Afternoon,

I have a 98 Park Avenue, 3.8 with just over 120k on it. Long story short, car has been mostly sitting for last 2 years, the intake manifold blew out and began leaking coolant into engine as typical for 3.8. I recently purchased the car and replaced upper plenum/gaskets, lower gaskets, plugs, coils, ignition module, and plug wires. All of this was with GM parts and gaskets except the upper plenum, original Delco part no longer available, so went with best reviewed aftermarket one I came across...it seemed alright, had the metal egr passage upgrade. Anyway, car now runs very well, oil pressure is ~60 at hot crusing and hot idles about 45, I've driven it about 500 miles now after fixing and it'* not using any noticable oil or coolant, avging 28mpg so I think all is good there and motor is in good health, this is the 4th car with the 3800 I've owned and it feels as they do/did. I also replaced the egr purge line from throttle body.

I've noticed when the engine isn't fully warmed up, and you are coasting to a stop off the gas, it fairly regularly trips the P0171 bank one too lean code. It never does (or hasn't yet) it on the gas or while driving or when coasting when its fully warmed up. As far as I can tell everything else on the engine is original but not sure. Nothing changes when it comes on, car runs and idles so smooth you can't even feel it running. I know this is a vague problem, but just wondering if anyone could give me an idea what might be up.

Thanks

I should also mention car did have about a quarter or so tank of I have no idea how old, but surely very old, gas in it when I got it. I did make sure to use proper torque spec and sequence for intake, this is the 3rd one I've done

Fill the tank all the way up and drive it around for a while. See if the problem goes away by the time you need to fill up again.

Bank one on your car is just the first oxygen sensor since there is only 2 (one pre and post cat)
If your O2 is original, its time for a change. They do become slow and I noticed an idle improvement on my 99 park avenue ultra when I switched to a new Denso sensor. I bought it for a 98 grand prix I used to have but never got to installing it. They sell sockets for o2 sensors which youll need, and if the threads are full of crud i would chase them personally. The sensor on my scan tool live data was visibly more responsive, and this could be your issue. If it comes back, time to look at other issues. Have you replaced the vacuum harness on top of your engine? and is the nipple on the back of the intake properly plugged (there should be an extra one if I recall my old 1998 correctly)