Solved a surging idle problem on this 3.8. Engine would start good cold and remain idling good until it warmed up for a few minutes at which time it would start surging and eventually die. It was throwing a P0172 code which is a rich mixture condition on Bank 1. After reading about all of the problems with the lower intake gaskets, belt tensioner cooling elbows, upper intake plenum gaskets and plenum, I decided to take care of these potential problems and address the idling problem if it remained after the above repairs.
Although the car ran much better after the Lower & Upper intake repairs the surging idle problem was still there. I took a fuel pressure reading and had 46 psi. After taking the fuel pressure reading, I pulled the vacuum hose off the fuel pressure regulator and noticed fuel coming out of the fuel pressure regulator vacuum port. Bingo! The diaphragm had failed and fuel was being sucked through the regulator and into the Throttle Body creating the surging idle condition and resulting in the P0172 code.
When testing for fuel coming through the fuel pressure regulator diaphragm, hold the vacuum hose about a 1/2" away from the vacuum port so it is still trying to pull on the regulator diaphragm. If it is leaking, you will be able to see the fuel migrating from the vacuum port to the hose.

 

Welcome to the forum.

Nice trouble shooting and diagnosing on your part