Hello,
I have been stumped by this valve for years and today will be solving the problem for good. Jeremy5848 has a good solution in a posting from 2012
http://www.peachparts.com/shopforum/diesel-discussion/318935-om606na-1996-97-pressure-control-flap-removal.html?highlight=om606+pressure+control+flap
He has the opinion that the valves purpose is to lower the pressure in the intake in order to suck more exhaust gas into the "lungs" through the EGR. Therefore if you have stopped the issue with the tar buildup in the system the valve serves no purpose.