Hi all, I'm a new 1990 W124 300D owner as well as a new member around here.I must say I love the car and I've already mad a couple of repairs. One thing i'm stuck on is a rough idling and in trying to repair it I've found there must be 5000 vacuum lines going every witch-away in the engine compartment. Is there a way to get rid of some of that stuff? I'm sure as these cars are over engineered most of the things that the vac lines attach to are unnecessary. Any help would be greatly appreciated.

You have an OM602.962 Engine if it was a Car made for the US Market.
Please note that I spell poorly sometimes.

On Mine with the possible exception of the EGR there is no un-needed Vacuum Lines.
I only know about mechanical reasons for a rough idle so I cannot comment on Computer Controlled stuff.

Also I am not sure exactly how your Fuel Supply System is equiped.
Air leaks on the Suction Side of the Fuel Supplyl System are a common source of rough idle.

O-ring go bad in Fuel Heater if you have that.

Fuel Inlet Hoses crack or get inelastic and don't seal. If you don't Have the Rubber Fuel inlet Hoses then you have some Plastic Hoses that seal with O-rings. The O-rings can be the issue.

Plastic Filters if you have that in the Fuel Inlet Lines crack. If you don't have the Plastic Primary Filter you have a small Filter inside of the Fuel Filter Housing that has an O-ring at bottom and top.

You might say that My Fuel inlets system has no leaks. But, we forget that there is Atmospneric Pressure around everthing and the Diesel Fuel has some viscosity to it and and in the face of that you might not get an external leak but Air can get in when the Engine is running.

The Fuel Pressure Relief/Overflow Valves have some tiny Plastic Parts inside that are know to fall apart or warp and that means at the Valve is not going to properly hold pressure. That is going to show up more at idle speeds.

The Fuel Supply/Lift Pump Valve could be causing the Problem. You can get a Rebild Kit for it but you will need to grind of file off a Crimp that is on the Outlet Fitting of the Fuel Supply/Lift Pump. There is a Do-it-yourself with some pictures on how to do that.

DIY Repair Links

http://www.peachparts.com/shopforum/forumdisplay.php?f=82

http://www.peachparts.com/Wikka/DoItYourSelf

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84 300D, 82 Volvo 244Gl Diesel

If you do the wastegate actuator swap… Do a search… You can eliminate all the vacuum lines on the passenger side of the engine.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife