Seat Toledo Diesel Engine Issue

Sounds like you've got a crankcase ventilation issue, also known as a PCV valve problem. I had something similar with my Seat. Not sure what year yours is, but these engines are prone to that. The valve gets clogged, causing pressure to build up in the crankcase. That excess pressure can force its way out through seals and gaskets, leading to oil leaks, rough running, and those emissions issues you're experiencing. First, you should check the PCV valve itself and the hoses connected to it for any signs of damage or blockage. Is there any noticeable oil buildup around the valve or hoses?

Thanks for the quick reply! Oil buildup around the valve... good point, I'll check that out. When you say 'blockage,' what am I looking for exactly? Just like, gunk and grime?

Yeah, gunk and grime is exactly what I mean. That valve is supposed to let pressure out of the crankcase, but if it's clogged, it can't do its job. Because you mention smoke, reduced engine power, and a check engine light, it's probably more than just a simple blockage. The valve itself might be defective. Honestly, at this point, it's probably best to take it to a workshop. They can properly diagnose it and replace the valve if needed. Running it like this will only cause more problems with emissions and possibly damage the engine.

Thanks a lot for the help. Took it to the shop, and you were right. The crankcase ventilation valve was busted. Cost me 360€ to get it fixed, but at least it's running smoothly again.