Sounds like you may need to adjust your BISS screw closed somewhat. Your BISS screw is allowing too much air through, thus your ISC is completely closed trying to lower your airflow at idle. You want the BISS adjusted so that at your warmed up idle speed you are at 30 steps on the ISC.

If you screw the BISS in all the way and it's still less than 30, then you definitely have vacuum leak(s). If that is the case, a boost leak test is in order.

It's also possible that the FIAV isn't shutting. That wouldn't show up in a BLT, just FYI.