intake manifold gasket?

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?

Yes, as you have probably already found out. Mine threw the same codes and really only when it was cold outside. Reason, these gaskets leak when cold. After checking all 4 O2 sensors, cleaning MAF sensor (if you have a K&N Filter the oil can cause it to gum up and throw these codes) I heard on a local car talk show my exact issue. Ford had a TSB about these gaskets being defective. Problem is, warranty has passed. It was too big of a job for me to take on with just a Haynes manual so I took it to a shop and $600 later, no codes, better gas mileage, better acceleration. I also gained some of these benefits from changing the o2 sensors and cleaning the MAF sensor but noticed significant performance once the gasket was replaced. Save your receipts. If enough complaints start coming thru to Ford as these engines are now starting to show this defect in large numbers, Ford may reimburse is a lawsuit is pursued. My garage said he has seen at least 20 this month. and its been REAL cold so they are showing the defect.

How about re-torque to specs & see what happens. I've found leaks by running an unlit propane torch set low, around the gasket, listening for an idle reaction.