i am a professional mechanic, my father and I own a shop which my father started 16 years ago,
Trouble codes PO171, PO174 are as you thought lean codes. Can be caused by a vacuum leak, fuel pump, fuel pressure regulator, and several other things. These codes are time consuming to find are even harder when they are intermittant. Without the proper tools your will never find this problem. Until you take it to someone who does you are wasting time, money and effort.