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What is Engine Knocking and What Causes it?

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Definition of Engine Knock

For the purpose of this article, we will be talking specifically about spark knock, which is caused by pre-ignition (the ignition of the mixture prior to the spark plug firing) or detonation (the spontaneous combustion of the remaining fuel/air mixture in the chamber). Check out this video of an F-150 for an example of spark knock.

Causes of Spark Knock

  1. Low octane fuel
  2. Carbon build-up in the combustion chamber
  3. Lean air-fuel mixture
  4. Ignition timing over advanced
  5. Incorrect spark plugs
  6. Combustion temperatures too high
  7. Faulty Knock Sensor

How To Fix Engine Knocking

The first step to resolving this concern is to find out why the engine is knocking in the first place. The first question to ask is, When did it start happening? Did you recently change to a new gas station, change fuel octanes, or noticed it started happening right after you filled the tank up? If you said yes, check out number one. Does your car have high mileage? If so check out number two. Did you have some work done at a shop recently? If so check out numbers, three through five. These questions can help narrow down why the engine started knocking, and then we can figure out how to go about fixing it. In this article, we will break down the different causes of spark knock and how to fix them.

Low Octane Fuel 

First up, let’s cover fuel octane rating. What is an octane rating? Well, fueleconomy.gov states,Octane rating is the measure of a fuel’s ability to resist “knocking” or “pinging” during combustion, caused by the air/fuel mixture detonating prematurely in the engine. In the U.S., unleaded gasoline typically has octane ratings of 87 (regular), 88–90 (midgrade), and 91–94 (premium)”. Most cars run off of regular (87) octane fuel. It’s affordable and does the job perfectly, but a lot of high end, luxury and sports cars require higher octane ratings. (Consult your owners manual for the exact octane rating your specific car is designed to use.) The reason for this is they use higher compression engines that make more horsepower. The trade-off is that these engines require a higher octane fuel in order to not knock. If you have one of these cars that require a midgrade or premium fuel and you are running a regular fuel instead, it can definitely cause a knocking concern.

How To Fix Low Octane Fuel

If you have already put regular octane fuel in your car, as a temporary fix, you can add an octane booster to your current tank. (These are readily available at your local auto parts store.) This will increase the octane of the currently installed fuel to a better range, allowing you to drive the vehicle until the tank is near empty, and then refuel with the proper rated fuel. 

Carbon Build Up In Combustion Chamber

Next up is carbon build-up; this is usually only present in higher mileage cars, but it can show up early from poor fuel quality. What causes carbon build-up? It is one of the normal by-products of combustion- this carbon forms on the top of the piston and in the combustion chamber. Check out these pictures of carbon build-up for clarification. The problem with build up like this is it increases the compression ratio by making the space inside the cylinder smaller.

How To Fix Carbon Build Up

There are several ways you can go about fixing this. The most cost-efficient way is to add a “pour-in” fuel additive to your fuel tank. These types of cleaners are very easy to use. Your local auto parts store will have various brands to choose from, and you simply follow the directions on the bottle and pour it into the fuel tank. Based on how bad the carbon build up is, this method can take two or more tank fulls of fuel with the treatment before you see results. The next option is an upper intake cleaning. This service is usually performed by a shop and involves injecting a cleaner either through the fuel injectors or intake manifold using a special tool designed to do this. This method gets much faster results, but it does cost more than a few “pour in” fuel treatments.

Lean Air Fuel Mixture

First things first with a lean fuel mixture, you may have a check engine light on for codes P0171 and P0174, a lack of power, stalling concerns, and not to mention the whole reason you’re here, engine knocking. This can be caused by a disconnected vacuum line, leaking intake gaskets, or a weak fuel pump. If the car is running lean, it can cause severe damage if you continue driving it.

How To Fix a Lean Air Fuel Mixture

If you have recently had some work done, consider taking it back to the shop that performed it and asking them to check it out. Often times, during service, a vacuum line can be unintentionally knocked off and cause this concern. If you haven’t had any work performed, consider checking for intake and vacuum leaks. Check out this detailed guide to finding vacuum leaks. If you don’t find any vacuum leaks check fuel pressure.

Ignition Timing Over Advanced

If your ignition timing is over advanced, it means the engine is firing the spark plug too soon. When this happens, the air/fuel mixture ignites too early and causes detonation, which makes the knocking noise you hear. This can be extremely destructive to the engine and should be addressed immediately. 

How To Fix Over Advanced Ignition Timing

If you have recently had some work done, such as a timing belt, distributor, or a tune-up, consider taking it back to the shop that performed it and asking them to check your timing. Most modern-day cars don’t have adjustable ignition timing but can have failed timing components. If the ignition timing on your vehicle is able to be adjusted, a shop can set the timing relatively easily. 

Incorrect Spark Plugs

If the spark plugs that are installed in your vehicle are incorrect they can become too hot and cause the air/fuel mixture to ignite prematurely, causing pre-ignition. This is extremely destructive to the engine and should be addressed immediately. Not only are there lots of different types of spark plugs, there are also different heat ranges of each type of plug. For more information on spark plug heat ranges check out this blog by spark plug manufacturer NGK.

How Do I Fix Incorrect Spark Plugs?

Simply put, install the correct spark plugs with the correct heat range. If you started noticing the concern after a tune-up, it is possible that the shop put in the wrong plugs for your vehicle or the wrong heat range of the correct plug. Consult your vehicle’s service manual and make sure the parts are correct.

Combustion Temperatures Too High

If combustion temperatures get too high it can cause the air/fuel mixture to ignite prematurely causing pre-ignition. This can be a side effect of carbon build up, lean air/fuel mixture, over advanced ignition timing, or incorrect spark plugs. It can also be due to the engine overheating. If the engine only knocks when it’s overheating, resolve the overheating concern and the knocking should go away.

How To Fix High Combustion Temperatures 

If the vehicle is overheating, resolve this concern. If it’s not overheating, check for carbon build up, lean air/fuel mixture, over advanced ignition timing, or incorrect spark plugs.

Faulty Knock Sensor

This is a much rarer cause for this concern, but it can be the cause nonetheless. Most engines are equipped with one or more knock sensors. These knock sensors monitor how much knock your engine is producing and if it becomes excessive, it will adjust the timing accordingly to resolve the concern. If the knock sensor isn’t working correctly, it may not detect the knock, which means the Engine Control Module (ECM) won’t know to adjust timing. This may set a check engine light, or it may set a code but not turn the light on. Check the system for codes P0325 and/or P0330.

How To Fix A Faulty Knock Sensor

Follow the diagnosis procedure for any knock sensor code you have set. If you have no codes and have exhausted all other causes, you may want to have a shop check out your vehicle at this point if the knock concern isn’t resolved. Some knock sensors are extremely difficult to reach and very labor-intensive to have replaced. 

FIXD Research Team

At FIXD, our mission is to make car ownership as simple, easy, and affordable as possible. Our research team utilizes the latest automotive data and insights to create tools and resources that help drivers get peace of mind and save money over the life of their car.

We’re here to help you simplify car care and save, so this post may contain affiliate links to help you do just that. If you click on a link and take action, we may earn a commission. However, the analysis and opinions expressed are our own.

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About the Author

FIXD Research Team

FIXD Research Team

At FIXD, our mission is to make car ownership as simple, easy, and affordable as possible. Our research team utilizes the latest automotive data and insights to create tools and resources that help drivers get peace of mind and save money over the life of their car.

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