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?


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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?

The M103 should have a smooth idle, and is a better engine than the M110 preceding it.

Vacuum leak may produce a fluctatuating idle speed. No real test but a matter of tracing the vacuum connectors to look for a loose connection. Spark plug cables can fracture especially if improperly plucked when disconnecting from the spark plugs.

I would not accept an answer that a M103 has an inherent rough idle. Even BMW straight sixes are smooth at idle.

Hope you sort out the problem. The M103 engine is an easier to maintain compared to the M110, and is miles more refined and economical.

I Know this kind of doesn't answer your question. But when I lived in Bradley Beach, I took my car to Nick Rossi, at German Automotive in Shrewsbury. The car was running terrible when I first got it. When he was done i.e air leak under intake manifold and few other adjustments, timing, CO and a few vacuum leaks I thought it would happen. I guess what I am saying is since you are in Redbank Check him out. Almost all he works on are 126 and 124's. BTW I have no affiliation with him. Just highly recommend him.

Did you replace the tranny mount when you did the motor mounts? If not you should.

Can you describe your idle problems more precisely? Surging? stumbling? intermittent miss?

I have had three straight 6 BMW's and the ones with the 2.5 litre always idled very smooth, its the bigger 3.5L engine that is know to idle rough and it is true they do idle rough (3.5L).

I'll have to try an M103, because I love my M110.

There is something wrong with your car!
 

I have a 1986 and the idle is very smooth. There are a lot of things that may cause a bad idle such as bad vacuum hoses/connections, fuel injectors, fuel filters, plugs, plug wires, fuel distributor, and the list goes on and on. Get it properly diagnosed and fix it.
Good luck!:cool:

Your car probably has a hormonic balancer between the engine and the transmition. Its job is to cancel the engine vibrations during idle. I have spent money on motor and transmition mounts but idle was rough. Then one day the hormonic balancer broke and replacing it made a big difference for idle. My car idles smoothly and doesn't vibrate at acceleration anymore.
I don't know how to check if it has gone bad. Maybe someone else here could help.

In order:

Engine mounts -- if not replaced, do so. Major improvement -- must have nearly an inch clearance between largest diameter of mount and frame.

Vac leaks. Idle control valve hoses are thew worst culprit, but all the vac lines have rubber hose connectors, and they are all probably bad.

Has the cap and rotor ever been replaced? Plug wires? Probably toast by now, again major improvement when replaced.

Injectors -- uneven fuel delivery at idle will cause rough running, cured by new nozzles. Usually cannot be cleaned because the internal filter is clogged. Purge the fuel distributor lines, too.

Worn valve guide seals or valve guides allowing air in around the injection system. Worn guides will require a valve job (typical on older M103s at about 150,000 miles, typically not a problem on newer ones).

Peter

m103 is smooth.

You are right about the BMW 3.3/3.5L motors though.
Idle always 'sounded like something must be wrong', but that was just their nature. Smooth once you revved em up...

The M103 2.6L in our 190E is dead smooth at idle despite having original engine mounts and tranny mount. I did a full tune up on it recently and a VERY slight idle shake was solved with new plugs, wires, etc.

The typical culprits for M103 idle shake have all been convered here. Check that tranny mount is it seems like the car is firing properly. If it's fine, check for vacuum leaks, EGR valve and tune up parts (cap, wires, etc). Failing all that, have a compression check run to see if maybe a cylinder has died.

Oh, and top off the tank with "known" good gas.

I have read in 103 Mercedes Literature [the service manuals and CDs, I believe] that the engines tend to idle more smoothly with the EHA disconnected. They go on to say there is no known cure, it's just the way they are. It was in the "programmed operations" section or something like that.

Both our 103s have a bit of rough idle; both are the same.

I drove a friend's 104 on a trip last weekend and you could not feel it running at all.

Maybe the difference is the KE vs the HFM fuel injection.

Most if not all 103s will show a hint of uneven idle roughness, and it's probably more pronounced on manual transmission cars when idling in neutral. The slight load that an auto trans places on the engine along with the damping effect of the torque converter will smooth out the idle, and the additional load of the A/C compressor will further smooth the idle.

The source of this -what I call a "designed in feature" - is two fold. First is the KE injection system. Continuous flow systems tend to have more cylinder to cylinder fuel distribution variance than solenoid injectors, especially at idle, and I also expect this is a root cause of the 103s higher than average emissions.

The second problem is that maintaining the average idle mixture at stoichiometric (14.7:1) rather than the 12.5:1 that the engine would prefer (and was typical on pre-emission cars) can create greater cyclic or cylinder to cylinder variation in the pressure-volume diagrams. In other words, there is enough cylinder to cylinder torque variation over the two revolution for all six cylinders to deliver a power stroke that the engine can develop a slight uneven torsional vibration that we feel as a slight uneven idle roughness. The combination of the "lean" idle mixture and cylinder fuel distribution variance conspire to create the slight uneven roughness that we often feel.

I noticed on both my former '84 2.3 and current 2.6 that both developed this characteristic slight idle roughness at about 40K miles. It varies in intensity from time to time, and always seems to idle better after a good "Italian tuneup". The slight roughness is primarily a cosmetic issue, but the emission issue is real. There is no "fix" - you just have to know how to play the emission test game.

The idle roughness is never noticeable until the engine temp achieves about 60C, which is about the time is goes into closed loop operation and the average mixture is maintained at stoich. When cold the idle is always butter smoother because the mixture is richer.

It's just the nature of the beast and why Mercedes eventually abondoned the KE system in favor of a full EFI system with solenoid type injectors.

Duke

Your explanation sounds very logical to me. I have exactly the same problem. My M103 has everything perfect except its idle doesn't look right to me. I once unplugged O2 sensor and noticed that my M103 idled better. The same effect went when the EHA was unplugged

I changed the injectors, cap, rotor, tightened all vacuum connections but still mild vibration at idle doesn't vanish.

If it is really nature of any CIS system maybe I just leave things as is.

Other M103 users, what are your thoughts?

vacuum leaks
 

i would disconnect and plug all vacuum sources one by one and check for a improvement.
one thing for certain my 103 runs smoother in cool weather rather than hot summers.
mak