What Does It Mean When an Engine "Knocks"?

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A combustion engine is supposed to run smoothly from start-up to shut-down. Occasionally an engine will produce noises that are difficult to diagnose. Sometimes, when you hear a strange sound coming from under your hood, you may not know what to do. A "knocking" sound is the most typical engine noise indicating a mechanical problem that can be fixed if diagnosed correctly and early.

Noted below are a few facts about why an engine knocks and what can be done to resolve these issues.

What is Engine Knocking?

Knocking in your engine, also described as pinging, can mean one of several problems. Some may be easily fixed while others may indicate major damage. The knocking sound often occurs when the air-fuel mixture is incorrect, which causes the fuel to burn in uneven pockets rather than uniform bursts. Left untreated, it can cause damage to the piston and cylinder wall. A knocking sound can also be caused by a lack of lubrication in the upper cylinder head area. This is typically a ticking sound created by valves and lifters that are loose or not receiving enough oil.

In general, the most common cause of engine knocking is related to the engine's ability to operate efficiently. Noted below are a 2 of the common causes of ignition-system and fuel-related engine knocking.

1. Fuel Has a Low Octane Rating

If you put in fuel with an octane rating too low for your vehicle, it can create a knock. An octane rating is the measure of a fuel type’s ability to resist premature detonation of the air-fuel mixture in the engine. The combustion causes the “knocking” or “pinging” sound you hear.

To prevent this from happening, use gasoline with an octane fuel rating at or above what the manufacturer recommends. An octane booster, which can be bought at an auto parts store, can help restore the correct octane rating and stop the knocking.

2. Carbon Deposits Restrict the Efficient Burning of Fuel

Fuels for vehicles are required to have a carbon cleaning detergent, but they may not prevent carbon deposits from forming. As fuel mixes with oxygen, it is burned. Since gasoline and diesel are made up of multiple carbon molecules, the residual of carbon will form on valves, spark plugs, and other components involved in the combustion process. This reduces the amount of volume inside the cylinder and increases the amount of compression.

In most cases, using a special fuel injector cleaner or additive designed to clean carbon deposits on engine components can resolve this issue.

2. Incorrect Spark Plugs or Wrong Spark Plug Gap

If any spark plugs other than what is recommended by the manufacturer is used, it can cause the knocking sound you hear. The spark plug has a certain heat range, which means it withdraws heat from the combustion chamber. Using the wrong part can prevent it from working correctly. It's also common for engine knocking to occur when the spark plug gap is not correctly set.

The spark plug gap is the place where the spark plug ignites the air-fuel mixture that helps power the car. Too narrow of a gap creates a spark too weak to perform this task, and too wide a gap might stop a spark from firing at all, or to only misfire rapidly.

Resolving these types of engine knocking problems is usually rather simple and something most car owners can do themselves. In other instances, it might be prudent to contact a professional technician to correctly diagnose the source of the knocking and recommend the right repairs.


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Jeffrey

25 years of experience
839 reviews
Jeffrey
25 years of experience
Honda Odyssey - Noise from engine or exhaust - Charlotte, North Carolina
Jeffrey nailed it. Other evaluations did not seem correct but his insight led to the correct diagnosis and complete repair. Thank you Jeffrey. Definitely will request him again.
Honda Odyssey - Noise from engine or exhaust - Monroe, North Carolina
Jeff always goes above and beyond to make sure we are satisfied with his work. We are pleased to work with him.

Travis

11 years of experience
309 reviews
Travis
11 years of experience
Mini Cooper Countryman - Noise from engine or exhaust - Saint Paul, Minnesota
It’s always been a great experience. Travis sorted the noise and listened closely for a recurrence. Then we discussed his next visit in a month or two for a serpentine belt replacement.
Mazda 3 - Noise from engine or exhaust Inspection - Minneapolis, Minnesota
Experienced, knowledgeable, and helpful. I had one of those "my car makes a weird noise" issues, and although of course I couldn't get the thing to happen while we zipped up and down the highway, Travis was thoughtful as he took in the details of my explanation of what I'd been hearing. He came up with a few different possibilities, ruled out a bunch of other ones, and by the time the shakedown drive was done we landed on a likely scenario that would explain things. Opening up the hood and checking the condition of a few filters, fluids, and plugs, Travis found some evidence that made the diagnosis even more likely. Now, I have a much better sense of what may be going on, some specific next steps in the plan, and a solid quote for additional work that needs to be done anyway - and going through the same process with the local garage might have taken days, and cost ten times more.

Walter

44 years of experience
59 reviews
Walter
44 years of experience
Honda Odyssey - Noise from engine or exhaust - Glenpool, Oklahoma
He educated me on what I needed to do to take care of my car. He is knowledgeable and so helpful! He’s the best!

Jeremy

18 years of experience
295 reviews
Jeremy
18 years of experience
Volvo S60 - Noise from engine or exhaust - Boynton Beach, Florida
Jeremy G. was very skilled, professional and courteous. My regular/former mechanic was unable to diagnose the problem with noise coming from under the hood. Yesterday, when I was going to drop the car off for the day for the mechanic to check everything as he told me I must, he called in sick. I reviewed the top-ranked mechanics rated online and found YourMechanic. So glad I did! Jeremy G. came determined what the problem was, assured me I would not be charged until the problem was resolved. We made an appointment for today, Jeremy did the repair and showed me the burnt-out part from the car, which is now running smoothly and quietly! I checked the general cost for the same repair/parts replacement and YouMechanic's was comparative and with the bonus of the convenience factor, which is considerable. I would not hesitate to recommend Jeremy at YourMechanic's.

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