Mercedes (and BMW) straight sixes have a rough idle "by design"?!

My car (a 1991 W126 M103 300SE) has had a rather rough and irregular idle since I acquired it from an MB dealer (with approx. 95,000 miles in August of 2002; it now has approx. 115,000 miles).
My current mechanic tells me that that idle is typical of Mercedes (and BMW) straight sixes. However my car's idle was PERFECTLY smooth once after being serviced (by Ditmire's team), although only temporarily. Which proves that the idle can be as smooth as I otherwise always heard and read a straight six's should be. Well, at least temporarily... Techron also helps, although not perfectly and also only temporarily.
So here is my question: Does any of you have a perfectly, and permanently smoothly idling straight six, and in particular an M103?
Among all the possible causes I have gathered by reading this (most useful and helpful!) newsgroup but haven't addressed yet are: + vacuum leaks: can they be tested for? + spark plug wires: but some claim that they are not a maintenance item?
+ timing chain: I've read that its state (stretch) can also affect the idle?
========================PS: Here is what I have changed so far and recently, with the main reason:
+ fuel injection: ---- fuel filter, because I wasn't sure when it had been changed before;
---- EHA, because it was leaking; ---- injectors, because they were leaking too, resulting in difficult starts; ---- O2 sensor, because it was original but is supposed to be changed every 60k;
+ combustion air: ---- idle control valve, because the car tended to stall when coming to a stop;
+ ignition: ---- rotor and cap, because the contacts were in bad shape; ---- spark plug caps, because there were blue sparks (i.e. leaks) around them in the dark; ---- spark plugs, because there was some serious knocking at idle due to a faulty set;
+ engine suspension: ---- engine mounts, because they were worn out, although surprisingly the new ones, installed just a couple of days ago, are not making any difference in the vibrations perceived at idle: do they need to settle down?
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This is BS. The rotating masses of staight six engines and of 6 cylinder Boxer engines (a.k.a. Porsche 911) are perfectly equalized. This is one of the main advantages of these engine layouts over V-type engines.
Kick your mechanic and look for a new one.
Frank
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This article explains why an inline 6 is smoother than anything at idle by design... expect a V12, which is as smooth.
http://www.e31.net/engines_e.html
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I have a '95 and with 110K and it has had a rough idle only once. While not the same engine, it is a straight six. The temporary roughness was due to a leaky intake manifold (caused when the head gasket was replaced). One of the ways it was diagnosed was by looking at the hydrocarbon level of the on the emissions test. Before the head gasket replacement it was 13. After, it was 210. I've also been told that you can also spray some carburetor cleaner around the intake manifold, etc. and see if the idle changes (if it does, there is a leak).
HTH.

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Vacuum leaks sure can be tested for and located. A vacuum leak is unmetered air seeping into the intake manifold. It can enter via a cracked or broken vacuum hose, defective intake manifold gasket or, unlikely, a cracked intake manifold. Listen for a hissing sound and then, if the sound source isn't apparent, apply some carburetor cleaner or propane to the suspected site. This combustible will be drawn into the engine and the idle will be smoother as the air / fuel mixture is improved. BE CAREFUL, this can flash so don't put your face near the combustible.
Spark plug wires are not a routine maintenance item but after 12 years they should be replaced, especially if there's a complaint like yours, and even before extensive checking for a vacuum leak.
Timing chains do stretch and wear but the idle would not be affected, IMHO.
My E320 idles very smoothly at about 550 RPM. I recently installed Bosch Platinum 2 (plugs with two gaps) and am very pleased with them.
I suggest you focus in the injectors, spark plugs and spark plug wires.
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Our '95 E320 idles very smoothly.
At idle, I can not tell if the engine is running.
If yours acts differently, then there is a problem.
Best,
Ross

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