Idle speed is set electronically on these engines and isn't adjustable. Should be 750 rpm or so in neutral, 500 rpm in gear when warm, higher when cold.
If it varies, car dies sometimes when coming to a stop, or idles rough, check the idle valve -- there are a large number of posts concerning idle problems on these cars!
Erratic idle is a sign of a vacuum leak, and in spite of all the electronic fuel injection and mixture control, vacuum leaks still can cause funny idle -- check all the rubber hose ends, etc!
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!