Mercedes-Benz: mystery vacuum coonection

wonder if anyone can identify this vacuum connection on a M102 engine ...
pics are at
www.yeknom.karoo.net

idle was rough until i plugged this vacuum connector tube with a screw ...
but since i doubt there is supposed to be an open tube on the inlet
manifold, i'd be grateful if anyone could tell me what was supposed to be
connected to this. couldn't fnd any evidence of a missing vacuum line.
many thanks.

That probably goes to the bottom part of the engine... above the oil pan...
I have accidently disconnected my C200's and take me two hours to find out.

Keith
E14 4NS
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