Lean codes are generally caused by vacuum leaks but there are other causes. Do you have access to a scan tool that can display real time sensor readings? That will help greatly. If you do then monitor the following parameters: short term fuel trim banks 1&2, long term fuel trim banks 1&2, O2 sensor voltage for both banks pre and post cat, rpm. Make two lists of the data, at idle and at 2500 rpm. Make sure the engine is warmed up. Post the data here and we will go from there