2018 Mercedes-Benz E300 Has Rough Idle

Resolve Rough Idle in 2018 Mercedes-Benz E300 for a Smoother Ride

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Reviewed by
Andrew Hajjar
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 2018 Mercedes-Benz E300 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 2018 Mercedes-Benz E300 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.

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.

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 2018 Mercedes-Benz E300?

When faced with a rough idle in your 2018 Mercedes-Benz E300, it's essential to adopt a systematic diagnostic approach to identify the root cause effectively. Start by checking for fault codes using a diagnostic scanner, as these codes can reveal specific issues within the vehicle's computer system. Next, inspect the spark plugs; ensuring they are clean and functioning properly is crucial, as worn or fouled plugs can significantly impact engine performance. Don't overlook the air filter; a clogged filter can restrict airflow, leading to an uneven idle. Additionally, examine the vacuum lines for any leaks or damage, as these can disrupt the air-fuel mixture. It's also wise to assess the fuel system, including the fuel filter and injectors, to ensure proper fuel delivery. Finally, take a look at the throttle body for any buildup that could hinder its operation. By following these steps in order, you can effectively diagnose and address the rough idle issue, ensuring your vehicle runs smoothly.

What are the common problems that could cause a rough idle in a 2018 Mercedes-Benz E300?

When diagnosing a rough idle in a 2018 Mercedes-Benz E300, it's essential to consider several common problems that could be affecting the vehicle's performance. One of the primary culprits is a dirty or faulty mass airflow sensor, which can disrupt the delicate balance of the air-fuel mixture necessary for smooth engine operation. Additionally, vacuum leaks in the intake system can create an imbalance, leading to an unstable idle. Worn or faulty spark plugs are another frequent issue, as they can cause misfires that result in a rough engine idle. A clogged fuel filter may also be to blame, as it restricts fuel flow, impacting overall engine performance. Furthermore, problems with the idle air control valve can disrupt the engine's idle speed, while a malfunctioning oxygen sensor can adversely affect the air-fuel mixture, compounding the rough idle issue. By addressing these potential problems through regular maintenance and timely repairs, you can significantly improve the idle quality of your E300 and enhance its overall performance.

How urgent is it to address the rough idle in my 2018 Mercedes-Benz E300?

When it comes to your 2018 Mercedes-Benz E300, addressing a rough idle should be treated as a priority rather than a mere inconvenience. A rough idle can indicate serious underlying issues such as faulty spark plugs, a clogged fuel filter, or a malfunctioning oxygen sensor, all of which can compromise your vehicle's performance. Ignoring these symptoms not only risks decreased fuel efficiency and engine misfires but can also lead to significant damage to engine components over time, resulting in costly repairs. For DIY enthusiasts, it’s crucial to recognize that regular maintenance and timely repairs can prevent these escalating problems. If you notice your engine idling roughly, consider conducting a thorough inspection of the spark plugs and fuel system, or consult a qualified mechanic for a comprehensive diagnosis. Taking prompt action not only enhances your vehicle's performance but also ensures its longevity, making it a worthwhile investment in your automotive care.

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