The thing i have noticed with autos and fuel cuts is it will only turn off the fuel when the torque convertor is locked up. In my old Lincoln, this would only happen if the transmission WAS in overdrive! It would be interesting to see if you can find where your TC locks up, or perhaps go that step further and put a switch on it, as some one else did on this site some time ago.
GasSavers_James 
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