Engine Knocking: Understanding Causes and Solutions

There are lots of parts in an internal combustion engine and many of them are moving very fast when the engine is running. It’s not unusual for an engine, especially an older one, to make some unusual knocking or tapping noises. What causes these knocking noises and are they easy to fix?

Engine knocking noises that come from the engine can be caused by bad spark plugs, poor quality fuel, a build up of carbon within the engine or a problem with the engine timing.

Sometimes the knocking noises from an engine can come and go, depending on the temperature of the engine or they may only be present when the engine is idling or when revving.

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When it comes to the diagnosis of engine knocking it’s important to identify the circumstances and environmental factors that may be present so that the severity of the engine knocking can be figured out.

What Are The Reasons Why Your Engine Is 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.

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.

6 Common Causes of Engine Knocking

Any issue that affects the combustion efficiency of the engine can cause engine knocking. It can also be caused by a lack of adequate lubrication within the engine that reduces the smoothness of the moving parts.

Here are some of the causes of engine knocking in more detail.

Cause 1: Spark Plug Problems

One of the most common causes 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 significant drop in combustion efficiency when they are worn out.

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 the 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.

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 an engine-knocking problem.

Cause 2: Fuel Quality

Another common cause of engine knocking is poor fuel quality. Poor quality fuel refers to any factor that affects the combustion efficiency of the fuel. This could be a contaminant such as water in the fuel or a 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 owner’s manual and see that you are using the suggested fuel octane.

Cause 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 plug 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 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. 

Cause 4: Incorrect Ignition Timing

Incorrect ignition timing will also cause engine knocking. 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 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. 

Cause 5: An incorrect fuel air mixture

The amount of air entering the engine should be closely monitored by the mass air flow sensor. If there is a vacuum leak or a problem with the idle air control valve then the air volume readings won’t be accurate and this can lead to a lean fuel/air mixture in the combustion chambers.

Problems with the fuel system such as a blocked fuel filter, a dying fuel pump or blocked injectors can also affect the flow of fuel to the engine, resulting in a lean mixture.

Bad oxygen sensors, damaged valve seals and blocked catalytic convertors can also affect the fuel mixture, causing a lack of fuel that will affect the combustion process and lead to engine knocking.

Cause 6: A Bad engine knock sensor

Most internal combustion engines are fitted with a special sensor that actually monitors the engine for abnormal knocks. This sensor, the engine knock sensor will register an error with the ECU if it detects knocking outside of a frequency range.

The engine knock sensor is usually located on the engine block and it uses a piezoelectric measuring sensor to detect knocking from within the engine crankcase. The ECU can use signals from this sensor to make adjustments to the engine timing to help manage the engine knocking.

A faulty engine knock sensor can cause engine knocking if it is not accurately monitoring the engine as it runs. If it is not operating properly then it will usually trigger a check engine light on the dashboard.

The best way to diagnose a faulty engine knock sensor is to run a diagnostic using an OBD-II reader to check for code P0325 that may indicate a circuit malfunction within the sensor.

What Does Engine Knocking Feel Like?

Engine knocking can feel like a knocking or tapping sensation in the engine, often accompanied by a metallic sound.

The knocking can be felt through the steering wheel or floor and is often more pronounced when the engine is under load, such as when accelerating or climbing a hill.

In some cases, the knocking may also be accompanied by a loss of power or a decrease in fuel efficiency.

What Next – How do you fix a knocking engine?

Depending on the cause of the engine knocking, it may be possible to reduce or eliminate it completely by carrying out a few basic maintenance checks.

The first step in resolving engine knocking should always involve running an engine diagnostic using an OBD scanner tool. This will allow you to view any engine error codes that have been stored in the ECU memory.

Sometimes engine knocking can trigger a check engine light and this may flash on and off depending on the temperature of the engine or when the engine is idling or when driving. An ECU readout may help you pinpoint whats going on as engine knocking can have many causes.

If there are no engine error codes, then there are a few other things to try before stripping down the engine to examine the cylinders.

1. Try using a higher octane fuel or fuel additive. Low octane fuel can sometimes cause engine knocking because depending on the type of engine or the condition of the spark plugs the fuel may be harder to ignite. Adding a fuel additive or choosing a fuel with a higher octane can sometimes eliminate engine knocking if the fuel burns easier and more thoroughly.
2. Check and change the spark plugs. Its worth checking the condition of the spark plugs to see if their condition is contributing to a problem with fuel combustion that may lead to engine knocking. If the spark plugs have not been changed in a long time they may be simply worn out or may be damaged by carbon deposits or oil if there is a problem with one of the engine seals.
3. Check timing belt for wear. The timing of the engine can also cause engine knocking, so checking the timing belt for excessive wear may help to resolve the issue. It’s also a good idea to check the pulleys and tensioners too. Worn out belt pulleys will usually make a noise and tensioners should also be in good condition. Sometimes auxiliary belts that run ancillary components such as the air conditioning and alternator can cause engine knock if they restrict the crankshaft rotation due to wear, so it’s worth checking them too.
4. Check the condition of engine sensors. It’s also a good idea to check engine sensors that are involved with the fuel air mixture. The mass air flow sensor can get dirty or lose its sensitivity over time, and this can affect how well the engine runs. Other sensors such as the oxygen sensors and the engine knock sensor will also affect the fuel air mixture as they monitor the engine when it’s running. It may be worth swapping out these sensors to see if this improves the engine and reduces or eliminates engine knocking.

It is possible that the engine knock is caused by damage or wear and tear to the engine. A common cause of engine knock that can’t easily be resolved is rod knock. Rod knock is not easy to fix and it usually involves stripping down the engine to get access to the engine rods and rod bearings so that they can be replaced.

FAQ – Engine Knocking

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