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What Is Engine Knocking?

A car engine.
A man filling up his gas tank. Lower octane gasoline can cause engine knocking.
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  • Written By: N. Madison
  • Edited By: Jenn Walker
  • Last Modified Date: 07 April 2014
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Engine knocking is a sound an engine makes that is often described as a metallic, hollow, hammering, or rattling sound. It is typically heard upon acceleration and is often the result of an irregular combustion process. For example, an engine's air-fuel mixture may ignite too early or spontaneously combust. In some cases, improper gas octane is the cause of engine knocking. Sometimes, however, the sounds may be caused by mechanical problems, such as defective bearings, rather than improper combustion processes.

Often, a problem with the combustion process is at the root of engine knocking issues. In some cases, this occurs when the engine's mixture of fuel and air ignite too early. The irregular explosions that result from such problems can lead to damaged pistons, damage to other parts of the engine, and premature engine wear.

Commonly, problems with an engine's air-fuel mixture cause engine knocking sounds to occur. There is an optimal mixture of air and fuel that is good for an engine. Too much air results in a lean mixture, which can cause knocking and damage to the engine.

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In some cases, gasoline problems cause the irregular combustion that leads to knocking sounds in an engine. This can occur because of problems with gasoline octane. Gasoline's octane rating signifies the amount of compression the fuel can handle before spontaneous ignition. When this spontaneous ignition happens instead of the normal ignition, knocking may occur and can cause damage to the engine. The lowest octane gases are typically the least capable of withstanding compression and are more likely to ignite this way.

Engine knocking may also occur because an engine gets too hot. In such a case, the air-fuel mixture may actually ignite without any inducement. Such a spontaneous explosion is possible when the engine's cooling system fails at controlling the temperature of the combustion chamber. The spontaneous combustion that happens in such a case is often referred to as pre-ignition.

Sometimes the sound of engine knocking is caused by something other than the typical combustion process problems. For example, some types of mechanical issues may cause these knocking sounds. Such mechanical issues can include defects in the crankshaft bearings, broken flywheels, and water pump bearings that are worn out. In some cases, a loose timing belt tensioner can cause knocking sounds, as can a problem with the air conditioner compressor. Likewise, an alternator's worn rotor bearings may produce a knocking sound.

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anon342187
Post 5

I have a 1995 Ford Escort 1.9 motor, and the fourth valve seat dropped. I had the engine completely rebuilt. The car still ran badly and had a pinging/knocking sound, and then the timing belt broke. The engine shop replaced the broken timing belt. Then I discovered motor oil was blowing into the air filter. An engine mechanic tore the engine apart and discovered the new seals had deteriorated and were allowing oil into all kinds of parts on the car and burning oil, even though the car wasn't smoking. He re-built the engine again with a new ring kit a second time. These are still the symptoms:

1. Pinging/rattling sound upon acceleration, especially bad uphill. (sounds like a diesel truck)

2. No power uphill, car really struggles.

3. Horrible gas mileage (I used to get 37-40 mpg, but now only get 23-26 mpg).

4. Car shakes and struggles to stay on when I turn on the A/c.

5. Rough idle, especially when the fan comes on, acts like it wants to stall when both the fan and A/C are on.

I have had this car to three Ford service centers and multiple mechanics. There are no codes to read, and no check engine light is coming on. The idle perks up in park or neutral, but the car shakes and runs poorly in drive.

These are the parts I have changed on the car since the engine was rebuilt: EGR valve, oxygen sensor, battery, alternator/regulator, PCV valve, idle air control valve. I have had the catalytic converter checked for back pressure, and it is ok. I have used fuel cleaners and higher octane gasoline. Could the problem be a bad throttle position sensor? Nobody has been able to figure this car out. Do you have any ideas? Please help.

PelesTears
Post 3

Can bad lifters or bent valves cause knocking and pinging? I drive an older Volkswagen and I am having problems with engine knock and ticking. What kind of problems can a stuck lifter cause to my auto engine? What kind of problems can broke lifters or stuck valves cause to my auto engine? I can do the basics as far as car repairs go, but I have never dug deeper than the upper intake manifold. How hard is it to take my lifters and valves out to clean them?

parmnparsley
Post 2

@fiorite- Carbon deposits can definitely cause detonation problems, but I would get a second opinion before paying for the fix.

A well-maintained vehicle that only has 50,000 miles should not have carbon deposits. Carbon build-up takes years to accumulate to the point where it would cause knocking. Most premium fuels also have a number of detergents added to prevent carbon build-ups.

It sounds like your mechanic might be taking you for a ride. I had a mechanic try to tell my wife something similar when she went to drop my vehicle off for service. Little did they know that she grew up in a race family, and knows cars good enough to know when something is wrong with it.

Anyway, carbon causes knocking because of the decrease in cylinder size. This increases the compression ratio beyond what a certain type of fuel octane is capable of handling. In these cases, the knock would start off mild, yet consistent.

They would become progressively worse over time as build-up increases. However, in most cases, the carbon deposits can be cleaned with a solvent applied to the cylinders through the throttle body.

Fiorite
Post 1

Can carbon deposits cause an engine knocking noise? I took my car to a new dealership for an oil change and they said I have built up carbon and my engine has a slight knock. I do not know what a knock sounds like, but the vehicle only has 50,000 miles on it and it has been well maintained. Is this new mechanic taking me for a ride, or should I fork over the $300 dollars for a carbon cleaning?

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