As one who bought my first M103 new, I never experienced any missing, roughness, or uneven idle until around 60,000 miles. Until then it idled so smooth that you couldn't feel it at all. As it got older, the idle started to get a slight and occasional miss to it and when you'd give it a slight throttle increase it almost seemed like it lost power, a slight hesitation.

I tried several bottles of Chevron Techron as the dealer suggested, no change except for the polluted oil necessitating early oil changes (per the Techron bottle).

I had the car tuned, of course. Also the ignition coil replaced, crank position sensor, and the fuel distributor. No change. RedLine "complete fuel system conditioner" or something like that did cure my problems, better than the part-changer mechanics did, permanently (with maintenance doses) and completely. I haven't seen as pronounced a problem nor change in my later (non-CIS) gas Mercedes (M104 for example), so it is apparently mostly a CIS issue.

I have had the same results in VW and Audi CIS cars. I think that it is just the nature of the CIS system, low flow rates with dirty injectors cause bad spray patterns at idle, it was covered in an SAE paper when Bosch started supplying injection systems to GM in the '70s and early '80s, and was blamed on the quality of fuel in the US vs the fuel in Germany. Carbon on the intake valve stems can also be a problem with CIS as the injector sprays into the intake port, at low airflow the carbon on the valve stem can absorb some of the fuel sprayed and cause mixture problems.

Also, the CIS system has simple O-rings sealing the injectors IIRC, a slight leak there will cause problems, was diagnosed as bad valves in my '77 Audi and only needed new O-rings around the injectors. Replacing the head was when they found the cracked O-rings.

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Gone to the dark side

- Jeff

Now, 'rough idle' is a bit subjective and vague term, that can feel different depending on what type of underlying problem you are dealing with (misfire, worn mounts, etc.).

In my particular case, in a 300E, 1990 with m103 and CIS-E injection, I generally have a smooth idle, but there is a slight irregular jitter that can be felt in the backrest, and in my hand resting on the door handle. It can best be described as a little jerk or jolt, on average once per second. It is inaudible, and there seems to be no dip in RPM at all. I am not suite sure if it is a misfire or not. Plugs, leads, dist. cap, rotor and injectors are all fairly new, so the usual suspects are eliminated. I also fixed a few small vacuum leaks, and the latest smoke test did not reveal any more leaks.

This thread is making me doubt whether it is an issue that can be fixed at all, or is inherent for my car.

/Viggo

I can't feel engine,sometimes,and think its dead,m104

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1999 w140, quit voting to old, and to old to fight, a god damned veteran, deutschland deutschland uber alles uber alles in der welt

agreed with the 104...103 not so much

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1989 300ce 129k
( facelifted front,updated tail lights, lowered suspension,bilstein sports, lorinser front spoiler, MOMO steering wheel, remus exhaust,stainless steel brake lines). (Gone)

1997 s320 154k (what a ride). Sold with 179k miles. Replaced with Hyundai Equus

1994 e320 Cabriolet 108k



1972 280se 4.5 153k Owned for 12 yrs, sorry I sold it


[/SIGPIC]

My M103 was always so smooth you could not feel it at idle, if you could feel it something was wrong...it would idle around 550rpm warm. My M116 V8 is the same way, warm idle is barely over 500rpm, so smooth you can't feel it. That is after doing all rubber parts on the engine and EGR delete, so there's zero vacuum leaks.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

My m103 was extremely smooth at idle, could not tell the car was running from inside unless you looked at the gauges. My newer bmw straight six was not as smooth as the m103 by a long shot, but that's an apples to oranges comparison


Sent from an abacus

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth

inline six's are supposed to be the best balanced engine.No need for balance shafts.Now if they have a hot cam,they will rock a little.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran, deutschland deutschland uber alles uber alles in der welt

What is the bad rap being given to the M110 engines?
I have one in my 83 280 SL and it is a beast, smooth and powerful.
It is a high rev engine and loves to be driven above 3500 rpms.
And it idles smoothly.
thanks
jz

Interesting observation about the M103 with a manual transmission having a rougher idle than the automatic version. I have noticed that the 87 with the manual does seem to vibrate a bit after its warm while the 91 is always super smooth and quiet.

I might also add (nearly 11 years later) that use of resistor spark plugs in M103s that were designed for non-resistor plugs alters the voltage wave form and reduces spark energy, which results in more misfires that increase idle roughness AND emissions.

Valve overlap is also a factor. The M103 camshaft is very similar in timing events to the sixties vintage L-79 Corvette 327/350 HP engine which pulls about 14-15" @ 750 in neutral and has a slight lope. My M103, manual trans pulls about 15" @700 in neutral.

If idle manifold pressure (ambient minus vacuum reading) is less than the critical pressure ratio of 0.528 (a concept from the science of fluid mechanics), flow across the throttle plate is sonic and choked, so the idle should be smooth, assuming no burnt valves, vacuum leaks, etc.

Sea level standard ambient pressure is 29.92" Hg. - call it 30", so if manifold absolute pressure is 15.84 or less, flow is critical. This corresponds to a vacuum reading of about 14". Above this manifold vacuum idle should be smooth, and less will begin to exhibit lope with increasing lope for higher overlap cams that make less idle vacuum.

The L-79 is on the ragged edge of critical and M103 is barely below.

Most M103s will idle smoother during warmup even after idle speed drops to the warm normal because the engine is still operating in open loop mode if coolant temperature is below 60C, which mainains the idle mixture at richer than stoichiometric. Once 60C coolant temperature is attained the CIS-E goes into closed loop, which drops the idle mixture to stoich. and the wiggles begin.

Duke

Yeah, Duke, I agree on the cam specs on the 103; definitely a high RPM spec with pretty tight LSA. Mine pulls about 16 vacuum, which at first made me look for vacuum problems, but turns out that's pretty normal.

Biggest issue I've had with idle over the last 25 years and 250k miles was by far motor mounts. Brand new off-brand mounts will rock and vibrate like crazy, making you think the motor is missing badly. I tried 4 different brands, they all shook the engine from the get-go and collapsed in a few thousand mile or less. Finally spent $150 on OE and problem solved.

Second is cheap injectors. I've bought about 3 sets in the last 25 years, and gotten some obviously pretty poor flow matches. Last set I got is pretty good, and she idles very smooth and gets better mileage. Don't know if OE, at 5 times the price, would guarantee good flow match; best bet might be to buy cheapies and get them tested.

DG

If my 300CE should have an uneven idle by design, it's gone wrong.

Damn.

RayH

A thread over a decade old. The OP never returned.
We might well recall a bit of a line from an old blues song:
..."don't start me talkin'"...

How about "deja vu all over again" as Yogi Berra would say.
But, one of the best things about this forum is the wealth of old info and history that we can resurrect and re-hash.
I actually find myself looking up old threads I posted years ago in order to remember history of what (and How) I repaired on my car way back then.

DG

My m103 idles just above 500 rpms smooth as silk. Changing out the IAC and airflow potentiometer with new OEM units really smoothed things out. My wife had a friend with one of these in high school. She noted that back then she always was amazed at how silent and smooth the car was. Almost like it didn't have an engine in it. 25 years later there still a m103 sitting in our driveway that runs smoother than a brand new Nissan four banger. Pretty amazing if you think about it.