Ben,
It could be something turbocharger related, but your OM-647 5 cylinder engine does not need turbo boost
to start to run at idle speed.
Yes, lack of turbo boost would cause black smoke at higher RPM, but not at idle speed.
You can set your Scan Gauge II to MAP and tell very quickly if you have boost/MAP or not, but you will need to
get it running first.
So, the no start issues is definitely something other than the turbocharger.
You need to find a shop that can scan it for stored DTC's that your Scan Gauge II cannot discover.
Scan Gauge II is a good driving aid, but not so good as a diagnostic scanner.
See if you can find someone with a DAD unit, or take it to an authorized Mercedes Benz or Freightliner Sprinter dealer
(a commercial Sprinter dealer shop if you can find one.
Have you checked the battery voltage....if you've been trying to start your Sprinter without success, the battery will
normally be pretty low.
Check/Change the fuel filter.
Check all the turbo hoses just to make sure they are all connected and don't have holes in them.
Chances are the MB DTC codes stored in your ECM will show a major fault somewhere that's preventing
it from starting until the fault is cleared.
You cannot find this sort of faults with your Scan Gauge II!
You need a factory level diagnostic tool to read the stored codes.
Where are you located?
Some here may know of a good commercial dealer or an independent shop that has the diagnostics capabilities
near where your are located.
Hope this helps,
Roger