I echo the use of ACDelco plugs......they work the best...

Sorry for a long post about my 3800 series II mpg problem below. Hope someone can help.
I have a '03 Park Avenue Base with 175k miles and running strong. A few months ago a bad igition coil caused me to misdiagnose and change the LIM and UIM gaskets. But after the gaskets job, I've been getting slightly lower MPG (25-27mpg crusing 60-70mph rather than 28-30mpg per ScanGauge2). I checked for vacuum leaks by spraying carb cleaner and did not notice any rpm changes. I confirm PVC valve is good and the MAP sensor on top of it is locked in place correctly. What else can it be? I did spray Mopar Combustion Chamber Cleaner while I did the gaskets...could this have cleaned out some carbon that was giving me bettering compression. I don't think so because for a few days after the UIM/LIM job, I did have noticeable better acceleration from 30 to 70 mph and now that power is not there anymore...so something happend during that few days (~100 miles). Also, during that few days, my spark advance would stay high (~30 degrees) even when lugging at lower rpm (~1800rpm) up small hills...now the spark advance would drop to below 20 degrees on the small hills (but still 30-40 degress on flat hwys). Obviously I don't know the mpg during that few days because it was too short to get an accurate mpg. I thought the new NGK G-power spark plugs that I installed during the UIM job might be the cause, but I put back the old/used AC Delco 41-101s and still the same. Anything else I can diagnose with the ScanGauge like MAP or */t or l/t fuel trims?

lots of seals have to be good when doing that job, did you replace inj orings/ map seals/tb/etc..

fuel trims should be close to zero

Jwfirebird, Thank you for the reminder about seals. I did replace the TB seal that had the all importnat seals for the coolant passages through the TB, but I didn't replace the MAP or injector o-rings. The MAP sensor on top of the PCV seems to have a good seal since the sensor requires some force to twist into place...but the kit does have a few O-rings left over so I will definitely look through them again and replace the MAP o-ring. For the injectors, I never disconnected the pressurized side...just lifted the entire assembly of 6 injectors still attached to the rails. But the kit also had the o-rings for the jectors so I'll replace those as well.
When I put in the LIM gasket, I used the black RTV that came with the kit for the left and right side flat gaskets(RTV'd top and bottom of those flat gaskets). Since there was still RTV left, I used a thin layer on the UIM gaskets especially around coolant passages. I even used the RTV to lubricate the 2 steel Dorman 47065HP elbows as well.
I had the Dorman 615-180 UIM for the job, but the mating surfaces to the LIM and TB were not smooth...had a lot of joint/seams from the mold from which it was made. Did Dorman go down in quality or did I just get a bad piece? Anyway, I re-used my old UIM since it seemed to have been already replaced by the previous owner...the stove pipe passage was in pretty good shape (not burned/disintegrated).
One question, after shutting off the engine, if I pull off a vacuum line (such as the one for the fuel pressure regulator), should I hear a hiss as it relieves the remaining vacuum? I'm not getting a hiss.

you only get noise if the engine is on. and i havent heard of any issues, i used the same stuff when i did mine. i would be nervous about using old upper they tend to get brittle and start to crumble. i would try shooting carb cleaner around the map to see if the engine changes pitch, all those things have to be sealed perfect for this engine to run right. and rubber seals get hard after a while and dont seal. they should be squishy.