Mercedes Bad Diesel Injector Symptoms: What You Should Be Aware Of

Giancarlo Perlas November 17, 2023

Of all the engine components, diesel injectors in your Mercedes-Benz are some of the most prone to creating performance issues. This is because of the nature of the diesel fuel and its proneness to leave behind carbon deposits when not fully combusted. This is why, as a driver, you have to recognize bad diesel injector symptoms and intervene adequately.

The injectors are integral for the proper functioning of the engine. They are responsible for atomizing the diesel fuel into the engine cylinders with the right accuracy and timing. This enables the engine to perform combustion and provide energy for the vehicle.

So, we will discuss some of the troubleshooting diesel injector pump issues and how to take adequate measures.

A person checking the injectors.
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Bad Diesel Injector Symptoms

You must know how the car functions normally to recognize the most glaring symptoms. Having an idea of how the engine runs normally, the sounds it makes, and the performances it can produce will enable you to immediately notice any change or inconsistency in its function.

A timely reaction to a problem with the injectors can be the difference between costly engine damage and a cheap one. A prompt intervention will also save you a lot of time, as extensive damage to the engine will require a much longer repair process. 

The problems arising from a late intervention can make your vehicle inoperable for a long time and cause various inconveniences.

So, experienced drivers frequently perform maintenance checks, as some symptoms are difficult to notice. Even if everything seems fine, staying vigilant and performing inspections is the proactive way to have a smoothly running vehicle.

You can discover the highest quality used fuel injection pumps at Goldfarb & Associates by searching through their reliable and inspected products. They take pride in offering their customers high-end products at the best possible prices.

Here is how to notice signs of faulty diesel injectors in your Mercedes:

1. Rough idling

Rough idling is one of the most common diesel injector malfunction signs. What causes rough idling is incomplete combustion, usually due to a malfunction in the injector. Clogging due to carbonization can prevent the injector from injecting the right amount of atomized fuel into the cylinder.

If you know your car well enough, you’ll have no problem noticing it’s going through some issues when idling. You’ll detect unusual vibrations and an uneven engine sound without smoothness and consistency.

Rough idling may indicate issues like clogged air filters and worn-out glow plugs. You will need to conduct a thorough inspection to determine that the rough idling is a manifestation of problems specifically related to the injectors. 

You’ll also need to check for other accompanying symptoms to help you diagnose more accurately.

2. Fuel economy

Problems with the diesel injector will worsen your vehicle’s fuel economy. As the injectors spray fuel with less consistency and accuracy, much of the fuel goes to waste, requiring more to be used to finish the production demand.

Diesel engines need a specific balance between the amounts of fuel and air to execute an effective combustion. With just one malfunctioning cylinder, fuel delivery can lead to an imbalanced fuel-air mixture.

Having more fuel in the mixture will lead to what is called a “rich mixture”, which is bad for fuel efficiency and increases emissions while delivering a bad performance. This can happen if the diesel injector gets stuck, spraying fuel longer than necessary.

Conversely, too much air in the chamber will lead to a weak mixture, which will also deliver poor combustion and cause overheating. This happens when the injector is clogged and doesn’t deliver enough fuel into the cylinder.

A mechanic holding car parts

Image credit: Henadzi Pechan on

3. Excessive exhaust smoke

Too much smoke is easily noticeable; it points to underlying issues with your vehicle. You can tell a lot about the problem by observing the excess smoke, with both the smoke’s color and smell indicating specific causes.

Excessive smoke usually indicates a rich air-fuel mixture, with a stuck open injector being the root cause. If you’ve noticed an excess of smoke, the chances are high that the fuel economy has already worsened and that there are other symptoms that you might not have noticed yet.

4. Difficulty starting

If you’re having problems starting the car, then the chances are high that you have underlying injector issues. 

In diesel engines, ignition occurs due to the right amount of air compression in the cylinder, which causes it to reach high temperatures and pressure. This should ignite the fuel in the cylinder, but only if there is the right amount of it.

A faulty injector won’t deliver the necessary quantity of fuel, which would cause problems with the ignition even if the pressure levels are sufficient. Repeated hard starting attempts may cause issues with the motor and its components, leading to premature wear.

Diagnosing the reason for your difficulties with starting the car is crucial as it will help you determine the next step in the process. Accurate diagnosis enables you to perform appropriate repair or maintenance procedures.

Besides injector issues, experiencing hard starting can also mean issues with different parts like glow plugs, fuel pumps, alternators, etc.

Maintenance Tips

Extending the vehicle’s life should be among your priorities, and it’s only possible through proper maintenance. You can take numerous measures to ensure that your vehicle remains in excellent condition for as long as possible and avoid diesel injector pump troubleshooting.

Here’s what you can do:

  • Regular inspection
  • Use quality fuel
  • Frequent filter replacement
  • Professional checkup
  • Staying vigilant for any symptoms
  • O-ring replacement
  • Timely intervention to any problems

Performing periodic injector cleaning with specialized cleaning solutions to remove carbon deposits and contaminants can improve the performance of your engine. It will ensure that there’s a timely and precise delivery of fuel. 

Also, the fuel filters require changing at the mileage-based frequency designated by the manufacturer.

An injector.
Image credit: 1001 slide on


The injectors perform a key role in the combustion process by injecting atomized fuel into the cylinder. The fuel delivery has to be on time and of the right quantity. Otherwise, the vehicle will suffer from decreased performance.

That’s why it’s important to be able to recognize indications of bad diesel injectors and intervene adequately. The most frequent bad diesel injector symptoms include rough idling, excessive exhaust smoke, and difficulty starting. Giving an accurate diagnosis requires examining the issue further and performing diagnostic tests.

Proper maintenance helps prolong the life of the car. It includes doing frequent inspections, using quality fuel, and handling any arising problems on time.

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About Author

Giancarlo Perlas

Giancarlo Perlas is an economist by profession with a career spanning nearly two decades. His professional journey has seen him assume vital roles in various government and private organizations, particularly in the blockchain and cryptocurrency sectors. Alongside his civic and corporate pursuits is his love for cars, particularly those made by Mercedes-Benz. In 2012, he found himself with like-minded individuals within BenzInsider. From then on, he used the platform as a way to share his passion with the automotive community. Follow his Facebook page at, X (formerly Twitter) @giancarloperlas, and IG @benzinsider. View all posts by Giancarlo Perlas →

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