Greetings / Dead Zone W124

[mwoodland]

Greetings folks.
Just picked up a Benz, just joined the list. I was a Benz mechanic through the ‘70s and early ‘80s, specializing in vintage restorations, 50’s through 70’s. Have kept semi-up to date on K and L Jetronics through the late 80s’ on other unmentionable Euro stuff.
Patient: 1990 300E, 118K, seemingly well maintained.
Recent repairs by P.O. New Oxygen sensor (2-wire), and new mixture adjustment “tower” on top of the fuel distributor. All to no avail, as far as I can tell.
Symptom(s):
1. Rough Idle
2. Mid-range hesitation/ Dead Zone, for about 1” of accelerator play, then the transmission kicks down, and it takes off like a scalded dog. Otherwise, going up a hill is a trial, the car slowing, and slowing.
I found that backing off the gas entirely, and pressing down again seems to reset the condition, and the car accelerates normally through the range that had just bogged down a minute ago.
I also noticed that, in some cases, when I backed off the gas slightly, the engine picked up Rs, and the car speeded up, kinda like it was over-carburated, overjetted, and running too rich, and by backing off, it would lean out, and run faster.
3. Idle speed does not respond to pulling the dip-stick (Should it?)
Diagnostics to date: Possible vacuum leak. By isolating circuits, one at a time, and testing with a Mity-Vac, everything seems to be responding appropriately (As far as I can tell without any manuals, schematics, resources, etc. Isolating and blocking vacuum ports one at a time didn’t help.
With engine idling, I shot all of the injectors/seals w/WD-40 to check for vacuum leaks, as well as all of the usual suspect spots.. no avail..
Actuation of EGR valve makes idle rougher.
Pulling the plug on the Idle Stabilization valve caused the idle to drop to 450 or so, and after reconnecting, the car had to be shut down and restarted to reset the ISV.
Any ideas would be appreciated, as would any suggested resources for technical info, manuals, etc.
Thanks in advance for your help.
I'm new to this list, so if my post is too long, or otherwise ill advised, I trust you'll let me know.
Thanks,
Mark

[KermitF]

Mark,

After a couple of years of working on rough idle, I replaced motor mounts and transmission mount earlier this year and idle on my '90 300E smoothed out immensely. Do the books/records you got w/car show last time these were changed out on your 300E?

[mwoodland]

So... Several hundred dollars later.
The (new) O2 sensor checks out, no vacuum leaks, per the 'smoke test'.
Everything checks out OK so far, but it had codes for EGR and O2 sensor.
I pulled the EGR tonight, and the diaphragm is leaking, but the valve itself is tight and not leaking, i.e. not contributing to an internal vacuum leak.
The Benz shop has exhausted the usual suspects, and is looking for a bag 'o dollars to start the next (as yet undefined) stage in diagnostics.
Has anyone had trouble with the throttle position sensor, or whatever its called, under the throttle linkage, and does anyone have a diagnostic routine for it?
Best regards,
Mark

[schumi]

Rough idle on these cars is something discussed with incredible regularity on this board. Do some searches for EHA adjustment. Another search for cleaning idle control valve. Also changing the rubber hoses to/from the ICV can help. There is also info on MBCA website on how to bump up idle 50rpm which can help with rough idle.

[mwoodland]

Sorry. The fact that I omitted is that the idle has smoothed out entirely.
Smooth as silk.
But the midrange is still a horror.
Stepping on the gas at 65 mph is like stepping on mashed potatos. It just bogs until it kicks down and takes off like a scalded dog.
The throttle has about 2" of dead / non-responsive travel.
Mark

[pesuazo]

That certainly sounds like a problem I had for about a month and just "disappeared" by itself.

The throttle microswitch, the throttle position sensor and the potentiometer are 3 suspects I was going to tackle, but the car has not had the problem for over 6 months.
So I am just waiting for it to happen again.