Check the 212F switch for a ground, and look for a shorted wire on the relay circuit. It has voltage any time the ignition is on. It is supposed to be switched on (to vent vac) when the temp is above 212F, the AC is on, or the car is in third above 1500 rpm, I think. May also be oil temp related, don't know.
You could also have a stuck relay, stays on when not energized.
Idle should be 1200 plus when cold, should drop to 800 at 150 F indicated on the temp guage. If you have a tired thermostat, you may be taking too long to get to operating temp. Vac should be applied on cold start, retarding the ignition timing.
If the vac retard isn't the problem, check the operation of the temp switch, the thermo time warm-up regulator, and the idle air warm-up valve. They can get tired and not shut off the extra air soon enough. And you will need to check both temp sensors for correct operation -- the one in the intake manifold and the one in the air intake horn.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!