2021 Mercedes-Benz E350 Has Rough Idle

Troubleshooting Causes of Rough Idle in Your 2021 Mercedes-Benz E350 and How to Resolve Them

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Reviewed by
Bo Fader
Written by RepairPal Car Genius
Updated January 14, 2025
0%
21% chance it's the
Spark Plugs
0%
15% chance it's the
Ignition Coil
0%
64% chance it's
Something else
The most common reasons a 2021 Mercedes-Benz E350 has rough idle are a vacuum leak, an issue with the spark plugs, or a problem with the ignition coil.
  • Spark Plugs: Worn or fouled spark plugs can lead to misfires, poor fuel economy, and difficulty starting the engine.
  • Ignition Coil: A faulty ignition coil can cause misfires, rough idling, and difficulty starting the engine due to inadequate spark delivery to the spark plugs.
Proper diagnosis will determine the exact cause of your symptom, saving you money on unnecessary repairs. The diagnostic experts at RepairPal Certified shops have the right tools and experience to solve any symptom.

How much does it cost to fix a 2021 Mercedes-Benz E350 has rough idle?

Common Repairs

The total repair cost will vary based on the condition of your vehicle and the number of repairs needed to fix your symptom. Proper diagnosis will save you money by finding the exact repairs needed to fix your car right the first time.

Brake caliper failure occurs in two different ways, and they create different symptoms.

The caliper can become stuck, in which case it won't release the brake pads. The seized caliper will cause the vehicle to brake constantly on one side, forcing the car to move toward that side. This pulling will be accompanied by a strong odor from one wheel.

Another way calipers fail is by leaking. If the caliper leaks, the brake pedal will feel too easy to push, and will sink all the way to the floor when held. Braking will become less effective the longer this goes on. There may also be brake fluid leaking onto the wheel.

Here are a few common symptoms of worn-out brake pads:

  1. Low brake pad warning light is on: Not all vehicles are equipped with this system, but if yours is, this dashboard indicator light will likely be the first signal it’s time to replace your brake pads.
  2. Brakes start to squeal regularly: This is often one of the first signs of worn brake pads and is caused by a metallic piece built into brake pads for this exact purpose. This part contacts the brake rotor to make a squealing or whining noise when the pads are low.
  3. Brakes make loud grinding noise: If not addressed early enough, your brake pads may get so worn that the metal backing plate of the pad grinds against the metal rotor.
  4. Brake pads look thin: You may be able to visually check your brake pad thickness to determine if they need replacement.

Brake rotors are mainly maintenance free, but when they have been subjected to abnormally hard use, the heat they absorb can cause the metal to turn blue, and cracking to occur. When this occurs, customers often specify they have lost braking power, or the vehicle's brakes are making noise. There may be no symptoms at all related to changing the brake rotors, since it is recommended to change brake rotors on most modern vehicles when performing a standard brake service.

Most often, a brake system inspection will just be a routine portion of manufacturer specified maintenance, and will be performed at mileage intervals. In those cases, there will likely be no symptoms. However, when a brake system makes a funny noise, causes vibration, is hard to use, will not slow the vehicle, prevents the vehicle from driving, makes the vehicle pull to one side, or leaks brake fluid, the vehicle will need a visit dedicated to the inspection of the brake system.

What are the initial diagnostic steps to take for a rough idle in a 2021 Mercedes-Benz E350?

When faced with a rough idle in your 2021 Mercedes-Benz E350, it's crucial to adopt a systematic diagnostic approach. Start by checking for fault codes using a diagnostic scanner, as these codes can reveal specific issues within the vehicle's systems. Next, inspect the spark plugs for any signs of wear or fouling, as these components are vital for proper engine performance and can lead to misfires if compromised. Following this, examine the air filter to ensure it is clean and unobstructed, as a clogged filter can restrict airflow and negatively impact engine operation. Additionally, inspect the vacuum hoses for any cracks or leaks, which can disrupt the air-fuel mixture and contribute to a rough idle. It's also important to assess the fuel system, including the fuel pump, filter, and injectors, to confirm they are functioning correctly. Lastly, take a look at the throttle body for any carbon buildup or obstructions that could hinder airflow. By methodically working through these steps, you can effectively diagnose and address the rough idle issue in your vehicle, potentially saving time and money on repairs.

What are the common causes of a rough idle in a 2021 Mercedes-Benz E350?

When dealing with a rough idle in your 2021 Mercedes-Benz E350, it's crucial to understand the common problems that may be at play. One of the primary culprits could be dirty or faulty spark plugs, which can cause misfiring and lead to an uneven engine performance. Additionally, vacuum leaks can disrupt the delicate air-fuel mixture, resulting in instability during idling. Another potential issue is a malfunctioning mass airflow sensor, which plays a vital role in regulating the engine's air-fuel ratio; if it fails, you may notice a rough idle. Clogged fuel filters can also restrict fuel flow, contributing to the problem. Furthermore, issues with the Exhaust Gas Recirculation (EGR) valve can negatively impact engine performance, while a faulty oxygen sensor can disrupt the air-fuel mixture, exacerbating the rough idle. Addressing these issues promptly is essential for maintaining the performance of your vehicle, and while some DIY fixes may be possible, consulting a qualified mechanic for a thorough inspection is often the best course of action to ensure a smooth and efficient operation of your E350.

How urgent is it to address the rough idle in a 2021 Mercedes-Benz E350?

Addressing a rough idle in your 2021 Mercedes-Benz E350 should be treated with a sense of urgency, as neglecting this issue can lead to more severe engine problems and costly repairs. A rough idle often signals underlying issues within the engine, fuel system, or ignition system, which can progressively worsen if not promptly addressed. For DIY enthusiasts, it’s crucial to recognize that ignoring these symptoms can result in decreased fuel efficiency and potential engine misfires, which not only affect performance but can also lead to significant damage to engine components over time. Regular maintenance and timely repairs are essential in preventing these complications, ensuring your vehicle remains reliable and efficient. By taking action sooner rather than later, you can save yourself from more extensive repairs and maintain the longevity of your E350, making it a worthwhile endeavor for any DIY mechanic.

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2021 Mercedes-Benz E350 Rough Idle: Causes & Repairs