So, 2003 Chevy Monte Carlo SS 3.8L, 150k on the clock. Not too long ago, it started to run like junk, chucked P0171, system too lean bank 1. After poking around, found a bad MAF. Changed the MAF, car ran and still does run fine. With the exception that at operating temp, closed loop, my long term fuel trims are at 16.4, short terms remain around 0. Long term improves when at higher RPMs, but usually stays above 10 anyways. Other then that, car runs fine.
Next, I was doing a PCS on a friend'* Oldsmobile Intrigue with the very same 3.8, and just for kicks and giggles I decided to plug my scanner in and check the fuel trims to verify the 16.4 issue was just occurring on my car. And sure enough, the long term fuel trim on the intrigue was at 16.4, short term was around 0. Next, my brothers car, a 1999 Oldsmobile 88 with the same 3.8 decided to throw P0171 just like my Monte did, and without doing too much diagnosis I checked the long term fuel trim, which was yet again at 16.4, short term was around 0.
I'm so confused with this, I've looked over all the engines, and sprayed around all the vacuum lines to try and find a vacuum leak and never found anything. What is the deal with these 3.8s? Has anyone else come across this problem before? And if so, how did you remedy it?
Thanks!

My vacuum leak was where the vacuum lines go to the HVAC controls. I had one line that crumbled from age.

Do a little research on Youtube for fuel trim theory.

Thanks for your reply! I'll definitely have to look up some fuel trim info and peak at the back vacuum line.