p0171 system to lean troubleshoot

[corollave]

hey i have 2000 corolla. i been have some issues trying to figure exactly what the fix is for this code. I have started with some of the more simplier thing like making sure maf sensor and o2 sensors are good and working. I have done a fuel pressure test and the pressure is good and within spec. I also did a vaccum test and noticed the engine is not creating enough vaccum, i have checked all the vaccum hoses and can find no leaks. This where i am little stumped not sure what fails on these motors when there not creating enough vaccum. Was just wondering if anyone know what generally fail on these motor when the vaccum is low.