Knocking Sound When Accelerating – Causes and Fixes

A knocking sound when accelerating can be caused by a number of things. Learn what they are and how to fix them in this article.

Knocking Sound When Accelerating

Any strange sound coming from the engine is enough to cause alarm. 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.

In this guide, we evaluate the reasons why you might hear knocking. We also look at how to fix it, the average cost of repair and discuss whether you should keep driving your vehicle.

Causes of Knocking Sound When Accelerating

If the car engine is knocking when accelerating, the octane of the fuel could be too low or the timing is bad. If the knocking noise is coming from under the car rather than from the engine, it may be due to a bad CV joint or driveshaft. A full diagnostic exam is needed to determine the cause.

Here is a more detailed list of the possible causes when your car is knocking when accelerating:

1. Low Octane

Your car engine is meant to work with a particular octane of fuel. The octane rating shows the measurement of how heat resistant the fuel is to knocking. The higher this number is, the further the piston can move toward the spark plug, thereby producing more output of power. 

If the octane number is too low, there will be pockets of air-fuel mixture exploding before the piston can reach maximum travel levels. 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. With this type of knocking, you might also have trouble keeping the car running. 

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. The ECU is left in the dark, unable to resolve potential problems, which could lead to a knocking engine in the long run. 

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

4. Drivetrain or transmission issues

If you hear that the knocking sound is coming from under the car when accelerating rather than from the engine, it could be due to a bad driveshaft or CV joint. It can also be a loose exhaust system.

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.

There could also be a problem with the differential or your transmission that is causing a knocking sound. These can be difficult to diagnose, so consult a mechanic if you’re not sure how to do it.

5. Lean Air-Fuel Mixture

When the mixture inside the combustion chamber becomes too lean, the fuel is lower than it should be in comparison to the air level. This problem can be caused by a number of issues, such as defective oxygen sensors, clogged fuel injectors or a failing mass airflow sensor.

Without the right amount of fuel flowing through the cylinders, the burning time is wrong. This problem causes the mixture to combust at the incorrect time. With the improperly timed detonations, engine knocking will occur, especially when trying to accelerate. 

6. Bad Spark Plugs

Spark plugs are required in a gas engine to ignite the air-fuel mixture. The engine isn’t able to run without this ignition source.

However, over time, the spark plugs become contaminated or fouled. That’s why you should change them at regular intervals to avoid problems. If the spark is delayed, the engine could start to knock, mainly when under a load, such as during acceleration. 

7. Carbon Deposits

Burnt fuel leaves behind carbon deposits. There’s no way around it. However, there are ways to prevent these carbon deposits from building up and clogging essential 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. Thankfully, most problems resulting from carbon buildup are simple enough to fix 

8. Low Oil Level

Another possibility is that the engine oil level has dropped too low. The oil level must be between the appropriate lines on the dipstick for the engine to run as it should.

If the level gets too low, the rod bearing from the piston rod to the crankshaft can have a gap. As it moves, this rod bearing makes a rattling or knocking noise. 

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

Fix Engine Knocking

1. Scan Engine Codes

If the engine knocking occurs with the Check Engine Light on, your first step is to use your professional code scanner. Your OBDII scanner will show you what engine codes are set, helping you determine what’s causing the problem. 

You can get readings from all of the appropriate sensors in real-time. 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. 

If the code scanner shows that a part has gone bad, you might need to replace it. Most sensors are simple to replace, but you shouldn’t attempt any fix that you’re unfamiliar with. 

2. Perform Engine Maintenance

If the knocking is occurring without a Check Engine Light, you might want to start by doing the regular engine maintenance procedures. Start with an oil change, ensuring that the system is filled up properly. You must use the right type of oil, as outlined in the owner’s manual.

It may also make sense to use an engine cleaner, injector cleaner or throttle body cleaner. These cleaners can clean out carbon buildup in the passages. There are also cleaners to improve the quality of fuel in the tank. With the right application, you can easily dissolve any carbon buildup to prevent further blockage.

It’s also possible that you need to replace the spark plugs. These can become damaged or contaminated over time. 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 Car Maintenance Tips (Extend the Life of Your Car)

3. Increase 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.

By increasing the octane rating, ignition can occur at the appropriate level. Temporarily, you can use an octane booster additive to get it up to the desired number. 

4. Look under the car

If you think that the knocking noise is coming from the driveline, you need to look under the car and check the CV joints and driveshafts. You may also want to check that the exhaust system is mounted correctly.

5. Visit 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.

In most cases, you will be charged a diagnostic fee for an evaluation. Ask upfront what you will be charged for this visit. In some cases, the diagnostics could reveal a problem that you may be able to fix on your own. 

Cost to Fix a Knocking Engine

To fix the knocking engine, you could be looking at anywhere from $5 to $500 or more, depending on what’s causing the problem. If you can simply put an additive in the tank, you have a minimal repair cost.

Otherwise, replacing sensors or spark plugs can cost much more. These repairs become even more expensive if the parts are hard to get to or remove. 

Knocking Noise FAQs

Can I Drive a Car With a Knocking Engine?

You could drive your car with the knocking engine, but you may not want to. Depending on what’s causing the problem, you could find yourself stranded with a broken-down car. If you must drive, take it easy on the throttle and be gentle. As soon as you can, have the vehicle checked out and repaired.

How long will a knocking engine last?

There is no way to tell how long a knocking engine will last, as it will depend on many different variables. However, if your engine is knocking, it will probably drastically shorten the life of the engine if you don’t fix the problem as soon as possible.

Can low oil cause knocking?

Yes, low oil can cause engine knocking. A lack of oil will create excessive heat and wear on the engine, which can lead to knocking. It’s important to maintain the correct level of oil in your car’s engine to prevent this from happening.

Why is my car knocking when I go over bumps?

If your car makes a knocking noise when you go over bumps, there is probably something wrong with your suspension. The most common parts that cause a knocking noise when going over bumps are a bad sway bar, tie rods, coil springs and struts.

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Magnus Sellén
Written by:

Magnus is the owner and main author of Mechanicbase. He has been working as a car mechanic for over 10 years, and the majority of them specialized in advanced car diagnostics and troubleshooting. Certified Automotive Diagnostic Technician.