Thalia EGR Valve Issue

Indeed, your symptoms are indicative of a malfunctioning EGR valve. The confluence of reduced engine power, illumination of the check engine light, concomitant with the emission of smoke during acceleration, strongly suggests a compromised exhaust gas recirculation system. The rough engine running, elevated fuel consumption, and the presence of jerking during acceleration further corroborate this assessment. The hissing sound you perceive may be symptomatic of a vacuum leak within the EGR system, exacerbating the aforementioned issues.

So, a vacuum leak would cause all that? I was pretty sure it was the EGR valve itself being jammed or something. Is there any way to be absolutely certain before I start ripping things apart?

While a vacuum leak can contribute to the observed symptoms, the primary issue, based on your description, remains highly suggestive of a defective EGR valve. The vacuum leak would only amplify the problems created by a faulty valve. To ascertain the definitive diagnosis, it is advisable to conduct a thorough diagnostic assessment at a qualified workshop. The technician can employ specialized tools and methodologies to accurately pinpoint the root cause of the malfunction.

Thanks. Took it to the shop, and you were spot on. EGR valve was toast. Cost me 355€ to get it sorted, but she's running like a champ again. Appreciate the help!