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I am surprised that nobody on here talks about reprogramming the ECU or fooling it by altering the readings that the sensors are feeding it. I would think this would be an easy way for a few more MPG.
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I have talked about altering the O2 signal quite a few times. In addition, I've actually done it on my daily driver. However, you cannot reprogram the closed loop behavior of engines equipped with a narrow band O2 sensor (don't get me started on EFIE!), only the open loop. Racers operate in open loop, hypermilers operate in closed loop. There are very few parameters available in most ECUs to squeeze out a few more mpgs. Some of these are idle speed, cold engine enrichment values, fuel cutoff thresholds and decel leaning. To lean out across the board, you need a wideband with a variable narrowband emulator.