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SORRY FOR THE LONG ONE
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Hi, I've been over quite a few Forums, searching for a similar problem, without result.
So, here I am!
I bought this Mercedes E320 Model Year 1995 W124 with the 104 3.2L engine.
The vehicle was not drivable at the time of pick-up.
Someone tried to repair the wiring from the Throttle Body and failed miserably.
Blank wires touching each other etc.. I towed it home.
Replaced the Throttle Body with a 2004 Stamped one.
Hey the car would run and drive. High Idle 1100-1300 no load.
Traced it down to the e-gas computer. Possibly shot input (Idle Switch)
A replacement fixed this one.
Now the engine idles great, has a lot of power (as long as it stays cool outside)
Failed smog with extreme high CO HO2 Nox in any category.
The CO was measured at 4.65% guess this one would be a nice suicidal Car.
Naturally the other numbers went out of wack too.
The Spark coil cover was stripped of it's threads put inserts in, it holds much nicer the coils in place, now.
Replace the spark plugs etc. etc.
Checked at random the wiring harness of bad insulation or other signs of foul play.
Nothing! Unless it's inside the connector!
Verified that the intake manifold has enough vacuum, was good.
During all of this the CEL was off. I didn't have any codes on port 8 / 14
But I had always a '6' in Perm.
As we know this is a Idle control problem. Idle was wonderful, so what was the problem?
Now, finally I get more codes, guess since the temperature rose a little.
I have:
'8' Coolant, I unplugged and re-connected that one.
'9' Air intake temp, I unplugged that one and put it back.

This is the one, it makes sense if my CO is at 4+% it can't adapt any-more.
I went and checked the signal from the ECM to the Fuel Injectors, open circuit (unplugged / no load) with an Oscilloscope to see if there was a possible short in the wiring and one or more would flood the intake port(s)
Nope, looked pretty good, although there was a increasing voltage from Cyl. 1 on 400mv - 9.2V The width is at about 3sec. since the engine was cold.
Does not really make sense to me, in the contrary, I don't know what the computer is sending?
"Careful" the voltages measured may or may not be correct!

Unplugged each Injector while running the engine. It acted just as I would expect the engine to do. Each cylinder reacts as if the is one missing and comes back as if all six are running.
Unfortunately I can't measure the fuel rail pressure since I don't have an adapter.
Unplugged the vacuum at the fuel pressure regulator, nothing coming out, not even a sweat.

I had the EGR taken out to see if there was any clogs, nope nothing stuck a piece of wire inside the tube nothing to be found the tube is clean.

I checked the O2 sensor with DVM and O'Scope, it appears to be doing just fine.
Went to a Muffler place to exchange the Cat, they didn't want to touch it, because of the high CO values. They said it will kill your Cat within a very short time and it will not knock down the CO by that much.
I might still need a new cat, but need to fix the rich running first.

That's pretty much where I am at!
I ran out of ideas and feel sorry for myself that I bought this bucket.
I doubt that a new fuel press. reg would do that much of a difference.

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Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert