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What Causes Engine to Knock: What You Should Know!

What Causes Engine to Knock: What You Should Know!

A knock can be unwelcomed especially if it’s from your engine. What causes engine to knock? If the smooth roar of your engine is replaced by a repetitive tapping or pinging sound that becomes louder and faster as you accelerate, this is a classic sign of engine knock.  Unfortunately it can be caused by a variety of different issues that can be tricky and difficult to diagnose. It can be from a worn bearing, lean air/fuel mixture down to a very low octane. We will discuss it further in this article.

Auto Repairs Are EXPENSIVE


What Causes Engine to Knock: 9 Common Reasons


Knocking occurs when the fuel in the cylinder ignites prematurely. By firing before the piston reaches top dead center, this premature ignition disrupts the engine's smooth rotation. The misfired piston then applies backward thrust to the crankshaft against the engine's momentum, resulting in the knock or ping you hear, as well as a slight hesitation that results in a loss of power. Knocking can cause damage to the piston's surface, the cylinder walls, or the crankshaft bearings, all of which are costly to repair.


To prevent knocking, modern computer-controlled injection systems can correct your fuel mixture. Because of the wide range of fuel quality, atmospheric pressure, and ambient temperature, as well as the possibility of a malfunction, every modern combustion engine includes mechanisms to detect and prevent knocking.


A control loop continuously monitors the signal from one or more knock Sensors (commonly piezoelectric sensors which are able to translate vibrations into an electric signal). If a knocking combustion's characteristic pressure peak is detected, the ignition timing is delayed by a few degrees. So this comes at the expense of engine performance. 


If the signal returns to normal, indicating a controlled combustion, the ignition timing is advanced in the same manner, keeping the engine at its best possible operating point, the so-called ′′knock limit.′′ Modern knock control loop systems can individually adjust ignition timing for each cylinder.


Depending on the engine, the boost pressure is controlled at the same time. This maintains peak performance while reducing the risk of engine damage caused by knock, such as when running on low octane fuel. But this “knock sensor” can also fail. So you can not always depend on knock sensors and again issues causing the knock can be tricky to diagnose. To lessen this predicament, here are the common issues and areas you can look into to determine what causes engine to knock: 

  • Issues with the spark plugs


The spark plugs of your car’s engine are responsible for delivering the electric spark that in turn ignites the mixture of fuel//air in the cylinder. In other words, spark plugs are critical to getting your engine started! Spark plugs, like other components in your vehicle, age and degrade over time


Most automakers recommend replacing spark plugs every 30,000 miles, but spark plug longevity is dependent on the condition and type of spark plug. What causes engine to knock could be the fact that you're not using a manufacturer-recommended spark plug or if your spark plugs have seen better days.


Faulty spark plugs, if not replaced, can result in a loss of engine power and fuel economy. To learn more about these little bolts of lightning, check out our handy guide to spark plugs. Fortunately, spark plug replacement is a relatively inexpensive repair.

  • Issues with the Belt Tensioners/Pulleys

Another possible source of engine knocking isn't related to the engine at all. It's possible that it's coming from the accessory belt. The engine turns a belt that is connected to various pulleys throughout the engine bay as it runs.


This belt must be tensioned just right so that it turns smoothly and quietly. If the belt becomes stretched, the tensioner fails, or one of the pulleys bends out of shape, you may hear rattling, clicking, and slapping noises that could be misinterpreted as engine knock. Fortunately, replacing a noisy accessory belt, adjusting the tension, or replacing a bad tensioner or accessory pulley can all be done quickly.

  • You’re using low octane fuel.


Because gasoline has different octane ratings, you have so many options when you pull up to the pump. The greater the octane rating of a fuel, the more compression it can withstand before igniting. If your engine was designed to run on high-octane fuel, using regular fuel may result in excessive engine noise.


High-octane fuel is more expensive than regular fuel. While saving a few dollars at the pump may appear to be a compelling reason to stick with regular gasoline with low octane, it could be what causes engine to knock so it might just be time to spend a little more.


Check your owner's manual first. What is the fuel type your car manufacturer is recommending, and is it the one you are using? If necessary, increase your octane level at the next fill-up or use an octane booster to improve performance. If this doesn't seem to help after a few fill-ups, your issue could be caused by something else.


Remember that using the incorrect fuel for an extended period of time can cause engine damage and reduce fuel economy. Cheaper gasoline will not save you money if you get fewer miles per gallon and may have to pay for engine repairs in the future.

  • You have worn bearings.


Rod knock is another type of engine knock. As the pistons in the engine move up and down, they turn the crankshaft, which ultimately sends power to the wheels. The rod bearings allow for smooth piston movement, but they can wear out or become misaligned over time.


As the bearings deteriorate, the pistons begin to rattle against the crankshaft, producing a similar knocking sound. To repair this issue, you may need new bearings or other work done on the pistons or crankshaft – parts that are located deep within the engine, making this a time-consuming repair.

  • Issue with the Knock Sensor.


Fortunately, engine knock isn't a common issue in modern vehicles because the air/fuel ratio, fuel injectors, and timing are all computer controlled. There's even a knock sensor that detects engine knock and alerts the Engine Control Unit, allowing it to automatically correct the problem.


This means what causes engine to knock can be issues with the knock sensor. Checking the knock sensor is an important part of the diagnostic process if you're driving around in a modern car with engine knock.

  • You have lean air to fuel mixture.


A lean air/fuel mixture in the engine can be caused by problems with the oxygen sensors, fuel injectors, fuel pump, or mass airflow sensor. A lean air/fuel mixture contains insufficient fuel and too much air. Without enough fuel in each cylinder, the mixture will not burn quickly enough, resulting in multiple detonations thus an engine knock.

  • Bad Timing


The computer controls the timing of the engine in most modern cars, that is, when the spark plugs will fire at what point in the engine's travel. However, if the spark doesn't fire exactly when it should, it can cause multiple detonations in the cylinder, resulting in engine knock.

  • Excess carbon build up.


To help prevent carbon deposits from clogging your cylinders, all fuel sold in the United States must contain carbon cleaning detergents. Sadly, some deposits continue to form. When this occurs, there is less space for the fuel and air to reside, resulting in increased compression. Changes in fuel compression, as you learned with fuel, can cause unpleasant knocking sounds.


Excess carbon buildup can cause issues with the combustion process and damage the cylinders of your engine. The resulting decrease in performance may also result in decreased gas mileage or overheating. Are you seeing a pattern here? So hire a professional to clean your cylinders. To be safe, engine manufacturer Briggs and Stratton recommends checking your cylinder for carbon build-up every 100 hours of operation.

  • You are not up to your vehicle’s regular maintenance.


If everyone could afford a new car, they would almost certainly do so. Even if you're not a mechanic, hearing a brand new, perfectly-tuned engine is almost magical. However, you will not be able to replace your car as frequently as you would like. That means your car is bound to accumulate miles, wear and tear, and require regular maintenance.


Depending on the type of engine knock (whether it's a true metal-on-metal knock or a pinging noise), it could be the result of neglecting regular maintenance. If you've recently purchased a vehicle (new or used), it's possible that maintenance hasn't always been performed on time.


Before you buy a car, look at its vehicle history report to see if it has received regular maintenance. However, you may not always be aware of your vehicle's precise history—or you may be the owner who has neglected standard maintenance.


In any case, failing to perform routine maintenance can result in broken or loose parts flip-flopping in the engine compartment. Of course, this isn't the only reason for engine knock but it’s a strong possibility.


Quick Fix to What Causes Engine to Knock


So, what happens if you start your car and hear a knocking sound from under the hood? Do you ignore it and keep driving in the hope that it will go away? Do you open the hood and try to figure out where the noise is coming from? Or do you immediately take your car to the shop? While it is always ideal to have your car checked by a professional there are actually some quick solutions that might be all that you need to fix what causes engine to knock. The great news is that in some cases, a simple solution is available. Here are three things you should do if your engine is making knocking noises:


Fill up with premium unleaded gasoline.


The problem could be the typical low-grade and least expensive gasoline at the pump. If you usually use this type of gas, switch to premium unleaded the next time. Higher-octane fuel can frequently assist in the correction of a knocking engine.


Pour in some fuel detergent.


You pour detergent into your dishwasher or washing machine to help clean your dishes and clothes. The same is true for fuel detergent. Although most gasoline contains some fuel detergent, if your engine is knocking, you may require something a little stronger.


It's similar to using stain-buster detergent to remove grass or grease stains from clothing. Using the right fuel detergent can help remove carbon buildup, which could be contributing to the knocking.


Try replacing the spark plugs and the wires. 


If your vehicle has ever been serviced, the original spark plugs may have been replaced with the incorrect ones. They may still fire, but not in the manner that they should. Making sure you have the correct spark plugs installed for your vehicle can often help to silence the knocking. While you’re at it, give your engine a tune-up and that includes replacing the wires.


Before visiting the service center here are a few basic do-it-yourself solutions to consider as soon as you hear that engine knocking sound. You can either troubleshoot it yourself or bring it in for an inspection. Repairing the source of a knocking engine sound as soon as possible can save you time and money.


Regular Change Oil


Change your vehicle's oil on a regular basis and keep an eye out for low oil levels. Worn-out oil and low oil levels can both contribute to poorly lubricated timing-related components near the engine's top.


Once you hear that knocking sound look into the issues mentioned above and try the quick fixes. They will surely be worth the try. Whatever your next step is just keep in mind that engine knocking is not something to be taken lightly. If you drive your car for an extended period of time with this problem, it can cause serious engine damage and more money down the drain.