The idle speed is adjusted with the large slotted screw on the front of the engine -- there is a hose from the left valve cover to the idle speed screw, then a y-shaped hose to the intake manifold.

Fast idle is controlled by a thermal valve on the right side of the crossover passage at the front of the intake manifold, has a hose to the idle speed screw assembly. If the valve is stuck, you will have fast idle all the time (and the idle speed screw won't do anything). A quick blow with the side of a hammer sometimes frees that stuck valve up, cheaper than a new one.

I would expect to replace the idle hoses, particularly the y-shaped one as any leak after then idle speed screw will cause a fast idle.

Take a look at the Vintage forum, too -- lots of us crazies have D-Jet engines.

Peter

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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!