A ‘knock-knock’ joke can crack a smile — but when your engine’s doing the knocking, it’s no laughing matter.
An engine knocking sound could mean anything from a faulty spark plug to excessive carbon buildup or even worn bearings.
And if you ignore it?
Well, you might be looking at some major (and expensive) engine repairs.
But don’t worry!
In this article, we’ll cover the top 10 reasons your engine might be knocking and what you can do to fix it. We’ll also break down the repair costs so there are no surprises along the way.
Let’s go.
This Article Contains:
- What Does an Engine Knock Sound Like?
- 10 Reasons Why You’re Hearing an Engine Knocking Sound (+ Fixes)
- Is It Safe to Drive If the Car Makes Knocking Sounds?
- How Much Does It Cost to Fix Engine Knocking Sounds?
- How to Avoid Engine Knocking Problems
What Does an Engine Knock Sound Like?
Engine knock (also called detonation knock) happens when the air-fuel mixture in the engine doesn’t burn properly or when metal parts inside the engine collide when they shouldn’t.
It isn’t always a loud knocking noise and can sometimes sound like a metallic ping, repeated tapping, or clinking of metal parts. The noise can also change in volume and frequency depending on what’s causing it, but it’s usually most noticeable when accelerating.
Now that we have identified the dreaded engine knock, let’s understand what’s really causing it.
10 Reasons Why You’re Hearing an Engine Knocking Sound (+ Fixes)
Here are the most common causes behind an engine knocking sound and their possible solutions:
1. Low Engine Oil
When your oil level is low, the engine’s moving parts don’t get enough lubrication, resulting in increased friction and heat. This causes metal components to grind against each other, creating knocking or clunking sounds.
Solution: Refilling the oil usually solves the issue, but it’s wise to schedule service and have a mechanic inspect the engine. If the oil level was low enough to cause knocking, it may have already caused some engine damage.
2. Incorrect or Low-octane Fuel
Using a lower-octane fuel than recommended can cause the fuel to ignite too early (pre-ignition), which leads to engine knocking. This premature combustion creates uneven pressure in the engine and causes damage to its internal components.
Solution: Check your owner’s manual for the octane rating required by your vehicle. There are three types of fuel options you can choose from:
- Regular: Octane rating 87 (most commonly used)
- Mid-grade: Octane rating 89 to 90 (more resistive to knocking)
- Premium: Octane rating 91 to 94 (usually used in high-performance cars)
3. Excessive Carbon Deposit Buildup
When fuel doesn’t burn properly, carbon deposits can build up inside the engine, forming black soot on parts like the spark plugs and cylinder walls. This can block airflow, which messes up the air-fuel ratio, leading to knocking and reduced fuel economy.
Solution: Here’s how you can clean carbon deposits:
- Warm the engine and let it run at 2,000 RPMs.
- Spray an intake valve and turbo cleaner through a vacuum line in short bursts.
- Once done, accelerate the vehicle a few times, let it idle for a minute, and turn it off.
- Allow the engine to cool down for an hour.
- Drive at highway speeds for 10 minutes to clear any remaining deposits and restore engine performance.
Note: If you’re not confident about performing a carbon cleanup, it’s best to call a mechanic.
4. Faulty Spark Plug
Spark plugs ignite the engine’s air-fuel mixture. If they’re old or worn out, they may not ignite properly, causing a knocking noise, also known as a spark knock.
Solution: To fix a spark knock, visit a service center for spark plug replacement.
5. Incorrect Air-fuel Mixture
Your engine needs the right air-fuel mix to run smoothly. An issue with sensors or fuel injectors can lead to a lean fuel mixture (too much air, not enough fuel), which can cause engine knock.
Solution: Schedule service with a mechanic to diagnose and fix an engine knock. Depending on the issue, they’ll check and replace the faulty oxygen or MAF sensor, fuel injector, or fuel pump. They may also inspect for vacuum leaks and reset or update the Engine Control Unit (ECU).
For older engines with carburetors, the mechanic will warm up the engine, locate the carburetor, and use a flathead screwdriver to adjust the brass screw clockwise until the engine idles smoothly.
6. Bad Knock Sensor
Modern cars have a knock sensor that helps detect engine knocking and signals the ECU to fix it automatically by adjusting the timing or fuel mixture. If the knock sensor is faulty, it may fail to detect the problem, triggering the check engine light and allowing engine knock to occur.
