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ok. my understanding of ecu operation is that during normal driving (part throttle) it is in closed loop mode - the engine senses the amount of air in, looks it up in a table, and adds fuel as per the table. it tries to maintain the magic air:fuel ratio of 14.7:1 for best power and efficiency of combustion. the o2 sensors in the exhaust monitor the products of combustion and provide feedback to the ecu - this feedback allows the ecu to trim fuel levels in order to maintain an afr of 14.7:1.
when the engine is working hard, ie in the higher rpm range, the ecu starts to worry about heating and detonation (the ecu can't control what kind of gas you put in, and can only modify timing a little bit). to address the issue it adds more fuel in an effort to cool the engine. this richens the afr to 12.5:1 or so. the o2 sensors only work in the 14.7:1 range, however, and can't provide any feedback to the ecu when operating outside of this value. as a result the ecu operates open loop and provides fuel based only on sensed air intake and fuel maps. my understanding is that, in open loop, it also ignores any fuel trim information compiled while operating closed loop.
so, my next question to you would be if you recall actual afr values when you encountered the lean condition at high rpm. I ask as this may not be a bad thing. in fact, this is the standard trick used by ecu 'tuners': they can't change ecu performance during closed loop operation (any modifications to the fuel maps would just get trimmed out) nor would they want to because the engine is already at 14.7:1. instead, what they do is lean-out the fuel maps during open loop operation to get closer to 14:7:1 (ie, set the engine to run at 12.7:1 instead of 12.5:1). you lose the benefits of the additional fuel cooling the engine, but gain power (more efficient combustion) and efficiency (less fuel used). this is why they often require that only high octane gas gets used with their tunes.
so, provided that the lean condition you encountered wasn't way out of whack, it wasn't necessarily a bad thing.
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Hybrid Mode
