Peugeot 806 diesel engine issues

Sounds familiar! I had a similar issue with my old Alfa Romeo 156. Same symptoms: rough starting, poor idling, and a noticeable drop in power. In my case, it was the fuel injectors. They were clogged up. A clogged fuel injector messes with the fuel-air mixture, which is why you're seeing those issues. Have you noticed any black smoke or a strong fuel smell?

Yeah, actually, now that you mention it, there's been a bit of black smoke on startup. And a slight fuel smell sometimes. Clogged injectors, eh? How did you figure that out? And what did you do to fix it?

The black smoke and fuel smell are telltale signs. I initially suspected it because of the symptoms, then a mechanic confirmed it. Injectors spray fuel into the combustion chamber. When they're clogged, they don't spray evenly. It's like trying to water your garden with a hose that's got a kink in it – you won't get even coverage. Honestly, I wouldn't mess with it yourself unless you really know what you're doing. Modern diesel injection systems are quite sensitive. I'd recommend taking it to a workshop and having them properly cleaned or replaced.

Thanks for the advice! I took it to the workshop, and you were spot on. Turns out, one of the injection nozzles was defective, most likely because it was dirty. They replaced it, cleaned the others, and now it's running like a champ again. Cost me 515€, but worth it to get rid of those problems!