What Is Engine Knocking? Causes, Solutions & More
The sound your car engine makes probably isn’t something you regularly think about — unless that familiar rumble sounds more like a dryer full of rocks.
Engine knocking is almost unmistakable once you hear it for the first time and signals that there’s something significantly wrong with the internal combustion cycle. Specifically, the exact timing of combustion happens too early in one or more cylinders.
The guide below explores engine knocking to help you learn more about what causes this premature detonation in your car.
Most modern vehicles are equipped with knock sensors to alert drivers of engine knocking, but diagnosing the root of the cause takes some detective work. Prompt engine knocking repair is critical to keeping your car functional.
What Is Engine Knocking?
Engine knocking occurs due to premature combustion in a fired cylinder. The spark plug ignites the air-fuel mixture before the piston has adequately compressed it and reached the top-dead-center position.
This early detonation or pinging causes an audible noise due to misaligned metal components within the engine that violently bang against one another outside of normal operating procedures.
Why Is Engine Combustion Timing Critical?
The combustion cycle that keeps your engine running consists of a multi-step protocol that begins with the piston traveling downward and ends with it traveling upward.
Here is the basic process that happens in an internal combustion engine:
- Intake: As the piston completes its downstroke, it draws in the air-fuel mixture to fill the cylinder.
- Compression: The piston begins the upstroke to compress the air-fuel mixture toward the spark plug tip.
- Combustion: Under strict timing conditions, the spark plug ignites the compressed air-fuel mixture, and the resulting combustion forces the piston head back down.
- Exhaust: The piston completes another upstroke, forcing the exhaust gases up and out of the exhaust valve.
The piston’s movement in relation to the spark plug’s ignition remains critical to optimal operating conditions. When engine knocking occurs, combustion happens before the air-fuel mixture is fully compressed. This results in an uneven burning of the air-fuel mixture and loss of ideal timing conditions.
Engine knocking doesn’t go away on its own. The knocking happens consistently as you drive, whether you’re accelerating to merge into traffic or maintaining speed. This phenomenon also increases the heat and friction within your engine, which can lead to stalling if left unaddressed.
Over a short period, engine knocking can reduce efficiency, increase carbon deposits, and result in a lack of adequate oil circulation. Any one of these conditions can lead to damaged cylinders, pistons, connecting rods, catalytic converts, valves, spark plugs, and, ultimately, a seized engine.
Depending on the extent of the damage, repair bills for engine knocking can go up to $4,000 or more.
What Causes Engine Knocking?
Uneven burning of the air-fuel mixture can lead to pressure spikes within the cylinder. Consistent pressure spikes increase the cylinder’s compression ratio, which strains numerous engine components.
Below are the most common causes of engine knocking:
- Incorrect spark plug gap
- Low motor oil levels or poor oil quality
- Fuel of low quality or low octane
- Incorrect ignition timing
- Worn piston rings
- High cylinder temperatures due to lack of cooling
- Connecting rod (bearing) failure
- Excessive carbon deposits
- Faulty fuel pump
- Loose accessory belt
Your car’s knock sensor can mistakenly alert you to engine knocking if it requires replacement. However, you should treat each instance of engine knock as if it were real. A skilled automotive technician can diagnose the sensor and check for engine-knocking conditions to verify your vehicle’s health.
How To Diagnose Engine Knocking
If the audible pinging characteristic of engine knocking doesn’t confirm the diagnosis, the diagnostic trouble codes that illuminate the check engine light should.
A diagnostic tool can capture the conditions monitored by the knock sensor, which alerts the engine control module (ECM) of any unusual vibrations in the engine block. If the ECM fails to adjust timing to alleviate the vibrations, it alerts the driver of a severe malfunction.
Diagnosing the root cause of engine knocking requires a thorough inspection of various components.
Follow the steps below to diagnose engine knocking:
- Verify spark plug gap
- Check oil level and condition
- Evaluate engine timing
- Inspect the pistons, connecting rods, cylinders, and surrounding components
- Perform an engine compression test
- Confirm the fuel pump is functional
- Double-check accessory belt tension
While you can perform some of these diagnoses on your own, it’s best to schedule an appointment with a trusted mechanic. Their experience with engine knocking can help identify and address the problem sooner and minimize any additional damage.
How To Fix Engine Knocking
The solution to fixing engine knocking lies in the information you gathered during your diagnosis. A thorough approach can help you avoid the time and cost of multiple engine disassemblies and reassemblies.
Here are common solutions to fixing engine knocking:
- Adjust the spark plug gap
- Replace spark plugs
- Change oil and look for metal deposits
- Adjust engine timing
- Replace worn pistons, piston rings, or bearings
- Drain the fuel system and replace it with high-quality fuel
- Add fuel system cleaners to a full tank of gas
- Tighten the accessory belt
- Increase engine performance with fuel additives
- Replace faulty knock sensor
In the best-case scenario, fixing engine knocking requires replacing a minor component. In the worst-case scenario, multiple issues contribute to engine knocking and require separate diagnoses and repair or replacement.
Protect Your Engine’s Longevity With High-Quality Products
Regular automotive maintenance can ward off engine knocking and keep your car running smoothly and efficiently. Investing in high-quality products can also contribute to greater engine longevity and performance.
Call us at 336-629-2061 today to speak with one of our friendly and knowledgeable staff members and discover how Cadence delivers value at every turn.
Sources:
Mysteries of the Air/Fuel Ratio (AFR) Explained | Haynes Manuals
How much does it cost to fix a knocking engine? | Jerry
Fact Sheet | A Brief History of Octane in Gasoline: From Lead to Ethanol | White Papers | EESI
How Does a Knock Sensor Work and Why Is It Important? | Edmunds