2017 Engine Knocking What is that Chattering Sound?

You may have heard it. When it's hot outside and you are slowly driving up a hill and, all the sudden, you hear a curious chattering sound emanating from underneath the hood. If you let up on the gas, the noise seems to go away, but push the pedal down and the chattering starts up again. What you are probably hearing is what many mechanics call "knocking." It's a situation where the air-fuel mixture in the cylinders self-ignites and "hits" the flame front generated by the spark plug. We will explain this further but the important thing to know is if you allow knocking to occur long enough, it can damage your engine.

What is knocking?

Internal combustion engines develop power by burning a mixture of air and fuel in the engine cylinders. When the fuel mixture is ignited by a spark plug, a piston is driven down and that motion eventually gets coupled to the vehicles wheels and makes it move. The problem comes when an engine is really hot and the air-fuel mixture ignites itself. According to Lustine Toyota of Woodbridge, a local Toyota dealer in Woodbridge, VA, this usually happens the same time that the sparkplug fires. Now have two separate flame fronts hitting each other in each cylinder.  Those flame fronts smashing into each other create such a violent collision that you can hear it outside the engine as knocking.

That's a problem

Flame fronts hitting each other does more than make curious chattering noises. Believe it or not, the pressure developed will eventually damage the pistons. In fact, it will burn holes right through the top of them. Scary part is that it doesn't actually take long for that to happen; Sometimes just an hour or so of consistent knocking. Let's take a look at ways to prevent knocking from happening.

Gas octane

SEE ALSO: Wheels done right: team SCHMIDT showcases its first project for the new year!

Your car's engine is designed to run on gas with a certain octane rating. This rating is set by the manufacturer of the car and is noted in the owner's manual.  If you want to avoid knocking, stick with the octane gas that the car maker recommends.

Deposits on the cylinder head

When a mixture of gasoline and air is ignited, there is a chemical reaction that produces CO2 and H2O. In practice, small amounts of other gases and soot are produced too, most of which are blown out the exhaust. However, the tiny amount of soot that stays behind eventually forms "carbon deposits." These are spots of carbon that glow red in a hot engine. When this happens, it can ignite the gasoline air mixture before the spark plug does. The result is knocking. Removing carbon deposits is something that requires a professional mechanic.

The wrong spark plugs

According to Edmunds.com, using the incorrect spark plugs is common. Since the spark plugs operate under precise conditions, the wrong plugs can mess up the combustion cycle and cause knocking. The solution to this is to remove the wrong sparkplugs and put in the right ones.


If you ever find your engine knocking, stop and find out why. In most cases, getting to a gas station and putting in a higher octane gas will cure the problem long enough that you can have a professional mechanic look at it.