Blog Car Care Advice Top 8 Reasons For An Engine Knocking Sound (+4 FAQs)
Car Care Advice

Top 8 Reasons For An Engine Knocking Sound (+4 FAQs)

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When the smooth rumble of your car is replaced by repetitive tapping, scraping, or engine knocking sound, there’s most likely trouble brewing under the hood. 

This shouldn’t be ignored as it can lead to a blown engine or other severe problems if not promptly addressed.

But what causes the engine knocking sound?

In this article, we’ll cover a few potential reasons you’re hearing an engine knocking sound and some frequently asked questions about the issue. 

This Article Contains: 

Let’s get into it.

8 Reasons You Hear An Engine Knocking Sound

Engines combine many moving parts, making a cacophony of sound and noise commonly associated with driving a vehicle. If you start hearing an out-of-the-ordinary sound, like knocking, tapping, or scraping, this might indicate a serious problem. 

Let’s take a look at a few reasons you might be hearing an engine knocking sound: 

1. Low-Quality Or Low Octane Fuel

All fuel types are designated a number based on their octane rating. 

The higher the octane value, the more refined a fuel will be. In other words, the greater the percentage of octane, the more controlled the air fuel detonation will be. 

When you put gasoline with low octane fuel rating into your car, it can cause the airfuel mixture to detonate prematurely, causing an engine knocking sound. 

2. Bad Knock Sensor

Most cars nowadays have a knock sensor that detects an engine knocking sound and sends information to the Engine Control Unit (ECU). The ECU then fixes the issue automatically. 

If your knock sensor is damaged or broken, the engine knocking can continue unchecked. And a failed knock sensor will trigger the check engine light.

3. Damaged Or Broken Crankshaft 

The piston rings in your cars engine move up and down inside the cylinders connected to the crankshaft. This is regulated by the engine timing system. 

The rods and bearings connecting the cylinders to the crankshaft are essential to maintaining the proper clearance between the cylinders and the crankshaft. 

When the crankshaft is damaged, and the clearance between the cylinders is not correctly maintained, a striking of metal may occur, causing a rod knock.

4. Faulty Or Wrong Spark Plug 

Spark plugs are responsible for generating the spark in the combustion chamber of your engine. This spark ignites the air fuel mixture, which provides power to the engine. 

If your engine has a faulty spark plug or spark plugs that don’t fit the requirements of the specific engine, premature detonation within the combustion chamber may result. This can cause a detonation knock in the car engine. 

5. Lean Airfuel Mixture 

Staying on the theme of combustion, the “spark” provided by the spark plugs ignites a compressed air fuel mixture. 

When there is a low proportion of fuel in this mixture, the result is multiple detonations and a loud knocking engine noise.

Be sure to check out the engine components that regulate the flow of air and fuel within the engine, like the fuel injectors and the mass airflow sensors — as these influence your air fuel ratio. 

6. Worn Bearings 

When your car gets older and experiences engine wear and tear, the bearings on the connecting rod between the cylinders and the crankshaft can start causing a noisy ride with a loud rod knock. 

As cars age, particles also build up in the engine, and combustion by-products like carbon deposits, dirt, and grime can form at the back of the rod bearing. These will cause damage to the connecting rod, replacing smooth movement with a piston slap or knocking sound. 

Call a mechanic to replace your rod bearing, as these parts are located deep within the engine. 

7. Stretched-Out Serpentine Belt 

As a cars engine runs, it turns a serpentine belt connected to various pulleys (powering different accessories) throughout the engine bay. This belt must run at just the right amount of tension to spin silently and smoothly. 

An overly stretched belt will be unable to maintain the right level of tension, resulting in a rattling, clicking, or squealing sound that can be mistaken for general engine noise. 

8. Non-Lubricated Cylinder Head 

All engine cylinders require lubrication. When a cylinder is running unlubricated, it will generate a knocking sound. 

A cylinder will typically lose lubrication when there’s an engine oil leak. Using generic oils with lower flashpoints can also cause poor lubrication in the engine. As such, many car manufacturers recommend using synthetic engine oil to lubricate the cylinder head. 

An unlubricated cylinder head can also cause damage to the piston rings and cylinder wall, making this a must-fix to prevent a piston slap. 

Tip: If you notice an oil leak, place an oil pan underneath your vehicle to capture any leaking engine oil before performing an oil change.

Still have some queries about the engine knocking sound?
Let’s go through the FAQ section.

4 FAQs About The Engine Knocking Sound

Here are answers to some frequently asked questions about the engine knocking issue: 

1. Can I Still Drive My Car If I Hear An Engine Knocking Sound?

A knocking sound is usually due to a problem with the internal components of an engine. Driving while you hear a knocking noise is not recommended. 

The more engine damage you cause to these internal components, the more difficult and expensive the repair job will be. You may even need to replace essential engine parts, which would be costly.

Most of the issues that cause an engine knock will illuminate the check engine light, so keep your eyes on that, and don’t let engine wear become a long-term problem. 

2. What Is A Carbon Deposit?

As fuel burns within the engine, it leaves a carbon residue, aka carbon deposit. 

These carbon deposits, or carbon sludge, mainly form within the cylinders. This build-up can clog your cylinders and perhaps an air filter or fuel injector, increasing the amount of compression within the combustion system. 

This directly affects your vehicle’s ability to detonate the air and gas within the cylinder, which could be an alternative reason you hear a knocking or ticking noise. 

Tip: Add an additive to your fuel injector to eliminate carbon deposits within your engine, or visit a mechanic to clean the deposits out of your engine. 

3. How Much Does It Cost To Fix An Engine Knock?

The cost of an engine knock repair varies depending on where the problem stems from within the engine.

If the knocking sound is simply from a bad knock sensor or poor ignition timing, the repair will cost you between $100 – $400. 

However, if there’s more severe engine damage, and you need to replace a cylinder rod, the price could start to travel upwards of $2000. 

4. How Can I Fix An Engine Knock?

Before starting any self-repair job, try performing these quick fixes: 

If this doesn’t help, start asking yourself questions like “when did the knocking noise start?” or “what type of knocking or ticking noise is coming from the engine?

You can also try checking for trouble codes by running a diagnostic scanner on your vehicle. 

If you’ve diagnosed the issue and know where it’s coming from, and the problem doesn’t involve critical components of your engine, you can try and attempt to fix it yourself.

If you can’t find exactly where the issue stems from, or aren’t comfortable attempting these repairs, let a mechanic do the job for you. 

Final Thoughts 

If you hear an engine knocking noise, try to address it ASAP to prevent additional damage to your engine components. Contacting a professional auto repair service is your best option to get that knocking engine resolved. 

That’s where AutoNation Mobile Service steps in.

We’re a mobile vehicle and maintenance repair service that makes repairing your car super convenient.

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