Just ran some checks with the OBD reader. The initial ST/LT Fuel Trim all fell in the +/-5% range as I ran from idle to 1500 rpm, 2500 rpm, 3500 rpm. The MAF flow increased from 4.35 to 15.92. The O2S1 reading (upstream of CatConv?) varied from .06 (lean) to .80 (rich) as I held the RPMs.
Of particular note, the ST Fuel Trim dropped to -16% (negative) after I dropped back to idle and the engine stumbled occassionally. Too rich…trying to go lean.
Not sure what to make of it. I was thinking maybe an idle vacuum leak, but that would be showing too lean?