While there are a few common cause for these codes on the vehicles ( I have a few videos on this already) this one is commonly overlooked as a potential cause of a rough idle, rolling idle and of course the lean codes P0171 P0174. Besides the quick fix in this video I show you how the pcm reacts to this tube being disconnected and why the codes set.

 

Depending on the model year of 4.0 SOHC engine you are talking about, I have good memories of changing the upper and lower intake gaskets, along with the updated timing chain tensioner and oil restrictor to add to the left head, A. K. A. 00M12. But the bonus is, it would also "fix" the lean codes.