Help needed: O2 sensor/multifunction temp sensor for '85 4kq

[Jpinkowish at aol.com]

Gas mileage in enthusiastic, but not nutty, mixed use driving recently 
dropped from 21-22mpg-->16mpg.

DATA
--200K miles on car, at least 100k miles on O2 sensor and multifunction temp 
sensor.
--Resistance of temp sensor(bottom of upper hose on front of engine) is 5200 
ohms at 68deg F.  Slight increase(maybe 5400 ohms) when car is hot.  Bentley 
says 2500 ohms and should go lower.
--When car is hot, coolant temp gauge in car only reads at 2nd bar(just below 
mid-range).  In cold weather, temp gauge reads between 1st and 2nd bars.  
Coolant thermostat is 87deg C and 18 mo old.  Car puts out good heat into the 
interior.
--O2 sensor heater resistance measures 5.5 ohms when cold and climbs to 6 
ohms after 5 min idle.  Bentley says 2-4 ohms.  Very slight occasional 
stumble at idle.  Revving and holding engine at 3-4000 rpm in neutral 
produces a more pronounced, but still slight stumble.  Car pulls strong to 
the redline.
--Engine idle is 1050-1100 rpm.  Been that high for several years.  I don't 
recall ever adjusting mixture screw.  Cleaning the ICV doesn't change the 
idle.  Swapping to another ICV doesn't change it either.
--Inside tailpipe is sooty, but no black smoke under heavy accel.

Car is aligned, tires properly inflated, I buy branded mid-grade fuel, and no 
dragging brakes.  All engine vacuum hoses, etc. are new within 10k miles and 
tight.

QUESTIONS
--Are both the MFT switch and O2 sensor kaput?
--Will replacing both get the mileage back up to snuff?
--Is the high idle a significant detractor on gas mileage?
--Do I need to get more electrical data?
--Do I need to pursue something else?

Look forward to your replies.

Jan Pinkowish
'85 4ksq
Bristol, CT