I've been around peachparts for a while. I have several 123, 126 and 210 diesels and I'm pretty versed with them. I recently purchased a 1990 420sel with 208k and runs fantastic. It has full records from it's first oil change to a complete engine overhaul 30k/10 years ago. I have one issue that I'm having some issues with and that is a rough start when it is cold.

It would take about 2-3 seconds of cranking to start. I replaced both fuel pumps, fuel filter and fuel accumulator, now she fires right up but still has a rough idle almost stalling for about 5 seconds then jumps up in idle and levels out. This ONLY does this on cold starts. If I do a warm start, the car fires up and idles perfectly. I am just trying to figure out where I should start looking under the hood. I seen several posts for warm start issues, but haven't found any with just cold start issues.

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Current fleet-
2015 BMW 750i xDrive
2009 BMW 535i xDrive
2004 Chevy Avalanche 4x4
1993 Mercedes 300SD
1993 Mercedes 300SE (parts car)

Previous Benz's-
2001 Mercedes SL500
1983 Mercedes 300SD
1980 Mercedes 300TD
1996 Mercedes E300D
1999 E300DT
1984 300TD-T
1988 Mercedes 560SEL (parts car)
1979 300SD
2003 E500
2004 S430 4Matic
2001 E55
2003 E430

I have exactly the same problem with my 1991 560SEL. So far I haven't found the cure, so I will be interested to hear any suggestions.

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown

The most likely culprits, in approximate order, are:
Cold start injector
Air flow potentiometer
O2 sensor

I tried to check some old threads for info. This 87 had the conditions cold and warm. They fixed it with new air distribution hoses and a rebuilt fuel distributor.
http://www.peachparts.com/shopforum/tech-help/217492-my-420sel-has-rough-idle.html

On this 91 420sel they we're having similar rough idle conditions and it was determined that issue was from a sticky valve that needed adjustment:
http://www.peachparts.com/shopforum/tech-help/17882-rough-cold-idle-1991-420sel.html

Hopefully this helps!
-Matt