We have a great 1993 300E (3.2 liter) with 120K that uses almost no oil between oil changes at 3-4000 miles.
Gas mileage has dropped from just under 30mpg to just over 20mpg recently.
The other day the check engine light came on. About the same time the idle changed from a normally smooth 600-700rpm to a pulsing or surging 600rpm to 1100or 1200rpm (only after warm-up)
I checked the spark plugs, they were even but sooty black. I then retrieved the following trouble codes:
1. #2 oxygen sensor inoperative (replaced 7 months ago)
2. #4 air injection inoperative
3. #10 voltage at air mass sensor too high or too low
4. #26 up-shift delay switchover valve open or short (replaced 7 months ago)
5. checked for vacuum leaks, none found (yet)
6. read lots of archived threads on rough idle and other

I cleared codes but #10 is back.
Seems like the coolant temperature sensor might be the source of the problem. If it is sending the wrong signal saying the engine is still cold, thus the rich fuel mixture, bad gas mileage and rough idle.

Anyone have a value @ temperature chart for the coolant temperature sensor for this model?
Anyone have any other suggestions?
I really don’t want to start throwing parts at this problem. Thanks for your input!!!