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If the car has been tuned for flex fuel you won't need to empty your tank. That's the idea of having a flex fuel sensor. It recognizes the ethanol content and blends the fueling/spark accordingly. So you can run straight gas, straight E85 and any mixture in between at any given time.
But I can't stress this enough, the car needs to be tuned for it. Yes, the ecm on these cars works with the flex fuel sensor but the sensor needs to be turned on. Also, like KingLT1 mentioned the spark tables need to be updated and the stoich table needs to be adjusted other wise you will be running extremely lean on ethanol.
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