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#1
06-21-2022, 01:26 AM
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1990 420sel rough start/idle
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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2022 Nissan Frontier Pro4x 1990 Mercedes 420SEL 1950 Packard 8 Previous German cars- 2001 Mercedes SL500 1983 Mercedes 300SD 2011 BMW 328i (manual) 2008 BMW 535xi (manual) 2006 BMW M3 (manual) 1980 Mercedes 300TD 2006 BMW 750i 1996 Mercedes E300D 1994 BMW 740i 2006 BMW 330xi (manual) 1999 E300DT Smoke silver/black 253k 2012 BMW 535i Xdrive M-Sport 1984 300TD-T 304k 1988 Mercedes 560SEL (parts car) 2010 BMW 550i xdrive 2017 Audi A4 Presitge 1979 300SD 313k 2003 E500 189k 
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#2
06-21-2022, 11:46 AM
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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
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#3
06-21-2022, 01:58 PM
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The most likely culprits, in approximate order, are:
Cold start injector Air flow potentiometer O2 sensor
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#4
06-23-2022, 03:56 PM
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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
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