you have 2 answers: put winds on descent forecast page aaand
it wont work 'cause its broken on level up.
on zibo, pmdg and real 737 the plane descends at idle. for this work with no problem, you have to insert the descent winds on descent forecast page (fmc, descent, descent forecast. below f310 you insert fl200, 150 and 100 and at or above 310, 200 and 100). on zibo this works fine and pmdg should do too, but on level up for me this only worked 1 time, the first time i did that. so, if the winds are not inserted, theorically the plane should "feel" the winds and and recalculate vnav idle descent without third-party help, but it also wont work on level up. results: the plane will try to maintain the speed on a low-angle slope/path (i guess u can understand what i want to say). it accelerates, then goes idle. it loses speed and again accelerates and its a loop.