I have a 1991 300D which runs pretty well, except for a somewhat rough idle. I have replaced the DV seals already, and checked the fuel system for leaks. The motor mounts have also been replaced. Diesel purge done. Next on my list are the injector nozzles, but I want to confirm first if they are the source of my rough idle.

Diesel Kleen or other fuel additives seem to reduce the vibration at idle, which is not a surprise. What IS surprising is that the idle smooths out dramatically in hot weather (above 80 degrees F). What could be causing this?

Is the idle improvement due to lower viscosity of the fuel? If so, would that indicate worn injector nozzles?

I have searched the forum and didn't find anyone with the same observation. Any help from the best & brightest would be appreciated!