'81 300SD Why is my engine purring now?

[tyl604]

For the last few days the diesel engine has settled down and just purrs at idle. Generally is tends to have a moderate shake; not quite as bad as an old '60's muscle car with a high lift cam but something like that. A few years ago I had my indy adjust the valves and increase the idle a bit and that helped a lot, but before that I would sit at a red light with a decent shake when, suddenly the engine would just calm down like something had clicked on to tell the engine to smoothe out. But mostly it just shook.

So the increased idle has helped but what could have changed suddenly to make it smoothe out so much at idle? I notice that it does idle smoother in colder weather and shakes more in hot weather. It has been pretty cold in Atlanta the last few days - say 24 degrees - but today is in the mid-40's. Could the colder weather make such a difference? Or is there some non-computer type gadget under the hood that is only working now and again to make it idle smoother?

Pretty happy now with the way it idles; just wish I knew how to make it last. 255K miles on the engine.