S80 Engine Power Loss

Sounds plausible. A faulty EGR valve can definitely cause those symptoms, especially the smoke and rough running. The hissing could indicate a vacuum leak somewhere in the system, which often goes hand-in-hand with EGR problems. Have you checked for any obvious cracks or loose hoses around the EGR valve and intake manifold? Also, when was the last time you had your catalytic converter checked? An overloaded catalytic converter can mimic EGR valve issues.

Thanks for the quick reply! I haven't specifically looked for cracks or loose hoses, but I'll definitely check that out. As for the catalytic converter, I haven't had it checked recently. Is there a way to test the EGR valve itself to be 100% certain before replacing it?

You could try manually operating the EGR valve (if you can access it) and see if it's jammed or sticking. But honestly, given all your symptoms and the age of the car, it's highly likely the EGR valve is the culprit. At this point, I'd recommend taking it to a workshop to get a proper diagnosis. They can run more detailed tests and confirm the issue before you start throwing parts at it. This could save you money in the long run!

Appreciate the advice! I took it to a local mechanic, and they confirmed it was indeed the EGR valve. New valve installed and the bill came to 355€. Runs like a charm again. Thanks!