Car: 1991 560 SEL 103K miles
Problem: temp gauge never go above the tick between 40 and 80 C even after 30 minutes of stop and go.
1st Diagnostic: Thermostat stuck open.

Spend three hours to get that ^&*^%$ thrid screw to get the T-stat out. Put the unit in a pot and cooked it. The 1st stage open at 82 C and 2nd stage open at 95 C, working perfectly.

Now what? Please advise how to check the temp sending unit?

Low engine temp seems to contribute to my car running high idle. Engine keep wanting to go to a higher RPM to get warm.