After installing the new rear speaker box, while waiting for the new interior, I've noticed some slight strangeness with my EZ EFI2.0 F/A in the GMC. I've been running only mid grade fuel instead of regular for a number of months now. A plug check showed some light ceramic sooting, but still looking good. The engine was running slightly rougher at cold start until it warmed, and I noticed exhaust was no longer looking as clean as it had in warm engine WOT acceleration. I had also begun getting sporadic 003 IRM readings. I changed WOT from 12.6 to 12.5 and the exhaust during WOT acceleration immediately cleaned back up. Since the cold start appeared rich, I took the idle F/A to 14.5 - leaving the enrichment setting where it was. Cold start has now settled to a smoother idle again. I also changed cruise F/A to 14.8 and accel fuel enrichment to "0". Timing and heat range still looks correct at the plugs, and the plug ceramics have only returned to a clean white coloration. The IRMs have gone back to the 000 readings I had grown accustomed to. I'm hoping this strangeness isn't signaling the O2 sensor going bad. Maybe there was just some carbon buildup somewhere in the old engine that is going away.