Who’s there? Three common causes of engine knock

A problem you should definitely fix
by Jason Tulio | Jun 23, 2019
PHOTO: DokaRyan/PIxabay

Hear that? That knocking sound from under your hood?

Okay, we hope you don’t actually hear anything aside from the sound of your engine running smoothly. But alas, as any car owner knows, things can and do break down as your car ages, sometimes leading you to suddenly hear strange noises that weren’t there before.

If you hear a sort of rattling sound from your engine bay, this could be something that is commonly known as engine knock. It can be caused by any number of things, but generally, it has something to do with the engine’s ability to operate efficiently.

What can cause engine knock?

Engine knock can often be a result of incorrect air-fuel mixture inside the engine, which then causes the fuel to burn in the cylinders unevenly rather than in synchronized bursts. Another reason could be a lack of lubrication in the upper part of the cylinder heads, resulting in the valves and lifters coming loose or not getting enough oil, which can lead to a certain ticking sound.

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Below are three common culprits leading to the dreaded engine knock, plus a few simple fixes you can do to address the problem.

1) The fuel you’re using has a low octane rating.

Notice how there isn’t just one type of fuel option whenever you pull up to a gas station? Instead, you have a variety of choices, each one assigned with its own octane rating. A higher octane rating means that kind of fuel can withstand more compression before igniting. If you use a type of fuel that’s lower than your manufacturer’s recommended rating, it can lead to engine knock at first and whole host of problems for your engine later on. Check your car’s owner’s manual for the recommended octane rating for that particular make and model.

A good rule to remember when choosing fuel (which we’ve shared before) is: The fuel brand along your daily route with the octane rating that gets you the best fuel economy for your driving environment and habits is the fuel that’s best-suited for your car.”

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2) There are carbon deposits in your engine.

Fuels have carbon cleaning agents in them, but they’re not a guaranteed solution to carbon buildup. As the fuel and oxygen in your engine mix and burn, carbon deposits can naturally form, and these stay on the valves and other areas. When this happens, there’s less room for the fuel and air, resulting in a change in the compression process. As with lower-octane fuel, this can cause engine knock. To fix this, you can opt for a special injector-cleaning additive, or you can take your car to a shop for more hands-on cleaning. 

3) You’re using the wrong spark plugs.

Spark plugs operate within a certain heat range, so using the wrong kind for your engine can prevent it from working properly. The combustion process is actually started by the spark plug, so using the wrong kind to begin with can lead to all sorts of problems. Again, check with your manufacturer to find out what kind of spark plugs you should be using.

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Another thing that can cause engine knock is when the spark-plug gap is not correctly set. The spark-plug gap is where the spark plug ignites the air-fuel mixture from the engine. Too narrow a gap, and you end up with a spark that’s too weak. Too wide, and the spark might either not fire at all or misfire entirely.

Take note that these are just the common causes for engine knock. To be completely sure about what’s causing that sound in your engine bay, it’s best you consult a mechanic to properly diagnose and fix your car.

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PHOTO: DokaRyan/PIxabay
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