My car has a similar problem : with a cold engine, as long as the glowplugs are on, it runs smoothly (on 92 W124, the glowplugs stay on up to 60 seconds after the engine starts, depending on the temperature). When the glowplugs turn off, it starts to misfire with unburned fuel smoke. The misfire is worse when the outside temperature falls. When warmed, it idles without any problem.

I've got my injectors rebuilded and although there was a notable improvement at high rpm, it didn't cured the cold idling problem.

I didn't change the glowplugs : current readings (AMP) are all within specs.

I was up to do the "delivery valve" job, but your story scares me a little.

From what I have read in this forum and in other sources, cold idling problem seems to be common on the 602 engine : I think the afterglowing time was extended on later models to get rid of that.

Keep us informed on your work.

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DR
1992 300D 2.5T 351 000 km