if that mini freeze plug ain't installed, the eng will run fine & psi will actually be a bit higher as there ain't no filter restriction BUT the eng life will be shortened drastically even with regular oil/filter changes cuz the oil ain't being filtered (the fresh oil does help greatly tho) & the owner will have no idea what caused its early demise until it is later torn down & the missing plug is noted. What Moper said, remove all of em & clean it head to toe several times. maximum attention to detail on every area.