Mercedes CLK diesel engine trouble

Smoke and reduced power definitely sound concerning. A bad crankcase ventilation valve, or PCV valve, can certainly cause those problems. Are you noticing any rough idling, or is the engine running smoothly when you're stopped? If the valve is stuck open, it messes with the air-fuel mixture, which impacts emissions and performance. Have you checked for any obvious leaks or damage around the valve and the hoses connected to it? It is a simple check that might save you a lot of money.

The idle is a little rough, now that you mention it, but it's not terrible. I haven't spotted any obvious leaks, but I'll take a closer look at the hoses. Assuming it is the PCV valve, is it something I can tackle myself, or am I better off taking it to a workshop? I'm reasonably handy, but I don't want to make things worse.

A rough idle strengthens the case for a PCV issue. While replacing it is generally straightforward, on a model of that age some parts might be stuck. If you're not comfortable with that, or if the hoses are brittle and might break, it may be best to have a workshop handle it. Given the emissions and performance problems, a professional diagnosis might be worthwhile. It could save you time and prevent further issues down the road. Also, they will have the correct tools. Is it worth the risk?

Thanks for the advice! I think you're right; I'll take it to a workshop. Ended up costing me 360€, but at least it's fixed now. It was indeed the crankcase ventilation valve. The smoke is gone, the power is back, and the check engine light is off. Much appreciated!