Experiencing rough idling in your 2018 Mercedes-Benz C-Class can be frustrating. There are several potential reasons for this issue, and addressing it requires a systematic approach. Here's how you can troubleshoot the problem:

1. Check the Fuel System:

• Ensure that the fuel filter is not clogged and is allowing proper fuel flow.
• Inspect the fuel injectors for any buildup or clogging that might be affecting performance.
• Verify that the fuel pump is functioning correctly to maintain adequate fuel pressure.

2. Inspect the Air Intake System:

• Examine the air filter for dirt or obstructions that might hinder air flow into the engine.
• Check for any vacuum leaks in the hoses connected to the intake manifold.
• Look for any issues with the mass airflow sensor (MAF) that can affect the air-fuel mixture.

3. Evaluate the Ignition System:

• Inspect the spark plugs for wear or damage, as worn plugs can cause misfires.
• Check the ignition coils for any malfunction that might lead to poor sparking.
• Ensure that the ignition timing is correctly set.

4. Consider the Exhaust System:

• Verify that the exhaust system is not blocked or restricted, which could affect engine performance.
• Check the catalytic converter for clogs or issues that may be causing back pressure.

5. Analyze the Engine Control Unit (ECU):

• Use an OBD-II scanner to read any error codes stored in the ECU that could indicate specific sensor malfunctions.
• Ensure all sensors are functioning correctly, including the oxygen sensors and throttle position sensor.

6. Examine the EGR Valve:

• Ensure the Exhaust Gas Recirculation (EGR) valve is not stuck open or closed, which can affect idle smoothness.
• Clean or replace the EGR valve if necessary.

7. Inspect the Belts and Pulleys:

• Check for any wear or damage on the serpentine belt, which can affect accessories critical to smooth engine operation.
• Ensure all pulleys are in good condition and operating quietly.

8. Schedule Regular Maintenance:

• Follow a regular maintenance schedule for oil changes, air filter replacements, and inspections to prevent issues.
• Keeping the vehicle well-maintained can help avert problems that cause rough idling.

If these checks do not resolve the rough idling issue, it's advisable to take your vehicle to a professional mechanic or a Mercedes-Benz authorized service center for a more in-depth diagnosis and repair. Accurate identification of the root cause is essential for effective fixing, preventing further damage to your vehicle.