I was at a shop, asked what they could do about a slight rough idle. Among other things, they recommended "BG" injector cleaner, dump it in a full tank of fuel. This is my first diesel, only had it 6 weeks, so maybe there's better stuff out there, but this stuff seems to work. It's not idling better, but my car is definitely quieter. Though it hasn't fixed the slight rough idle, it's probably just one of several steps on the way to rehab.

1991 300d, 90k