I went to the tech session yesterday thinking my O2 sensor was bad. Lead tech at dealership hooked up their analyzer to the X11 connector. It read 2%(I am not sure units) and did not vary. He said it was too rich and needed more lambda. He took a 3mm allen and adjusted (near the air intake) the mixture more lean. The final result was 4.5 varying up to 5.5. Thus the sensor is fine. (I let a shade tree style indie touch the richness several years ago after fixing a water pump.) Perhaps the broken thermostat and the excess richness caused the apparent sudden drop-will need to monitor mpg and report.
For A264172's benefit I checked my X11 with my multimeter. I got 6.75v up to 7.5v. So it varies now.
Re O2 sensors- both Bosch and NTK have toll free number to call for tech info. The 13953 sensor used at one time one the Ford Mustang(Parts people now rec 13942 shorter cord for the Mustang) now rec on E 150 econoline can be spliced in though it has a 6 ohm heater instead of 4.5 ohm. Not sure if the ohm diff matters. NTK had a $29 one with a short cord(theoretically bad as solder will be outside cabin) and correct 4.5 ohms.
Many Thanks to all who helped.