Mercedes-Benz S400 Has Rough Idle

Troubleshooting Causes of Rough Idle in Your Mercedes-Benz S400 and How to Resolve Them

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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 S400 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 S400 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 S400

When addressing a rough idle in your Mercedes-Benz S400, it's essential to adopt a systematic diagnostic approach. Begin by checking for vacuum leaks, as these can significantly disrupt the air-fuel mixture, leading to an unstable idle. Next, utilize an OBD-II scanner to identify any error codes that may indicate issues with spark plugs or ignition coils, which are common culprits of misfires. If misfire codes are present, consider swapping the coil packs between cylinders to pinpoint any faulty components. Additionally, inspect the fuel system by monitoring fuel pressure at idle and during slight accelerations to detect any irregularities in fuel distribution. If your vehicle has high mileage, replacing spark plugs can be a proactive maintenance step. By following this methodical approach, you can effectively diagnose and resolve the rough idle issue, enhancing your vehicle's performance and driving experience.

What Could Cause a Rough Idle in a Mercedes-Benz S400?

A rough idle in your Mercedes-Benz S400 can stem from several common problems that are often manageable with some DIY effort. One of the primary culprits is a vacuum leak, which can disrupt the delicate balance of the air-fuel mixture, leading to an unstable idle. Inspecting the vacuum lines for cracks or disconnections can be a straightforward task that may resolve the issue. Additionally, worn or faulty spark plugs are notorious for causing misfires, which can manifest as a rough idle. Replacing spark plugs is a relatively simple maintenance task that can significantly improve engine performance. Another potential issue is a malfunctioning ignition coil, which can also lead to misfiring and rough idling. Testing the ignition coils for proper function and replacing any that are defective can restore smooth operation. By addressing these common problems, you can enhance the performance of your S400 and enjoy a smoother driving experience.

Why is My Mercedes-Benz S400 Idling Rough and What Should I Do?

When your Mercedes-Benz S400 is idling roughly, it’s crucial to act swiftly to avoid further complications. Ignoring symptoms like excessive shaking, fluctuating RPMs, and decreased fuel efficiency can lead to more severe engine problems, potentially resulting in costly repairs. A rough idle often indicates underlying issues such as vacuum leaks, faulty spark plugs, or ignition coil malfunctions, which can compromise your vehicle's performance and safety. For DIY enthusiasts, conducting a thorough inspection is essential; start by checking for vacuum leaks and examining the condition of your spark plugs and ignition coils. Regular maintenance, including timely replacements of air filters and monitoring sensor performance, can significantly reduce the risk of rough idling. By addressing these issues promptly, you not only enhance the longevity of your vehicle but also ensure a smoother and safer driving experience.

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