These are some common causes for it running lean, as you know;
- Unmetered air leaks, torn intake boot, leaking intake manifold or gaskets&seals
- Crankcase vent system fault, system valve stuck open
- Low fuel pressure, pressure regulator malfunction
- MAF malfunction, sending incorrect signal information
Unmetered Leaks, but there's more; includes metered air leaks. It can be leaks post/after turbo...up to and including the intake manifold.
If under boost, wouldn't a leak post turbo cause the engine to run rich? Or not be able to meet requested boost levels? I've checked all connections from the air filter to the throttle body. Valves were cleaned in February and a new manifold gasket was installed at the same time.
Crankcase function; sort the possibilities of leaks which you plan to do.
Where else besides the front PCV and rear breather should I check? I'm holding just under 20 Hg at idle.
Fuel pressure; can be fouled/dirty injectors. Failed (low pressure)regulator in the filter or failed high pressure regulator in the hpfp. Fuel data should sort the pressures. The difficult test with VCDS is pressure bleed off because each time we use VCDS, the ignition is turned to run and the pump primes. This is one way to check for bleed off but not many have this set up. Also, it is for checking difficulties when starting, but the concept is ideal for checking drops in fuel pressures in both the high and low sides of the fuel system;
Brand new injectors installed back in February. New fuel filter last year. Running latest rail sensor (red/orange) which was installed last year. Pressure sensor on the APR HPFP was replaced when it was installed maybe 2-3 years ago. When the LPFP primed this morning before turning over the engine, pressure reached 4.8 bar.
MAF; you've already dealt with.
Yep.