MB specs non-resistor plugs for the M103 engine and there are lots of posts here and elsewhere about not using resistor plugs. Don't know why a dealer would install resistor plugs, but I personally would return the resistor plugs. I've heard that US dealers can still supply the correct H9DC0. I've also heard that as recently as last month, Phil at Peach Parts had them in stock. As an alternative, lots of guys like the NGK BP5EFS plugs, also non-resistor. I've used NGK in my M103 for quite a while with very satisfactory results. MB Classic Center in Irvine can source those for you.

Sorry your 103 is misbehaving after all you've put into it, but I can't agree that it's a hard motor to get running good. Mine now has 200K miles, and the idle is so smooth I have to look at the tach to make sure it's still running. Mine still has original accumulator, fuel pressure regulator, IACV and vacuum lines, CPS and temp sensor. EHA and fuel distributor were replaced 21 years ago under warranty, so you might as well say they're the original ones, too.

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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