As per the other post:
Check your odometer first, then duty cycle/EHA current or CO. It's very possible that you have a fuel problem or bad O2 sensor and you are running very rich (black sooty tailpipe). Fuel distributor failure will result in exessive fuel delivery at all speeds. Expensive fix, alas.
Your rpm is normal.
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!