Knocking Sound When Accelerating – Causes And Fixes

It is alarming to hear unusual sounds coming from an engine. What’s most troubling is when there’s a knocking sound from the car when accelerating. The noise could be minimal or deafening, depending on what’s causing it.

We will examine the possible reasons you may hear someone knocking. Also, we discuss how to fix it and the average repair cost. Finally, we decide whether or not you need to keep driving your car.

Reasons for Knocking sound when Accelerating

The fuel octane or timing could have been too low if the engine knocks when it accelerates. A bad CV joint, or driveshaft could be the cause of the noise under your car. It is necessary to perform a complete diagnostic exam in order to identify the source.

Below is a list that will provide more details about the causes of your car knocking while you are accelerating.

1. Low Octane

A particular type of fuel is required to run your car's engine. Octane ratings indicate how resistant fuel is to heat knocking. This number indicates how far the piston can travel toward the spark plug and thus produces more power. 

Low octane numbers can lead to pockets of explosive air-fuel mixture that will cause the piston not to reach its maximum travel. What’s left is a knocking sound and a reduction in engine performance. 

RELATED: 6 Causes of Engine Knock (& How to Fix It)

2. Bad timing

A car’s spark must be timed perfectly for proper energy distribution. If the spark doesn’t occur at the right time from the ignition coil, detonation occurs.

This problem causes engine knocking that’s most noticeable as the speed rises and you accelerate. You might have problems keeping your vehicle running due to this knocking. 

3. Defective Knock Sensor

Today’s cars have an engine knock sensor installed. It’s meant to detect a potential fault and set a code in the computer, so you can have it fixed before a problem occurs.

If the knock sensor fails, it won’t be able to detect knocking situations. It is left in darkness, unequipped to fix potential issues, and could eventually lead to a knocking engine. 

READ MORE: 5 Symptoms of a Bad Knock Sensor (& Replacement Cost)

4. Problems with the transmission or drivetrain

It could also be a CV joint problem or a defective driveshaft. A loose exhaust system can cause it as well.

If the sound is more noticeable when you turn the steering wheel and accelerate at the same time, it’s very likely that the issue is due to a bad CV joint.

A knocking sound could be caused by a fault in your differential or transmission. These can be difficult to diagnose, so consult a mechanic if you’re not sure how to do it.

5. Lean Air-Fuel Mixture

The fuel level is reduced when the mixture in the combustion chamber gets too dry. There are a variety of reasons for this problem, including defective oxygen sensors or blocked fuel injectors.

Burning time can be affected by the insufficient fuel flow to the cylinders. The mixture will combust incorrectly due to this problem. Engine knocking can occur when the engine is trying to accelerate. 

6. Bad Spark Plugs

In order to ignite the mixture of air and fuel, spark plugs are necessary in a gasoline engine. The engine isn’t able to run without this ignition source.

Over time spark plugs can become fouled or contaminated. That’s why you should change them at regular intervals to avoid problems. When the spark isn't working properly, your engine may start to knock under load. 

7. Carbon deposits

Carbon is left behind by burning fuel. There’s no way around it. There are however ways you can prevent carbon deposits from accumulating and clogging vital parts. You should follow through with all scheduled maintenance in a timely manner and you want to use a carbon cleaning agent if you think there’s a problem. 

If carbon builds up in the cylinder, there won’t be enough room for the air-fuel mixture to push through, leading to engine knocking situations. Most issues caused by carbon buildup are easy to resolve. 

8. Low Oil Level

The engine oil level may have dropped to a low point. For the engine to function properly, it must have oil levels that are between the indicated lines on the dipstick.

The rod bearing connecting the crankshaft to the piston rod can become brittle if the level drops too low. The rod bearing's movement causes it to make a noise, either rattling or knocking. 

RELATED: Engine Ticking Noise? (Causes & How to Fix it)

Repair Engine Knocking

1. Search Engine Codes

Your professional code scanner should be used if the engine is knocking and the Check Engine Lamp on. Your OBDII scanner will show you what engine codes are set, helping you determine what’s causing the problem. 

In real time, you can see the readings of all sensors. You should also run these readings under various loads to see what’s going on. It’s important to cross-reference these readings with the information found in your service manual to see what’s considered normal. 

The code scanner may indicate that the part is damaged and you should replace it. Most sensors are simple to replace, but you shouldn’t attempt any fix that you’re unfamiliar with. 

2. Do engine maintenance

If you hear the engine knocking but don't see a Check Engine light, it might be worth starting regular engine maintenance. You should start with an oil change. Make sure the engine is properly topped up. You must use the right type of oil, as outlined in the owner’s manual.

You may want to also use an injector cleaner, engine cleaner, or throttle body cleaner. These cleaners are able to remove carbon buildup from the passages. These cleaners can be used to enhance the quality of the fuel inside the tank. You can quickly dissolve carbon deposits to stop further blocking.

It’s also possible that you need to replace the spark plugs. Over time, they can be damaged or contaminated. If you haven’t changed the plugs at the appropriate time, the delay of spark could lead to knocking, but it’s an easy fix.

RELATED: 11 Tips for Car Maintenance (Extend Your Car's Life)

3. Increase the Fuel Octane Rating

If you aren’t using the right octane rating for your car, it’s time to increase to a premium option. Read the owner’s manual to figure out an appropriate octane rating for your application.

You can increase the octane rating to allow ignition to occur at the right level. You can temporarily increase the octane rating by using an octane booster. 

4. Take a look under your car

You should inspect the underside of the vehicle to make sure the driveshafts and CV joints are in good condition. It is also important to verify that the exhaust system has been properly mounted.

5. Consult a professional

If the problem seems beyond your comprehension, it’s best to visit a professional mechanic for more guidance. By messing with things you don’t understand, you could create more problems.

A diagnostic fee will usually be required for any evaluation. Inquire upfront about the cost of this evaluation. You may find a problem you can fix yourself in some cases. 

Repairing a Knocking Motor: What is the cost?

To fix the knocking engine, you could be looking at anywhere from $5 to $500 or more, depending on what’s causing the problem. You can repair the engine with a simple additive.

It is possible to spend much more on replacing spark plugs or sensors. If the parts are difficult to reach or removed, these repairs can be even more costly. 

Knocking Noise FAQs