The dreaded EDS (Electronic Diesel System)
[one of the computers behind the Battery] controls the Idle speed.
The classic test of the ELR
(That's the device on the very back side of the Injection Pump,with the wires you've already traced)
is to unplug the two wire connector(with the engine warm and running) and see if the idle changes.
I almost suspect that one or more of your New Glow Plugs isn't functioning...
'Rough idle at Start with a cold engine is one of the big diagnostic clues for
that Malady.Easy to test with a multimeter at the 5 aperture plug on the
Pre-Glow Relay (On the front side of the Driver's wheelhouse[Hump in the
engine compartment for the left front wheel] ) DG has posted a DIY on
Glow Plug Testing:
http://dieselgiant.com/glowplugrepair.htm
[It's on an older OM617 series Diesel,BUT almost everything applies]
[Your Pre-Glow Relay is Electronic,it does not have the 80 Amp strip fuse!]
When you say new Glow Plugs...Only Beru or Bosch are worth installing!
What did your Primary fuel filter (Little in line plastic see through) look like?
'Any Black or Dark Brown Goo or specks in it?
If so your fuel system has the FUNGUS
('Grows in the Diesel/Water interface of your nice dark,cool fuel tank)
Which might be part of your problem...
This will fix the FUNGUS:
http://mystarbrite.com/startron//content/view/94/124/lang,en/
It sounds like "Snake Oil" ,But it's not It works.
see video:
http://mystarbrite.com/startron//content/view/82/112/lang,en/
Once you've sorted out this anomaly, GSXR has a "Website" devoted to all
things W124 :
http://www.w124performance.com/images/
There is also a modification he spearheaded to eliminate a lot of the EGR
crap that disables Turbocharger Boost at:
Wastegate actuator swap for OM602.962?
It's a long read but packed with information.
If you go to Mercedes EPC (ELectronic Parts Catalog) website and register
they'll give you a year of service for free[They want a US based credit card
number to prove you're a customer...no charge]:
https://epc.startekinfo.com/epc/subscribe.jsp
This is the Mercedes Benz Club of Russia's Illustrated EPC site [Free]:
http://www.detali.ru/cat/oem_mb0.asp?TP=1
AND here's the 124.128 (It's a 250D in Europe) page:
http://www.detali.ru/cat/oem_mb1.asp?TP=1&F=124128_15S&M=602.962&GA=722.418&GM=717.437&
The RU site,as it's known,is more user friendly(AND usually updates Parts#s quicker)
I found this on another forum:
http://www.benzworld.org/forums/w124-e-ce-d-td-class/1421010-online-w124-service-shop-manual.html
This is a link to a "Czech and Slovak Federative Republic"
(ČSFR, with the short forms Československo in Czech and Česko-Slovensko in Slovak)
website that has the actual MB service CD s for the W124 online.
CD1:
http://mb.auto.pl/index.php/Dok.-serwisowa-CD1.html
CD2:
http://mb.auto.pl/index.php/Dok.-serwisowa-CD2.html
More reading materiel:
Bleeding Fuel Lines on 1993 300D W124
http://www.peachparts.com/shopforum/search.php?searchid=4168951