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I would guess an overly-rich mixture that the O2 sensor is compensating for, at least to some extent. You could confirm that with a Wide-Band O2 meter on a decent dyno, or potentially if you have access to a WBO2 that you can temporarily install in the car.
It could be some other cause, and not related to going open-loop. There are other things that depend on load, like ignition timing and so on. (Depends on what sort of engine management/fuel system/spark system is on the car, as well.)
-soD
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