Mercedes-Benz 420SEL Has Rough Idle

Diagnosing Causes of Rough Idle in Your Mercedes-Benz 420SEL and Solutions for a Smoother Ride

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Reviewed by
Devin Cabugcal
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 Mercedes-Benz 420SEL 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 Mercedes-Benz 420SEL 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.

Troubleshooting Rough Idle in a Mercedes-Benz 420SEL

When troubleshooting a rough idle in your Mercedes-Benz 420SEL, it's essential to adopt a systematic diagnostic approach that begins with the simplest potential issues before moving on to more complex ones. Start by inspecting for vacuum leaks, as these can significantly disrupt the air-fuel mixture and lead to an unstable idle. Carefully examine all hoses and connections for any signs of wear or damage. Next, turn your attention to the spark plugs; ensure they are clean and in good working condition, replacing any that appear worn or faulty. If the spark plugs are fine, test the ignition coil, as a malfunctioning coil can cause poor engine performance and contribute to rough idling. Additionally, consider checking other components such as air filters and sensors, as these can also impact engine performance. By following this methodical approach, you can effectively identify and address the root causes of rough idle, ensuring your vehicle runs smoothly and efficiently. Regular maintenance and prompt attention to any warning signs will help prevent future issues and extend the life of your engine.

What Are the Common Causes of Rough Idle in a Mercedes-Benz 420SEL?

When experiencing a rough idle in your Mercedes-Benz 420SEL, it's essential to understand the common problems that could be at play. One of the primary culprits is a vacuum leak, which can disrupt the air-fuel mixture and lead to unstable engine performance. Additionally, issues with spark plugs or ignition coils can significantly affect how smoothly your engine runs; worn or faulty spark plugs may fail to ignite the fuel properly, while a malfunctioning ignition coil can result in misfires and poor acceleration. Low engine compression is another factor to consider, as it can indicate deeper mechanical issues that may require more extensive repairs. Furthermore, a faulty fuel distributor or injector can lead to improper fuel delivery, exacerbating idle problems. Lastly, an incorrect fuel mixture adjustment can throw off the engine's balance, resulting in a rough idle. By diagnosing these common issues, you can take the necessary steps to restore your vehicle's performance and enjoy a smoother driving experience.

Why is a Rough Idle in a Mercedes-Benz 420SEL a Cause for Concern?

A rough idle in your Mercedes-Benz 420SEL should not be taken lightly, as it can signal serious underlying problems that require immediate attention. Ignoring this symptom may lead to a cascade of issues, including diminished engine performance, increased emissions, and reduced fuel efficiency. For instance, a failing ignition coil can exacerbate these problems, potentially resulting in stalling or difficulty starting your vehicle. Moreover, neglecting a rough idle can escalate into more significant and costly repairs down the line, jeopardizing the reliability of your car. Therefore, addressing a rough idle promptly is crucial not only for maintaining optimal performance but also for ensuring your safety on the road and preventing further damage to your vehicle. Taking the time to diagnose and resolve this issue can save you from more extensive repairs and enhance your driving experience.

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