Running rich - 99 3800

[einstein]

I have the Elmscan (Scandata) PC scanner, and I'm trying to learn how to diagnose the rich running situation on wife's 99 3800. We got car with 69,000 miles 3 years ago, some unknown previous owner tuned the car some - Pacesetter headers and I imagine a PCM tune.

Car ran great from 69,000 until the last few months. Now it's over 95,000 miles. Runs rich, loads the 3 O2 sensors until they foul, then car stalls when slowing down.

What I don't know is how to read the data on fuel to determine what has gone wrong. It seems to behave as if a sensor tells it that engine is cold, meaning "deliver more fuel". The engine coolant tempurature at PID SAE 0x05 seems right at around 206º.

Scan tool shows Short term fuel % trim - Bank 1 & 2 @ -19.53%; Long term fuel % trim - Bank 1 & 2 @ -22.66%. These numbers seem out of whack to me, but I don't know where to find specifications.

Wondering too about grounding issues causing some weirdness....

Any ideas are appreciated!