5 reasons why diesel engines make knocking when idling

Several issues could cause a diesel engine to make knocking sounds or other abnormal sounds when the engine is on and running or idling. A few of these issues are problems with the fuel injectors or dirty motor oil slowing down moving parts. It could also be faulty glow plugs or an old engine that needs flushing and service. Even when there is nothing wrong with the engine, diesel engines are loud. 

why a diesel engine makes knocking sounds when idling

There is an issue with the fuel injectors.

The fuel injector injects fuel from the fuel tank into the engine cylinders. Then it will mix with the compressed air inside the piston chamber and then ignite. When the fuel injectors are not working correctly, they can cause the diesel engine to misfire. It is the misfiring that makes knocking noises.

Not enough motor oil or dirty motor oil coating the moving parts of the mechanic engine.

Motor oil eventually becomes dirty, so a mechanic needs to change it out. The knocking sounds from a diesel engine could be the result of dirty oil. The oil clogs up the many small moving parts of the engine.

The glow plugs inside a diesel engine no longer work

Diesel engines have small devices called glow plugs. These plugs cushion the sounds coming from the piston chamber. Worn-out glow plugs are not doing the jobs they need to and the pistons can now make much more noise.

The engine is old or worn out and the moving parts are dirty or coated with grim.

A worn-out or old diesel engine will make many more noises than a diesel engine that has barely hit 30,000 miles. So the knocking sound could just be all the different parts trying their best to work together.

There is no issue; you are just not familiar with the sounds that a diesel engine makes. 

A diesel engine is much louder than a gasoline-powered engine, even when the ends are both the same size. If you are not familiar with all the unique sounds that a diesel engine makes, it will take a while to get familiar and comfortable with a diesel engine.

What part of a diesel engine makes the knocking sound?

The main cause of knocking sounds coming from inside a diesel engine has to do with the fuel injection system. A diesel engine is similar to a gasoline engine, but they are not the same. 

In a gasoline engine, both fuel and air enter into a piston chamber. The fumes from the fuel mix with each other. The mixture is then compressed inside the piston chamber by a piston. Then the piston compresses the fuel mixture. 

The compressed fuel mixture is set on fire by a spark plug. This is how an engine turns liquid fuel into mechanical energy so that the gasoline-powered car can operate. 

In a diesel engine, fuel and air must also mix inside the piston chamber. But, air enters the piston chamber first. Then the piston compresses the air. After that, the fuel moves into the piston chamber.

Finally, the compressed air and the gasoline fumes ignite to create energy.

The knocking sounds coming from a diesel engine are the result of the pistons compressing the air inside the piston chamber. 

How to fix each of the problems that produce a knocking sound

The fuel injector does not work as it should. It releases too much or too little diesel in the chamber.

An unusually loud knocking sound coming from a diesel engine could be the result of too much or too little seal in the piston chamber. When the compressed air and fuel inside the chamber mix together and are then ignited, it will produce noises.

But the noise will change and become louder when the compressed air to fuel mixture is not the correct level. 

Along with an unusually loud knocking noise, the poorly created mixture will also cause the engine to rattle and shake.

There could be drawing or clogged oil deposits clogging the injectors. But washing or replacing the injectors can solve the issue.

The engine is old, so some parts may need replacement or service

The efficiency of both a gas-powered and diesel-powered engine drops with age and use. If an old diesel engine is not cleaned or serviced in a while, grime can build up around the smallest moving parts of the engine. 

When this happens, these moving parts will not be able to work as effectively as they would if they were clean.

It may also be time to have a diesel mechanic perform an engine flush so they can remove all of the dirty oil that is circulating throughout the engine.

Replace the glow plugs

Replacing the glow plugs around the internal combustion system will mute any strange or loud sounds coming from the engine. You can hire a mechanic to replace the glow plugs or you can do it yourself if you are particularly handy around the engine. 

Just be sure not to snap new glow plugs when installing them.

Motor oil is very dirty, so a mechanic must perform an engine flush.

The amount of damage filthy motor oil can cause to a diesel engine can be irreversible if motor oil is not changed for a long time. Have a mechanic perform an engine flush and replace the motor oil with new clean oil. 

Not only will the knocking sound stop or lessen, you’ll find that the rest of the engine will also perform better. 


There could be many reasons why a diesel engine makes knocking sounds when it is idling. The fuel injectors may not be working because motor oil deposits or grime are inside the injector. Injectors may require replacing or washing.

You should flush out the diesel engine once the mileage reaches around 60,000 miles or 100,000 kilometers. Also, do not underestimate all the different sounds that a diesel engine makes that a gasoline-powered engine does not. Diesel-powered engines are much louder than gasoline-powered ones.

  • Eric Williams

    I'm the founder of Daily Car Tips. I wrote articles in the automotive industry for more than 10 years, published in USA and Europe. I love sharing my knowledge and insights with fellow enthusiasts. Join me on this journey as we explore the exciting world of cars together!

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