Have a vacuum gauge with a cone shaped rubber attached to it. Pull the dipstick and idle the engine, should read about 19"/Hg, but am at 1,000 feet above sea level, will be about an inch less for each thousand feet above sea level.
If reading less than this, not sure how much, shop manual keeps this a highly classified secret, but was only reading about 7"/Hg, this set that P0171 code, also a class A code, need a scanner to reset it.
There is a venturi in the inlet to the turbo that create this vacuum, and also a pressure sensor in the crankcase system that also covers all that air space clear up to the camshaft cover and also includes the PCV system. Shop manual does not say when that pressure sensor clicks.
Like Robby said, he also has been around the block once or twice, that vacuum leak can be anywhere. PVC is nothing more than a check valve, only lets air flow toward the turbo inlet, if you kick in the turbo, positive pressure, if that check valve leaks, will kick up that code. Can be as simple as that oil filler cap is loose, question is were to find it. Mine had a minor crack in the left back corner of that disk in the camshaft cover, that was easy to find. Could have epoxied that, but what the heck, PT warranty was still good, let them put on a new camshaft cover.