After a year of fighting and searching..I finally found and fixed!!!

I got a new OBD ll reader that has live data display...I was able to see that my fuel trims were way too high. Seemed to rule out electical/grounding problems. I decided to look for vacuum leaks again, started spraying carb cleaner looking for vacuum leaks..I found that one of fuel injectors on the front of the engine had a leak at the o-ring seal to the manifold.

I pulled the fuel rail and much to my surprise I found that the drivers side rear injector had no o-ring at all. At first I was afraid that I had knocked the o-ring off removing the fuel rail, but the more I looked at it I could tell that the o-ring hadn't been there for quite a while. Replaced all 6 o-rings fuel trims back to normal. I don't know if I should be happy with or mad at myself. This missing o-ring must have happened when I did the LIM in March of 2007.

The crazy thing is that the car ran well and would set the CEL and reset itself. Sometimes it would take 150-200 miles of driving before the CEL would set. I would have thought that a missing o-ring on an injector would have caused a bigger problem.

Spit

Glad to here you found it. But have a look here, https://www.gmforum.com/t284036/ and make note of the 4th solution.

No doubt I missed this big time!!

I had checked (or so I thought) all around the manifold and the vac hoses with carb cleaner... I had found what I thought was some leaking around the Upper intake manifold where the throttle body bolts on, and I sealed that up with RTV sealant. Now I kind of wonder if it wasn't the missing o-ring on the injector where the cleaner was getting injested since it was the rear injector closest to that spot. I had checked it with vacuum guage hooked up to the line that goes to the transmission and as I remember I had 17-18 inches of vac..that held steady..

It was the message that you reference that lead me to try spraying the injector o-rings, I guess I was lucky that one of the front one had a split/sliced o-ring that caused it to run rough when sprayed or else I might not have pulled the fuel rail and found it... The one that was missing is kind of hard to get to...close to the EGR and throttle body. I think I was afraid to spray to close to the EGR because of the heat. I didn't know that one was missing until I pulled the rail

Thing is I got this car in a rush...found the LIM problem..rushed right into fixing it and never really knew for sure where the 171 started... since it was for my 85 year old father in law who lives 150miles away . I wanted everything right, so as soon as I got it I did plugs, wires, air & fuel filter,trans service & filter., Then I found the leaking LIM , once the LIM was fixed the water pump started to leak so I changed that, I changed both oxygen sensors after the code started to trip. Then I changed the fuel sending unit when I noticed how it didn't read the tank level correctly, both rear ball joints when I heard it creaking. Gave it to him and he drove it maybe 5,000 miles in a year and a half.

The goofy thing was that the car would throw the light and turn the light back off with the missing o-ring!!! For the most part is ran great. I got the car back a year ago and got to spend more time with it. I had to change the elbow at the belt tensioner and the oil sending unit..but just kept getting that 171 every 3-4 days. I would get 22-23 MPH even with the fuel trims out of whack. I can't wait to run a tank or 2 and see what I get now. I would hope for 2-3 MPH better.

I have seen other postings where the P0171 starts after a LIM change...the injectors are really a good place to look!.

This site is great..it'* given me a lot of good advice.. Thanks

Spit