[alphawawa]

This may be the best bang for the buck "hack" (meant in the best way possible, as you did) I've ever seen for a BMW repair. Thanks so much for the detailed documentation - great job.

I ended up using a minimal amount of Permatex Ultra Black gasket maker instead of silicone, as I figured there was no doubt that it can withstand the heat cycling involved.

I'll report back here if this hack fails in any way. Fingers crossed.

UPDATE 1: This seemed to work for about 25 miles of driving, but then the squealing sound returned during auto start/stops. I used the smoke test to see if the newly placed cap had a new tear, but it seems to be working just fine, so further diagnosis pending.

UPDATE 2: Since it was relatively easy, I decided to remove the cap and use a good ole' German high temp silicon sealer from Victor Reinz. This solution lasted less than 5 miles of driving even though the smoke test again revealed that there was no sucking occurring at the cap, as the squealing returned again (only during Auto Start/Stop Shutdown)

UPDATE 3: Finally succumbed to trying high temp JB Weld for plastic on the cap, and the squealing remained. Ended up paying a local indie shop to replace the cylinder head cover with an OEM unit. So far, no squealing, and no indicated leaks from the front or back main seals either, so not clear what issue was fixed. Maybe the spring constant of the Amazon CPV is different than that of the OEM-integrated one? Someone else indicated there were other similar pressure-adjusting features of the cylinder head cover, but I could not find any on the OEM cover.

CONCLUSION: This was an easy and inexpensive attempted fix but did not resolve the squealing during Auto Start/Stop shutdown.