i just tried plugging my valve cover breather hose, and my engine never stalled out after 5 minutes, but when i let go, i got sprayed with oily exhaust!

is this good or bad?

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1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350