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vehicle maintenance

Why Can I Hear a Knocking Sound in My Car? Possible Causes and Solutions

For car owners, nothing is as alarming as starting up your vehicle and hearing strange car sounds. There’s a high chance that those sounds will require a lot of time, money, and labor. 

This is particularly true with car engine knocking sounds. It’s important to be aware of the many factors that could be behind knocking noises so that you can prevent and manage them.

In this article, we highlight everything you need to know about engine knocking and why you should never ignore that knocking sound in car engines.

What Could Be Behind the Knocking Sound in Car?

Car sounds are not unusual — some of them are normal, while others could signify that something serious is happening to your car. Car engine knocking sounds belong to the latter category. 

Your car engine making knocking sounds could mean something is causing some serious engine issues. Unfortunately, the engine is a very vital and expensive car part — fixing engine knocking issues won’t be cheap. 

What Is Engine Knocking?

mechanic checking knocking sound in car

Engine knocking occurs when the fuel in your engine’s cylinders does not burn evenly. Ideally, with the right proportion of air and fuel, fuel burns in small regulated pockets. This way, the fuel burns evenly at regular intervals, instead of at all at once. Once each pocket burns out, it creates a little spark that ignites the next fuel pocket.

However, if the air/fuel ratio is off, fuel burns unevenly and the sparks are produced at the wrong intervals. This produces an annoying knocking sound and could potentially damage the pistons and cylinder walls of your car engine.

What Causes Engine Knocking?

We know that engine knocking happens because of improper fuel burning. However, this is usually due to your engine’s response to other factors. Luckily, some of these factors are quite easy to spot and can be fixed on your own.  

However, if the knocking still persists, it might be in your best interest to take the car to an auto shop and have them look at your engine.

Here are some of the most common issues behind engine knocking:

1. Faulty Knock Sensor

Most modern vehicles have an automatic engine knock sensor. This sensor detects if the air-fuel mixture proportion and fuel injectors are working properly.

However, a knock sensor can be faulty and fail to detect changes in the air-fuel ratio or fuel injectors. This will lead to your engine knocking. When you hear an engine knocking sound in your car, the best-case scenario is that you have a faulty knock sensor.

2. Bad Spark Plugs

old and new spark plug

The spark plug produces the spark that ignites the air-fuel pockets in the engine’s cylinder. If the spark plug is worn or faulty, it won’t ignite when it’s supposed to, and this could cause an engine knocking noise.

As time passes, spark plugs become more prone to wear and tear. It’s a good idea to change your spark plugs regularly. Most car manufacturers recommend a spark plug change every 30,000 miles. However, this heavily depends on the type of spark plug, your vehicle make, and your model.

If your spark plug is really old or you’re not using a manufacturer-recommended one, then this could be the reason behind your knocking engine. You can either have them changed by yourself or take them to a repair shop for a replacement.

If you have had the plugs changed recently and your engine is knocking, then you may have gotten a bad spark plug or the problem is somewhere else.

3. Low Octane

Your engine knock could be because you have fuel with a low octane rating in your engine. This means you’ve been using the wrong type of gasoline to fuel your car. 

Every car make and model has its own specific fuel requirements. Some cars, especially sports cars, are built for high octane fuel, while others require low octane gas. High octane fuel can withstand more compression before it ignites. If you use low octane fuel for a car that’s meant to run on high octane, you could damage your engine.

Before you fill up your tank for the first time, carefully look through the owner’s manual to figure out the kind of fuel your car needs. As tempting as it may be, don’t attempt to save some gas money by buying lower octane fuel for cars that require high octane. In the long run, you’ll spend more on engine repairs. 

4. Carbon Deposits 

A carbon build-up in the combustion chambers can change the fuel compression and cause your engine to produce knocking sounds. Ideally, gasoline should have carbon-cleaning agents to prevent the build-up of carbon deposits. However, some carbon deposits still form. 

The more these deposits build up, the more compression is required for the air-fuel combustion process. In the long run, this could cause engine damage. 

If you suspect the knocking sounds are because of a build-up of carbon deposits, get a trained mechanic to clean out your engine’s cylinders. Don’t skimp on car maintenance, and take your car to the auto shop to check for carbon deposits regularly.

5. Worn Bearings

The rod bearings in the engine make piston movement smooth and clean. However, once those bearings begin to wear, you can hear the sounds the piston rods make when they rattle against the crankshaft.

The fastest way to fix this problem is to install new rod bearings in your engine. Sometimes, if the damage is extensive, you might also need to fix the pistons or crankshaft. You should replace your engine bearings after 60,000-80,000 miles.

Final Thoughts

The first thing you should do when you hear a knocking noise is take your car to a mechanic for a complete engine check before the problem worsens. Driving around with that knocking sound in your car engine is not only dangerous, but it could also cause extensive and expensive damage to your car. 

Remember that regular car and engine maintenance is key to preventing your engine from knocking. Also, don’t forget to pay attention to your check engine light.