New ignition parts, new filters, new vacuum lines at idle air valve (and possibly the idle air valve itself), and have fuel pressure & volume tested and injectors inspected. While you're at it, inspect the OVP and fuel pump relays. Those are basic maintenance items that may well have been neglected and are necessary in any case for a properly-running M103 motor, fuel economy and performance. There's a good chance that fixing those things will also likely correct your hard start/idle issues.

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't