Mini Cooper Diesel Engine Issue

Sounds very similar to what I experienced some time ago with my Mini. Do you notice any rough idle, in addition to the smoke and check engine light? The PCV valve is part of the crankcase ventilation system. It regulates the pressure and redirects blow-by gases back into the intake manifold to be reburned, instead of leaking to the atmosphere. A leak in that system can cause all sorts of issues. Was the smoke black or blueish?

Yeah, the idle was a bit rough sometimes, now that you mention it. The smoke was more on the black side when accelerating. What exactly is 'blow-by gases'? And if it's the PCV, is it something I can fix myself or should I just bite the bullet and take it to a shop?

Black smoke usually indicates a rich running condition. 'Blow-by gases' are unburnt fuel and exhaust gases that escape past the piston rings into the crankcase. If the PCV valve fails (part of the crankcase ventilation), it can definitely cause those symptoms. I replaced mine back then, however, I think you will need specific tools to correctly diagnose and replace the valve. It is not a pleasant job without them. Given the symptoms and the potential consequences of incorrect repair, my advice is to get it professionally inspected. I went to the workshop and the problem was solved.

Update time. Just got the Mini back. It was indeed the crankcase ventilation valve. Cost me 360€, ouch, but the engine's running smooth again. Thanks for the heads up and confirming my suspicions!