Hi, since I was working to tidy up the engine a bit, check the vacuumhoses, change oil etc, I removed the oilfiller neck, since it is about the dirtiest part in the bay and wanted to clean it.
When I looked in the case with a flashlight to see what it looked like in there, I saw a small plastic cap laying in there.
It does fit in the oilfillerneck base bottom, see pic, so I guess that is where it belongs.
I took out the Haynes manual but I cannot find this part mentioned.
What is it for ? Why is there a capped hole in there ?
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Looks like stopper for the breather valve, part # 022 115 486. #9 in diagram

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Great, that is the one. Thanks ! I will have to check if nothing is missing there...
I cannot find it in the Haynes, where did that diagram come from ?
From the partnumber you mentioned I found post
http://www.914world.com/bbs2/index.php?showtopic=19995
so it happens to others as well :-)

What does the valve in there do ? regulation of crankcase pressure ?

Ok, checked all in the diagram and all is there.
So I cleaned it up, all back together, install new gasket and the plug in.
Small job, always nice to have some result and another thing off the to-do list.
oh...and nothing else broke while doing this, that is also a good thing
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Great to see that you have well sorted the breather! Not sure what the valve does, likely it is for gases to flow only one direction. The parts diagrams and info are available at the Porsche website here:
http://www.porsche.com/all/media/pdf/origi...914_KATALOG.pdf

PCV, or Pressure Control Valve. It's for crankcase ventilation. It allows gases to escape the engine case.