I just got done swapping and testing the PCMs, and for sure that isn't the issue. When I put the stock PCM back in, and started the engine, the PDM started out in open loop fueling mode, and wasn't adjusting fuel trims (later this also happened when reinstalling the ZZP PCM as well). About ten seconds later it switched to closed loop and started to adjust, but was still asing for 16-20% short trim, and 16.4% long trim fuel. After that I swapped the ZZP PCM back in. The short fuel trim now bounces around at idle, instead of staying steady at 20%, but the car hasn't gotten to relearn for an hour or more yet.