jamfu

My 2000 f150 is giving me a p0171 code which is a lean mixture. A buddy from work put his computer on it and the o2 sensors are working properly. Furthermore, I have checked the vacuum tubes coming from the iac for leaks with some starting fluid and the engine did not rev any higher. I also sprayed around the intake manifold as best i could, and still nothing. I am not really satisfied with that though. So, I was wondering if any of you have heard of that gasket going bad on the 4.2 l or any other engine?