Why Is My Engine Knocking?


If your engine is making a knocking noise, it is nothing to ignore. Engine knocking usually comes from an issue relating to the burning of fuel. It might be from bad spark plugs, a poor mixture of fuel and air, fuel that is too low octane, and a handful of other reasons. One thing is for sure, if left untreated, that knock you are hearing could cause some serious damage to the most expensive part of your car, the engine.

There are quite a few different noises that can be considered engine knocking, and each is caused by a different issue under the hood. Because of this, diagnosing engine knocking isn’t always simple. However there are 7 common reasons that engine knocking occurs.

7 Reasons Engine Knocking Occurs

  • Bad spark plugs
  • Improper timing
  • Poor mixture of fuel and air
  • Worn bearings
  • Bad belt tensioners
  • Broken knock sensor
  • Too low octane fuel

Before we get into the nitty gritty of each of these reasons, let’s discuss why car engines make a knocking noise usually.

What Is Engine Knocking?

Fixing engine knocking is something you can only accomplish if you have a base knowledge of why the knocking noise is happening in the first place. Engine knocking is used to describe the noise an engine makes, but a lot of noises do fall into that category and each will be caused by something different. Regardless of the exact knock, the noise is a sign that something wrong is happening under the hood and you want to get the issue addressed as soon as possible.

There are different situations where a car will make a knocking noise, some where the car is idling and the knocking stops immediately when the car is put into a higher RPM, some where the engine is only knocking when the vehicle is accelerating.

It can be scary to have your car begin to make such odd noises seemingly out of the blue, but if you are diligent on getting the car to a mechanic you can get the issue diagnosed and fixed in no time.

Generally, knocking takes place when there is an issue with the evenness at which fuel burns in your engines cylinders. When there is a proper mix of fuel and air, the fuel will burn in small even pockets for controlled energy. When the mix of fuel to air is off, the fuel will burn all at once causing a larger influx of energy which causes the knock.

When fuel ignition is going well, each pocket will burn, creating a small shock that will then ignite the next pocket of fuel, and so on. Engine knocking takes place when that fuel burns unevenly and there is a sudden influx of power. Engine knocking happens when fuel burns unevenly and those shocks go off at the wrong time. This influx is likely making the noise you are hearing and could be causing damage to your engine.

If you are currently experiencing this issues, know that you’ll likely never deal with this problem again. This has become an issue that is very rare among car owners, especially those who own newer cars.

The good news is that engine knocking has become relatively rare among those who own cars that are on the newer side. You shouldn’t have to worry too much about learning how to fix engine knocking if you have a car manufactured within the last decade. But there is still a chance it could rear its ugly head, and you need to know what to do if it ever does.

The Different Types Of Knocking

Detonation Knock

A detonation knock happens when the fuel in the vehicle is not burning evenly. This tends to be the most common form of engine knocking. When the car is burning efficiently, the air and fuel will burn in small pockets and in a controlled manner inside the cylinders. When the fuel burning isn’t even, the influx of power from the explosion will create a fireball which will create the knocking noise.

Rod Knock

The less frequent form of knocking is known as a rod knock. The pistons of the car will travel up and down in the engine block. The pistons turn the crankshaft which transfers power to the wheels and moves the car forward. The piston movements are facilitated using the rod bearings. When the rod bearings begin to wear out, the pistons will knock against the crankshaft making the noise many call engine knocking.

What Causes Engine Knocking?

While cars do make weird noises, engine knocking is not one to brush off. If you are hearing a noise that sounds similar to knocking, you likely have an issue under the hood that could cause some significant damage if not resolved in a timely fashion. We will cover the 7 likely causes that your engine is knocking and what they mean for you and your car.

  • Bad spark plugs
  • Improper timing
  • Poor mixture of fuel and air
  • Worn bearings
  • Bad belt tensioners
  • Broken knock sensor
  • Too low octane fuel

Too Low Octane

Car engines designed for high performance and some are rated for high octane fuels. In these car engines, you may hear engine knocking if the fuel you use for them has too low of an octane rating. The higher the octane fuel the more evenly the gasoline and air mixture will burn. As we discussed, with a detonation knock, the cause of the noise is the fuel burning at an accelerated rate. In some cases a simple mistake of filling up with regular instead of premium can cause the engine knocking. Be sure to check what type of fuel your vehicle is optimized on and give it that.

Bad Timing

The timing of the engine is the point when the sparkplugs fire and ignite the fuel. The exact timing is important and is controlled by the car’s computer to optimize the efficiency of the vehicle. If the spark plugs are not firing when they should, it can lead to the uneven burning of fuel and multiple explosions within the cylinders leading to the engine knock.

Lean Air/Fuel Mixture

Sometimes an issue with the oxygen sensor, fuel pump, fuel injectors, as well as the airflow sensors will contribute to an error with the ratio of air to fuel that your vehicle uses. The air to fuel ratio is critical for the car driving efficiently, but it is also important for preventing engine knocking. You need to make sure the mixture isn’t too much air, otherwise the mixture will burn too slowly.

Bad Knock Sensor

Because the computer in modern cars are so advanced, fuel knocking is less common of a problem than it used to be. One of the components that helps to make this possible is the knock sensor. The knock sensor is set up to detect engine knocking and correct it before it is able to cause any damage.

So if your knock sensor is faulty, it will allow the knock to continue. A mechanic will be able to diagnose if your knock sensor needs to be replaced.

Worn Bearings

Worn bearings produce the rod knock. The pistons of the car will travel up and down in the engine block. The pistons turn the crankshaft which transfers power to the wheels and moves the car forward. The piston movements are facilitated using the rod bearings. When the rod bearings begin to wear out, the pistons will knock against the crankshaft making the noise many call engine knocking. You may need to purchase and install new bearings or have other work done on the pistons and crankshaft to solve this issue. Don’t have sticker shock, know that these parts are extremely expensive to replace do to the labor a mechanic must put in. The parts are buried deep within the engine compartment and it takes some time to get to them.

Bad Belt Tensioners Or Pulleys

One of the causes for engine knocking may not be coming from the engine at all. The belt or belt tensioners may be your issue. As the engine runs, it pushes a belt that connected to various pulleys and components of the vehicle. The belt must be stretched tightly to do it’s job quietly.

If at any point the belt becomes loose it can cause some odd noises to be audible from cab of the vehicle. You can fix this problem by simply replacing the belt.

Faulty Spark Plugs

The spark plugs deliver the spark to the fuel and air mixture, igniting it. Your car would not have power without functioning spark plugs. Over time your spark plugs break down and 2 things can happen. They can stop lighting the fuel at all and your vehicle can become sluggish or lose power entirely, or they can improperly fire lighting all the fuel air mixture in your cylinder causing the engine knock you are hearing.

Spark plugs should be replaced every 30,000 miles or so, though they can break prematurely or last well beyond this mileage count.

3 Easy Ways to Fix Engine Knocking

Fill up with high octane fuel. The typical regular gasoline at the pump may be the problem. If you are experiencing an engine knocking you should try upgrading your fuel to see if that fixes the issue. If it does, you should still consider getting the car to the mechanic to have it looked over.

Use fuel detergent. Fuel detergent may help solve the knocking problem as well. Fuel detergent is included in most gasolines, but using a higher concentration of it may fix your knocking problem. Adding the right fuel detergent can help remove carbon build-up that could be part of the knocking problem.

Replacing spark plugs. It is possible that a mechanic in the past replaced your spark plugs with an improper variety. Maybe they fire but not quite the way that they need to. The spark plugs may also just be bad and firing at the wrong time. Replacing the spark plugs is an easy and cost effective way to solve your knocking problem.

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