Mercedes CLK running rich issue

I experienced similar difficulties with my Mercedes CLK a while ago. The symptoms you describe align closely with what I encountered. Have you examined the condition of your spark plugs? Rich running conditions can foul them rather quickly. Also, consider inspecting the intake manifold for any leaks, as unmetered air entering the system can skew the fuel-air mixture. Defective fuel injectors are indeed a potential cause, but before condemning them outright, you might want to explore these simpler possibilities.

Thank you for the insights. Regarding the spark plugs and intake manifold, would a visual inspection suffice, or are there specific tests I should perform to ascertain their condition definitively? Furthermore, if the fuel injectors are indeed the issue, is it feasible to test them individually, or is replacement generally the recommended course of action?

A visual inspection of the spark plugs can reveal fouling or unusual deposits indicative of a rich condition. For the intake manifold, listen for vacuum leaks with the engine running; a hissing sound is a telltale sign. While individual fuel injector testing is possible, it typically requires specialized equipment. Given the mileage on your vehicle and the symptoms you are experiencing, it might be prudent to consult with a qualified technician to diagnose the root cause accurately. It is likely a defective fuel injector.

I appreciate you sharing your expertise. I heeded your advice and took my CLK to a reputable workshop. After thorough diagnostics, they confirmed that a faulty fuel injector was indeed the culprit. The total repair cost, including parts and labor, amounted to 515€. The new fuel injectors have resolved the issue completely.