I had a few mechanical issues that I took care of on the 06' R53... it fired up and then all of a sudden just died afterwards. The original issue was fixed but now I can't get it to idle...

I changed the battery, that's a new AGM... but it struggles to start/idle. And when it does idle.. I can't raise the RPM.

Something I noticed was that... at idle, it is holding like 4psi of boost. This is indicated not only by my boost gauge but by the ECM datalog.

I am also noticing that the throttle-body isn't doing anything... it's holding steady at 7.5%... or at least that's what it's reporting.

So at this point, I am convinced that my throttle body has probably crapped-out on me... but my question is... why would it be reading boost? The BPV is brand-new directly from BMW... I vacuum tested the diaphragm and spring pressure and it is hooked up correctly.

Thoughts? Why is it giving me boost at idle?

 

The BPV closes when there's no vacuum, usually under acceleration or WOT. If your throttle body remains almost fully closed at idle, you won't have much vacuum.

You would be right about the throttle body being faulty. There's no idle air control valve on those, it's the electronic throttle body that's acting as the idle control. That's why your car stalls when the throttle fails as it won't open enough to allow the car to idle normally. Sometimes, you can turn the car off and on again and the throttle body will start working again, but that's really just a way to confirm that it's dead and you have to replace it anyway.

When mine failed, I bought one from RockAuto, I believe the oem supplier is VDO.

 

So it wasn't the throttle body...

I am getting P1689 - Electronic Throttle Control Monitor Level 2/3 Torque Calculation Error

And there's not too much help online for that...

 

I don't know if it's relevant, but try re-synchronizing the drive-by-wire throttle.
Key in, ignition on, engine not running, floor the gas pedal & hold for 30 seconds, ignition off, key out.