This is probably too general for an easy answer, but I'll try anyway:
We've got an '89 420 SEL with 216,000 on it and just recently (say in the past couple of months), when we first start the car, it idles very rough. It sputters like it's misfiring. Runs very rough. After about 20 minutes of driving, it settles down and runs like a charm. Smooth idle; smooth acceleration, purrs like a kitten.
Then if you park the car for a while and then go out later on, once again, the same symptoms occure and continue for about 20 minutes. Then it'll smooth back down. It actually seems to be worse on wet days and cold (if you can call South Carolina cold) days, but none the less, it does do it upon initial start-up, regardless. It's had the timming chain replaced at about 130,000 miles and has had a new fuel filtre installed at the first of the summer.
Any help will be appreciated
Thanks
'89 420 SEL
'86 560 SL
'79 300 SD (gone, but not forgotten)
'98 Sportster

You need to check and see if you have electronic control of the fuel injection system (CIS-E)

If you do not, the car will run in FOM (fixed operating mode)- no assitance (enrichment) for cold start.

If I recall, with the key in the position just before engaging the starter (KOEO), you should read 70% at x11/3 diagnostic port with your multi-meter if engine recognizes the "E" potion of the CIS-E system. While running, your duty cycle may be 'stuck' at 50% (FOM). There are other codes as well, that are represented by 10%, 20%, 30% etc readings.

do some searches on the subject for some expert posts.

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'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E

I had started a thread a while back inquiring about what I preceived to be a "cold start" problem. I don't think that's what the problem is. What happens is; that many times when the car is first started, it misses and sputters as if there were some sort of electrical problem. It'll continue to do this for 10 minutes or so. Then it'll clear up for say, another 10-15 minutes. Then it starts again. Maybe goes for another 5 minutes missing and so forth, maybe 10. Then it'll clear up again. It seems that the problem is worse on initial start up and especially when it's cold and/or damp. But it does it, none the less, even on warmer days, so I can't say in all honesty now that it's a "cold start" problem. It seems to be intermittant, though. Sputtering, stumbling at idle and acceleration. Even cruising.
Now, does anyone have any clues? We only use premium gas.
Thanks

Check your spark plug wires. The best way I have found to do it is with an inductive timing light. When it's missing, clip the pick-up on each wire and see if they are all firing. I had this exact problem on my 560 and it turned out to be a bad plug wire.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe