I had idle problems on my 1985 280GE. I took it to a reputable mechanic who could never diagnose and fix my problem. I went to another mechanic who fixed it perfectly. Problem was 3 injectors and valve lash adjustment. These m110 motors will run real crappy when valves not adjusted correctly.
I also had an issue with cold idle staying too high and not idling down when warm. This turned out to be a vacuum leak in the plethora of "air-slide" tubes under the intake manifold.
One final note. My compession had variances of 15-20 and I am no mechanic, but I think valve lash may affect these readings. So here are some things to consider and keep in mind that all "good" mechanics are not created equal.
Mike

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1981 300SD (171,000 miles)
1985 280GE (205,000 miles)
1995 Dodge Cummins Turbodiesel (278,000 miles)