Engine knocking refers to the metallic sound an engine can produce when pressure is applied to the gas pedal. The noise may be a hollow knocking sound or it can be more of a rattling-type sound—in either case, it's not a sound you want to hear.

When an engine knocks, it releases pollution into the environment in the form of nitrogen oxide (NOx) and raw, unburned hydrocarbons (HCs). These two chemicals are poisonous gases that show up as yellowish-brownish in a polluted sky. They can also cause respiratory problems like asthma and emphysema—a knocking engine is never a good thing.

There are some common causes for an engine to make these sounds.

Incorrect Combustion Process
An engine can knock because the combustion process is incorrect in some way. A "spark knock" is the result of combustion occurring too early. In a properly-firing cylinder, the flame front starts on one side of the piston and burns across the top to the other side, which creates a rapid and evenly-expanding gas that pushes down on the top of the piston. When the combustion process occurs too early, there is a random explosion in multiple locations on top of the piston; when this occurs, the flame front inside the cylinder collides with itself and does not burn evenly. An ignition timing that is too far advanced from the proper specification will create a spark knock because the combustion is initiated too early. 

An engine can knock because it is too hot; this is another uneven combustion scenario that is caused by the air-to-fuel mixture "lighting off" by itself. If the cooling system does not keep the engine's combustion chamber temperature in check, the air-to-fuel mixture will begin to spontaneously explode. This is called "pre-ignition" or "pinging."

In addition to cooling system problems, pinging can be caused by improper gasoline octane, an overly lean air-to-fuel mixture, or a lack of proper exhaust gas recirculation. The exhaust gas recirculation system (EGR) was created to neutralize engine knocking by adding a small amount of exhaust gas to the air-to-fuel mixture going in to the combustion process, which limits the peak combustion chamber temperature.

Internal Mechanical Problems
Internal mechanical problems can also cause engine knocking. One such problem stems from excessive clearance inside the bearings in the connecting rods that transfer the downward movement of the pistons to crankshaft rotation. Each time the piston changes direction, there is a knock from the metal hitting metal. This is often referred to as a "rod knock." It is usually very rhythmic; it increases with engine speed and intensifies with engine load.

Other mechanical problems that lead to engine knocking are:

• Defective main crankshaft bearings
• A cracked or broken flywheel or flex-plate that attaches the engine to the transmission
• A worn water pump bearing
• A failed or loose timing belt tensioner can knock when the timing belt slaps against it 
• An air conditioning compressor can knock when it is failing or icing up
  
• An alternator with worn rotor bearings can knock when the pistons fire
  

Be sure to properly inspect and verify the root cause of the knocking sound, as it can be tricky to pinpoint. You don't want to replace the entire engine only to find out that the knocking was coming from a cracked hub on the transmission flywheel or flex-plate. Ouch!

Daniel Dillon has 22 years of experience as a licensed Smog Technician in California. He helped write test questions for the California Smog Technician Exam, and has performed Consumer Assistance Program and gold shield diagnostic work for the state. He also was an instructor for SnapOn Tool Corporation.

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