It would make sense for the PCIII to feed a constant voltage to the O2 sensor port on the Ecu so that the Ecu doesn't try to adapt or throw a code. Then the PCIII could control the mixture because the ECU would be blind to the changes being made. The Ecu would think everything was fine and stick to the stock map, which is consistent with my experience of it not fighting the PCIII.
Originally the PCIII kit came from one of the BMW dealers (Santa Fe BMW?) and you just disconnected the O2 sensor and it always ran open loop. One of those kits came up on FM and I called the dealer and spoke to the guy who developed it. He said that they stopped producing it when DynoJet came out with a closed loop kit.
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2005 GS1150 Adventure - Range Rover, 2002 DR650 SE - Land Rover
"Not all who wander are lost" - JRR Tolkien