1999 GMC Jimmy ST FTRM1 is +50 @ idle FIXED!!
 

This is from my 1999 GMC Jimmy. It has a rough idle and trouble code P0171. With the engine running, the ST FTRM1 is at 50. When I snap the throttle or increase engine speed, it drops to 0.
I have done a thorough vacuum leak check of the engine with a smoke can. Nothing significant. A slight leak around the PCV hose but I wouldn't think it would cause this large of a lean condition.
When checking online, I have found a long list of possible problems. I was hoping one of y'all might have had this problem in the past and may give me some direction as to what to start with first.
My plan is to first check fuel pressure, then if that checks out, put some air to the pressure side of the intake plenum where the fuel feed rail enters to check for bleed down. If that's ok, I will change
the Fuel Pressure Regulator(which is a bigger job than I want to do right now. Are there vacuum lines that go to the transmission that could affect this? https://vimeo.com/user107172285/revi...818/8d2f0b23b5

How does it perform while driving? Starved of fuel at all?

Edit P.S.: How’s your LTFTs?

It performs great off idle but LT FTRM is +25.

That’s huge. Both banks?

No, just bank 1. I really feel like I still have a vacuum leak but I haven't been able to find it yet.

A lean condition on only bank 1 rules out a lot, and makes diagnoses a lot easier. A vacuum leak other than the lower intake manifold gasket on the bank 1 side would effect both banks, Fuel pressure issues, regulator, etc would also effect both banks, so that rules that out.
I would say your most likely suspects would be a bad manifold gasket, or a bad O2 sensor on the bank 1 side.
I suppose it’s possible that you could have some weird internal problem with the central injection unit that only effects the pop injectors on the bank one side, but I’ve never heard of that happening. A fuel leak in any one pop off hose would cause misfire, and you didn’t say it was doing that.
You could try running the engine and spaying brake cleaner around the bank one manifold gasket while watching for O2 sensor changes on that side with your scanner.
Also, swap O2 sensors around and see if your condition changes from one side to the other.

Thanks for your reply
This is how I was thinking. Other than fuel injectors and internal parts, all those things are common. Funny that I just read this after I swapped O2 sensors to see if the problem, followed a sensor.
And KaPow!! It did not follow the sensor and everything is normal. Fuel trims, MAF and O2 sensors are in the normal range. So, maybe the Left bank O2 sensor was grounding or was loose.

There ya go, easy fix