Toyota Repair: 2000 Toyota corolla P0171

Question
Hello! Ted Back again after a year. Got a frustrating problem. Your previous P0446 answer worked. 2000 Toyota corolla P0171.  My mechanic and I in the last 9 months have replaced the o2 sensors.  He has a scan tool and does have years of experience.Tested injectors and compression (all 160),replace intake gasket, No leakage in exhaust system.  Went a way for a long time.Last 2 weeks same problem. It seems that when now I hit even a slight bump the engine code comes on. Talked to Toyota national customer service twice. Absolute nothing other than a sales pitch to buy there parts. Went to local dealer service manager wants to start replacing all hoses. I could do that. Also would like to go ahead and replace the basic Evap hoses under the hood Evap VSV and Purge VSV by air filter box). But cant find the sizes. (4mm or 5/32?)Toyota national or dealer didn't know which I found very disappointing since it's a Toyota car.. They said use the trial and error method. Can you direct me again hopefully. And with the hose sizes? Copied a checklist and  it's down to PCV hose, Cool temp sensor, hoses or ECM. Or what you think.

Answer
I would recommend that you remove the heat shield from the exhaust manifold and recheck for a possible a cracked exhaust manifold especially where the welds are, a persistent Po171 code after trying a few things can be caused by even a small exhaust leak, not uncommon on this engine.