Solution: Visit a mechanic to replace the bad knock sensor.
7. Poor Ignition Timing
In modern cars, the computer controls when the spark plugs fire during the piston’s movement. If the spark fires at the wrong time, it can lead to multiple explosions in the cylinder, resulting in engine knocking, illuminated check engine light, and poor fuel economy.
Solution: You can adjust the ignition timing through ECU tuning or remapping. If you still have timing issues, visit an auto repair shop for a proper diagnosis.
8. Worn Rod Bearing
Rod bearings sit between the connecting rod (which links the pistons to the crankshaft) and the crankshaft itself, allowing smooth movement as the crankshaft rotates. If the rod bearing wears out or gets damaged, it can cause excessive clearance and a knocking noise (rod knock).
This happens when the connecting rod hits the crankshaft, producing a rod knock, especially at higher RPMs or under load.
Solution: Have a trusted mechanic replace the worn engine bearings since they’re deep inside the engine block. If the damage is severe and has affected other engine components, you may need an engine rebuild or replacement.
9. Piston Slap
A piston slap happens when there’s too much space between the piston and the cylinder wall. As the piston moves up and down during combustion, the extra clearance allows it to hit the cylinder wall, causing a knocking or tapping sound.
Solution: An engine rebuild or replacement at a service center may be necessary to restore proper piston-to-cylinder clearance. Regular maintenance, such as oil changes, is crucial to maintain adequate lubrication and prevent piston slap.
10. Bad Belts or Pulleys
The serpentine belt powers various pulleys in the engine bay and requires proper tension to run smoothly.
If the belt becomes stretched, the tensioner malfunctions, or a pulley is misaligned, it can produce rattling, clicking, and slapping noises. These sounds are often mistaken for engine knock but are caused by issues with the belt or pulleys, not the engine’s combustion.
Solution: Have a mechanic replace the bad serpentine belt. They’ll also replace the pulleys if they appear misaligned or damaged.
Wondering if you can keep driving with a detonation knock?
Find out next.
Is It Safe to Drive If the Car Makes Knocking Sounds?
No, driving with a knocking engine noise isn’t safe and is a risk you shouldn’t take.
Here’s why:
While your car might still run with a knocking engine, it’s a warning sign of serious trouble brewing under the hood. The longer you keep driving, the more engine damage you risk, and it won’t be long before things go from bad to worse.
For example, if the engine noise is caused by low oil levels, continuing to drive can damage critical parts like the engine bearings, cylinder walls, and even the entire combustion chamber. This can eventually lead to catastrophic engine failure or a complete engine seizure.
Curious about the cost of engine knock repairs?
Keep reading.
How Much Does It Cost to Fix Engine Knocking Sounds?
Fixing engine knocking can range from relatively minor fixes to expensive repairs, depending on the specific issue. Here are some estimated repair costs (including labor):
- Engine oil change: $35 to $125
- Spark plug replacement: $50 to $750
- Fuel injector cleaning: $60 to $100
- Serpentine belt replacement: $90 to $250
- O2 sensor replacement: $110 to $600
- Carbon buildup cleaning: $175 to $450
- Knock sensor replacement: $200 to $400
- MAF sensor replacement: $250 to $470
- Piston ring replacement: $1,800 to $3,500
- Engine rebuild: $2,500 to $5,000
To avoid engine knocking and the resulting auto repair costs, you can always take certain preventive measures. Let’s check them out.
How to Avoid Engine Knocking Problems
Follow these six tips to steer clear of engine knock troubles:
- Use higher-octane fuel with carbon-cleaning additives to prevent buildup in the combustion chamber and other engine components.
- Replace spark plugs and wires during a tune-up to ensure proper ignition and reduce engine knock.
- Get regular oil changes and monitor oil levels to ensure proper lubrication.
- Keep your car’s software updated for optimal engine performance and timing adjustments.
- Avoid overloading the engine by following recommended weight limits and driving practices.
- Inspect the car’s cooling system and replace the coolant to prevent engine overheating, which can lead to knocking.
Knock Out Your Engine Troubles with AutoNation Mobile Service
Fixing unusual engine noises early on and staying on top of your car’s maintenance schedule keeps your engine in top shape as well as prevents hefty repair bills.
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