What Does It Mean When an Engine "Knocks"?

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 3 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.

3. 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.

Source: (YourMechanic)