Volvo Repair: 2K S80 T6 Lean-biased B1-S1 02 Sensor, obd2 scan tool, intake manifold gasket

Question
Roger, I hope you can help.  The fuel trims are pegged at the upper limit on Bank 1 and I'm not sure where else to turn.  I've replaced front and rear sensors on that Bank.  New MAF sensor.  Replaced intake manifold gasket and leak tested and found no intake leaks.  Replaced the three port exhaust gaskets on Bank 1 and the manifold-to-cat flange and double checked the torque on the oxygen sensors.  New plugs and the coils were swapped with those on cylinders 4-6.  Adding propane doesn't have ANY effect on the trim (+25%).  I've got a decent OBD2 scan tool, but nothing like VIDA/VADIS that will read the actual sensor feedback on the front (before cat) sensors.  I've even recently swapped fuel injectors 1-3 with 4-6 - still no change.  I know this isn't everyone's favorite Volvo, but it's a nice ride when she's well.  Any advice?  Dealer was stumped, as well, but I can't say they let their best guy look at it.

Answer
Joe,
It seems that you have plenty of expertise to intelligently attack this problem, so I'm not sure if I can be of much help. The only thing that occurred to me was the possibility of one or more vacuum leaks. Have you carefully traced all your vacuum lines to see if there is a fault in any of them? I wish I had more help to offer.