixture is burning in a single detonation within each cylinder. However, the detonation knock occurs when that mixture detonates in more than one place at a time, leading to a knocking noise.

One such cause of this knock is too low an octane for the engine. If you have a performance engine, you need a higher-octane rating than most cars. High octane burns uniformly, thereby preventing that knock.

This problem is an easy fix, just requiring you to use a higher octane fuel the next time you fill the car.

How to diagnose rod knock

Follow the below-given steps to diagnose the rod knock:

Check the state of the oil and oil level
  • Locate your engine oil dipstick and pull it out.
  • Clean it with a rag
  • Check the color of the oil. If the color is muddy or dark, you have dirt engine oil.
  • Reinstall the dipstick and pull it out to re-inspect it.
  • Find out the state of your engine oil and level. Dirty engine oil can cause system wear, damage, and engine knocking.
Drain the engine oil and inspect it

Get an oil catch pan close to avoid messing up the entire garage with oil. Before draining it, look at your engine cap to know the oil viscosity your engine uses. This is important so you don’t replace the old oil with the wrong oil type.

Before draining the old oil, get a transparent container to take one cup of oil so you can take a closer look. After removing the sample for close inspection, drain the rest of the oil.

If you see metal shavings in the engine oil, it is an indication of engine components’ wear. The metal shavings could come from con rod, crank bearing, or other inner engine components, causing the knocking sound.

If you fail to carry out regularly scheduled oil changes, the oil will get old and start to erode the brass layers on the bearing. This happens mainly on the bearings far from the oil pump because they are getting low oil flow than the bearings closer to the pump.

You should also check the color of the oil. If it is dark brown, the oil is old or dirty and needs to be replaced. And if the oil is too light, consider replacing it. If you want to know how to extend the life of an engine with rod knock – it’s pretty simple. Always change oil at the right interval.

How to fix a rod knock

A rod knock is a sign that a problem lies in the interior components of an engine.

Follow the below-given steps to fix a rod knock

Step 1) Carry out Oil Change service

Slide under your vehicle and place a catch pan directly under the draining plug. Get the sizeable socket size and wrench. Locate the draining plug on the crankcase and loosen it to drain the oil.

Drain the oil completely and reinsert the plug (the draining plug is also called a sump bolt)—Torque the sump bolt to the manufacturer’s spec.

Remove the oil filter with an oil filter wrench by gripping the oil filter and turning it clockwise. Fix in a new filter and torque down to recommended specifications. Replace the engine oil with the same oil viscosity rating or recommended oil.

Keep checking the oil as you pour in the new oil if you don’t know how many quarts your engine takes. A four-cylinder engine typically takes around 4-5 quarts.

Step 2) Remove the crankcase and inspect the cod rod bearing

A loosen connecting rod cap will cause knocking noise. Inspect the bolts holding the con rod with the lid and re-torque them. Consider inspecting the cod rod bearing and replacing them as needed.

Step 3) Pour fuel-injection cleaner into your gas tank

Grimes and dirt might make their way into your gas tank when you refill your tank at the gas station. When the fuel pump is transporting gas to the engine, it sends that dirt and grimes along, causing engine knocking.

Cleaning your fuel injection system will clear out those grimes and resolve the rod knock. Do not use a fuel injection cleaner if you have older vehicles that use carburetors. Instead, get a carburetor cleaner and spray it in the carburetor.

Rod Knock Repair Cost

The repair cost of the rod knock varies according to the repair type and type of the car. The average repair cost of the rod knock is from $1900 to $3100. Usually, the job consists of replacing the seals, gaskets, connecting rod bearings, cylinder head bolts, and flushing out the engine and cooler lines.

FAQ Section

What is Rock Knock?

Rod knock is the sound of one or more of your rods “knocking” against the crank as it changes direction through its rotation. 

What is spun bearing?

Spun bearing refers to the outcome of unfortunate circumstances in the engine cylinder bores. Spun bearing results when there is a lack of lubrication in the engine, and the crankshaft rotates with the bearing instead of turning inside the bearing.

This is usually caused by connecting rods or nuts stretching due to improper bolt torturing during an engine rebuild. Several other factors, such as lack of lubrication, excessive heat, and high operating loads, can cause the bearing to wear. There are several other factors that can cause a spun bearing.

Is it worth fixing rod knock?

Rod knock is a serious issue with your engine—it means the engine is not functioning properly. If you start to notice rod knock, this isn’t a problem that will just resolve itself. Therefore, you must take immediate action and replace the rod bearing while also fixing any other parts associated with the sound.

Is rod knock serious?

Rod knocking is a problem with the internal engine moving parts. Once it occurs, you’ll hear knocking or banging sounds, have low oil pressure, and check the engine light in your instrument cluster. If you fail to track down the cause and fix it in time, it will result in catastrophic damage to the engine.

Can I Drive a Car with a Rod Knock Sound?

If a rod knock sound is coming from your engine, you should not be driving the vehicle. If you continue to drive the vehicle with severe rod knock, this can result in complete engine failure while you are driving.

Can an oil change fix rod knock?

If your oil pressure is low or you have a low oil level, you’ll likely hear a splattering, tickling, or ticking noise from the engine. If this noise is from the valves, or lifters, adding or doing an oil change will stop the noise. But unfortunately, if the noise is rod knock, an oil change will not stop it.

What are the signs of Rod Knock?

As your car has rod knock issues, it gives one of the below-given signs:

  1. Knocking Sounds
  2. Low Oil Pressure

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