We have replace egr valve & solenoid, cleaned mass air flow, replaced IAC, both O2 sensors, DPFE, and new plugs & coil packs. All intake gaskets have been replaced. No vacuum line leaks. Read codes last night after a week of driving. PO171 system too lean band 1, PO174 system too lean band 2, PO401 EGR flow insufficient. We put a gauge on the fuel pressure rail and got 60+ lb. Fuel pump was replaced several months ago. When engine was turned off the gauge lost a lb a minute and was 0 after an hour. Could this be a bad injector? We cleared the codes and let set overnight. Started this morning and had the same problem. Surging and would not idle unless RPM's were over 1,000. There was a popping noise in the engine during the first couple minutes of running. It stopped as engine warmed up. After 15 minutes it would finally idle but the engine was still surging. It always has that surging even after warm up and at higher RPM's. The codes did not reappear this morning. They will reappear as the car is driven.