Mercedes SL rough running and hissing sound resolved

I understand your concern regarding the issues you are experiencing with your Mercedes. I recall a similar situation with my previous vehicle, a BMW, although it was equipped with one of their diesel engines. The check engine light illuminated, and I noticed decreased performance. A mechanic indicated it could be related to the catalytic converter, but the problem was indeed the EGR valve. Have you checked for any error codes using an OBDII scanner? This might help confirm your suspicion before proceeding with any automotive repair.

Thank you for your prompt response and sharing your experience with the BMW. Indeed, I did utilize an OBDII scanner, and it returned a code indicative of an EGR valve malfunction. I am curious, in your previous experience, did you observe any specific nuances in the engine's behavior that definitively pointed towards the EGR valve as opposed to other potential causes? Any further details you can provide regarding your troubleshooting process would be greatly appreciated.

In my case, aside from the check engine light, there was a noticeable decrease in fuel efficiency, and the engine seemed to hesitate slightly when accelerating. My mechanic mentioned that a faulty EGR valve could also impact the catalytic converter over time, so it's worth addressing promptly. Given that you already have a trouble code pointing to the EGR valve, it seems quite likely that it is indeed the problem. Considering the symptoms and the error code, the best course of action would be to consult with a qualified technician. They can perform a thorough inspection and confirm the diagnosis. It may save you time and prevent further damage.

I appreciate the advice and the confirmation based on your experience. I followed your suggestion and took the vehicle to a trusted workshop. The diagnosis confirmed a defective EGR valve, and the repair, including parts and labor, amounted to approximately 355€. The symptoms have now been resolved. Thank you again for sharing your insights and guiding me toward a successful resolution.