BMD engine oil consumption, burnt spark plug

Hello,

I'm currently trying to repair a Polo with a 1.2L BMD engine. Because it wasn't running well and had no compression, we found cracks in the exhaust valves after disassembling the cylinder head.
We have installed the head and assembled everything. The customer drove 500 km and consumed 2 liters of oil in the process.
Now I had to have it towed because it wouldn't start. The third spark plug was completely burned out! Compression test okay! Leakdown test okay!
Kerzen und die 3. Zündspule erneuert und er lief super. Dann 20km über Land gebrettert, alles was geht. AEverything's great!
Then the engine ran for 15 minutes while stationary in the workshop, and suddenly it completely filled the workshop with oil vapor coming from the exhaust! ????
Okay, I've completely disassembled the engine! The entire intake manifold is full of oil. A pressure test performed right before disassembly showed fluctuating leaks on cylinder number 2, but within the acceptable range.
The engine block is now on the workbench. Now: VW prohibits disassembling the engine block because it is supposed to be stressed, and the crankshaft bearings will then corrode the engine. However, in order to access the connecting rod bearing bolts, the block must be split!
I also suspect a defect in the piston or the rings.

Okay, I read that if the catalytic converter creates too much back pressure in front of the exhaust valves, it can cause the valves to burn out?

Regarding the engine block:
We disassembled it to remove the pistons.
The cylinders exhibit minimal scoring. The pistons and piston rings were not broken or damaged. However, during assembly, Volkswagen precisely aligned the grooves of the oil scraper rings on all pistons. I have now corrected this misalignment.

We put the block back together like this.
Since there were no specified torque values, we used the old connecting rod bolts (20 Nm plus 90 degrees) and the motor block housing bolts (25 Nm plus 90 degrees).
We removed the catalyst for testing purposes.
So far, the engine is running well.

Have you checked the crankcase ventilation system, or its filter/oil separator? I seem to recall that, unlike with a diesel engine, there's one branch before and one branch after the throttle valve that leads into the intake manifold.

Could the engine knocking have been caused by frozen condensate that remained cool during driving and only thawed while the engine was idling in the workshop, leading to a release of oil?

Hallo, die Kurbelgehäuseentlüftung habe ich als erstes erneuert und alles geprüft. Fazit der Überarbeitung des Motors: Totalschaden. Nach längerer Zeit im Stand mit Leerlaufdrehzahl hat er plötzlich angefangen Öl zu verbrennen und eine Zündkerze zerstört. Dadurch sind so viele Teile in den Motor gelangt das alles Schrott ist. Beim Besuch bei einem Motorbauer in Nähe Lüneburg kam die Erleuchtung. Die Kolben bekommen feine Haarrisse unter dem 2ten Kolbenring. WWhen the engine is warm, the crack opens, and oil is drawn in through it, leading to a knocking combustion that, in turn, blows out the spark plug.
I just bought a refurbished engine for €1099, and it's running great.
However, there is still a vacuum of approximately 200 millibars in the engine at idle.

Thank you for your feedback!