This is a cry for help.
I've been trying to fix this rough idle for sometime now, and I've read many posts on the topic.
First thing I replaced were the two rubber hoses that go to the ICV, this helped.
New air filter, new plugs, (NGK regapped to 040 as per this forum) wires, cap, rotor... again this helped.
Oil and filter changed at regular interval (10,000 kms).
Buzzing noise from fuel pump fixed by replacing pump, along with fuel filter.
Replaced OVP relay and fuel pump relay.
New EHA along with eighth of a turn of screw, this helped.
New throttle cable. Lubricated throttle linkages. Adjusted bowden cable.
New O2 sensor.
New engine mounts. New coil. New voltage regulator.

The fuel injectors were replaced (supposedly) about 18 months ago.
I say (supposedly) because I'm not 100 % sure if the mechanic actually did replace them. (He was a crook and went broke because he lied to customers)
Maybe the injectors are old (how can you tell)
All along I've tried to keep the mixture reading at approx 50% as per duty cycle readout at x11 plug. (non California model)

Car starts instantly, drives perfectly, but till tonight, had a rough idle.
(along with a strong smell of exhaust fumes) It was not possible to drive with any of the windows down. The exhaust smell was too strong. Also the exhaust sound was a deep rumble, and because it was rough, it almost gave the impression that the motor was not running on all 6, not a smooth continuous sound.
Tonight, I pulled the ICV and cleaned it again (just to be sure) and then leaned the mixture screw down to 18% as per duty cycle readout. This had the effect of smoothing out the idle to near perfect and reducing the smell of exhaust to almost non existence. (rumble sound gone from exhaust now) sounds smooth (as it should.)
Took it for a drive, went like a charm... and lately, I've been watching the 'economy' guage and also watching the amount of fuel consumed. Tonight the 'economy' guage was hardly in the red.
Now I know 18% is lean, but the idle is absolutely perfect (well nearly).

So what does this mean.?
Does this point to something else not working.
18% duty cycle readout = smooth idle at the moment.
50% duty cycle readout = rough idle at the moment.

Another mechanic (a fairly reliable one, I'm led to believe) told me about twelve months ago that the fuel distributor was faulty and needed replacing.

Would a faulty fuel distributor display these symptoms.?

Car accelerates smoothly, no hesitation or misses, does not stall, plenty of power.
As I mentioned previously, starts instantly, hot or cold.
I apologise if I've missed a post somewhere that covers this.
Thoughts and opinions most welcome.

did you ever get any opinions on your problem as mine is similar everything is great except idle, sharp response no misses great power, just a bit of rough at idle in gear (like at a light) neutral just a hair of roughness at 1bout 700 rpm in gear only drops 100 or so and I have done most of what you have plus valve job

Have you replaced your injectors?

I still (well have always had) a slightly rough idle. If I run a bottle of injector cleaner through the tank it almost disappears. Injectors are original as far as I know.

Mounts (all three) have been replaced) along with EHA etc. Valve job 65k ago.

I think I will replace the injectors and see what happens.

__________________
'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E

injector cleaner doesn't seem to change much

The one thing that finally helped improve my idle was to re-gap the spark plugs (non resister) to .040.
The improvement to idle smoothness was outstanding.

sounds like maybe the Cat should be looked at-

__________________

Timothy Evans
TimEvans35@gmail.com
CELL 310 990 6780


1985 380 SL
2010 CL 6.3 AMG
*2009 CL 550 Sport 4matic
New addition-1984 380 SE
Newest Edition 1986 300E AMG

Hello,

I had a similar problem on my 1989 300ce. Replacing the spark plug wires fixed it for me.

According to the original Mercedes hand book the spark plug gap should be.032in. It also states Bosch, Beru, or Champion.

Gavinbr

If your economy gauge is "barely in the red" you have a vacuum leak. The economy gauge should be pegged to the full left at idle. Find that leak and fix the last of your problems.

Look behind the engine in front of the firewall. On my V8 (1988 560SL) there is a primary vacuum tap into the intake manifold at that location. That is a location of max heat and oil and the rubber connector deteriorates often. It is often overlooked since it is hard to see.

I have a 420 SEL and I had a similar issue but no exhaust smell and it was a cascading effect from bad vacuum lines. Also there should be a R16 resister near your break booster wrapped in electrical tape I pulled mine and the car runs MUCH better. Use 91 gas or better. Just pull the resister right out, it retards timing for our crap gas. If it starts to die it will offer more resistance.