After driving to work I noticed the idle on my 350SD was very rough. It happened suddenly. There was no gradual degradation in idle.

That morning I drove over some rough ice chunks in my driveway that shook the car up pretty good. I thought it might be some dirt in the fuel tank washing off the sides and clogging the filters. So, I replaced both fuel filters. They were dirty, but replacing them didn't help.

Here are the symptoms.

1. On start the idle is very rough. If I give it a little fuel to bring the RPM up the idle smooths-out.
2. Under normal driving conditions the car is just fine and the engine is smooth.
3. When I come to a stop the idle is smooth at first, but when the RPM drops below about 750 (to about 600) the idle gets rough again.
4. If I give it a little bit of fuel to get the RPM to 750 the idle smooths-out.

Here is what I am thinking:
What is the normal RPM for idle? Is it somehow adjusted too low?
Are my fuel injectors dirty and not as well-balanced, which is causing the rough idle? (It has been about 60K miles since I rebuilt them). I should probably clean and re-balance the injectors, but that is a lot of work if they are not the likely cause.

What are your thoughts?

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1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside