Mercedes GLK Diesel Engine Trouble

I had similar symptoms on my 2012 BMW 320d. Reduced power, check engine light, and rough running point towards a potential EGR valve issue. The hissing sound could indicate a leak in the EGR system, explaining the smoke and jerking. Have you checked the EGR valve itself for carbon buildup? A diagnostic scan would confirm if the EGR valve is the culprit. These malfunctions are common in diesel engines.

I haven't inspected the EGR valve yet and I am not sure of its location. Can it be cleaned, or does it need replacement? What diagnostic scan tool would you recommend to check the error codes? I'm trying to avoid a big bill if I can check it myself.

Cleaning the EGR valve is sometimes possible, but replacement is often more reliable. A generic OBD-II scanner should retrieve the error codes. For Mercedes-specific codes, a scanner with Mercedes-Benz compatibility is preferable. Given the hissing sound and smoke, I would lean towards replacement. I'd suggest getting it checked at a workshop and ensure they check the catalytic converter as well. It's important to address this issue promptly as it affects vehicle emissions.

I took the car to the workshop, and you were right. It was the EGR valve. The mechanic confirmed it needed replacement. The total repair cost was 355€. The car now runs smoothly, the check engine light is off, and the smoke is gone. Thank you for your accurate diagnosis and advice.