Engine Troubleshoot

What Is Engine Knocking (and What Causes It?)

Engine knocking causes
Matt Taylor
Written by Matt Taylor

Engine knocking is a noise that comes from the engine bay and can be caused by bad spark plugs, poor quality fuel, a carbon build up or a problem with the engine timing.

So as you can see, engine knocking is not just caused by bad quality fuel and can in fact point to a more serious engine problem.

What Is Engine Knocking?

Let’s start by defining what exactly engine knocking is. Ideally, during the combustion process, all fuel burns in the chamber producing little to no pollutants. 

However, combustion is only perfect in laboratories under heavily controlled conditions. In reality, the Fuel Injection Electronic Control Unit (ECU) does its best to achieve the best combustion process possible.

Even when it’s not perfect if everything goes as intended the results are within an acceptable margin.

Engine knocking, also known as “pinging”, occurs when the combustion process efficiency drops below those acceptable margins.

engine knocking causes

In an ideal situation, fuel is burned completely during the combustion cycle

The “knock” is a micro-explosion that happens spontaneously before or after the spark plug fires.

Engine knocking is an issue that should be addressed as soon as possible because it could potentially cause severe damage to your engine.

What Causes Engine Knocking?

Now that you have a better idea of what is engine knocking we can take a look at what causes it.

Basically, any issue that affects combustion efficiency can cause engine knocking.

Here are the top causes of problems with engine combustion efficiency:

1. Spark Plug Problems

Arguably, the most common cause of engine knocking is faulty spark plugs.

Spark plugs are disposable by design, so it shouldn’t be a surprise that they can cause a significative drop in combustion efficiency when they are worn out.

symptoms of bad spark plug

Worn out spark plugs can cause engine knocking

Nevertheless, not all spark plug problems are caused by their normal wear. Each engine is designed with a specific spark plug heat range in mind. If you use an incorrect heat range, this could have a negative influence on combustion efficiency and produce engine knocking. 

The same applies to spark plug type. There are many spark plug designs and materials ranging from Iridium, Platinum, Copper, double Platinum, and Silver. You can improve engine performance by using a “better” spark plug. Check out my article on the best spark plugs for more information.

However, if you do the opposite, and install a low-grade spark plug, this will certainly hinder the combustion process.

Finally, there is another factor, the spark plug gap. Most spark plugs come pre-calibrated from the factory yet it’s always a good idea to check that calibration prior to installation. An inadequate spark plug gap will destroy the combustion process efficiency, producing a huge engine knocking problem.

2. Fuel Quality

Another common cause of engine knocking is poor fuel quality. By “poor quality” I mean any factor that affects the combustion efficiency. This could be water in the fuel or low-octane fuel.

If you notice excessive or continuous engine knocking the first thing you may try is filling the tank using a higher octane fuel. You can also try using an octane booster for a while and see if it has any positive affects 

If the knocking disappears then you know for sure you have a fuel problem. Before asking the mechanic for a diagnostic, it's a good idea to check the owners manual and see that you are using the suggested fuel octane.

3. Carbon Deposits

Since the combustion process isn’t perfect, carbon deposits will build-up over time, mainly caused by unburnt fuel.

Intake and exhaust valves, the combustion chamber, piston heads, and spark plugs tips will inevitably be affected by carbon deposits. This can be a real problem in diesel engines, as the fuel combustion efficiency of diesel is much lower than petrol.

So as long as combustion efficiency is kept within the acceptable margins, these carbon deposits will be minimal with little effect on overall performance.

Unfortunately, almost any fuel delivery or ignition system issue will have a direct impact on combustion efficiency and will increase carbon build-up. Spark plugs will be the first victims of carbon deposits. However, carbon build-up will also appear in other areas like the intake manifold, and throttle body.

This is due to oil seeping past intake valves seals as well as the PCV valve (crankcase ventilation). The sum of all these factors will decrease combustion efficiency and produce engine knocking. 

4. Incorrect Ignition Timing

Incorrect ignition timing will also cause engine knocking.

What is ignition timing? Ignition timing has to do with the activation of the spark plug.

For the timing to be correct, the spark plug must fire at exactly the right time. A “retarded” ignition means that the spark plug was released after the piston reached its top dead center (TDC).  On the other hand, an “advanced” ignition means a spark plug fired before TDC is reached. 

The right “timing” is crucial for combustion efficiency. As engine RPM increases the ignition needs to be advanced to buy some additional time for the combustion process.

That’s why a precise ignition timing is so important. Older engines with adjustable timing are more sensitive to this issue, yet, modern engines equipped with Electronic Control Units (ECU) could also be deceived by incorrect signals coming from its sensors and therefore make incorrect adjustments. 

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About the author

Matt Taylor

Matt Taylor

Hey, I'm Matt, co-founder of The Motor Guy.

I've been a car fanatic all of my life. I have worked as a car technician for over 15 years, diagnosing and repairing all sorts of cars. I now devote my time to writing about car maintenance and car detailing.

I also love spending time restoring old BMW motorcycles and trying to outsmart my rescue border collie, Millie.