Idle surging on W124 under load (especially in reverse)...
 

My very first post involved trying to solve this problem. Trouble was, it was so intermittent that I couldn't define when it occurred.

Other more serious problems cropped up since, so I had to deal with those first. Problem went away briefly, so I thought it was just a fluke, but it's back again with a vengeance!

Since I have taken my W124 offline to "exorcise" it of its little demons, I though I would try to attack this problem once more.

Idle is fine when transmission is in neutral. When I shift in "reverse", the idle cycles forcefully between 500 and 1000 rpms...if you look at the car from the outside, you would think I was having trouble engaging some "low-rider" hydraulics...

Click it back to "neutral", all is well.

This sometimes occurs when in "drive" but not very often.

If I need to look at vacuum hoses, which ones should I be checking? Some of them I've seen described in other posts don't seem obvious...

ASSumming the car is a 1993 300E that has electronic idle control & throttle body, I would have the idle speed computer tested for codes. The neutral safety/back-up switch has an imput to this control unit, and if doesn't send correct signals on what gear you have selected then you could have idle problems in that gear selection.
Also many times the throttle body & butterfly will have so much CRUD on them that they can't adjust the idle speed properly. THIS cleaning can fix this surge problem as well.

Hope you don't need new electronic throttle body, $1,090.

I am assuming this is the throttle body located under all of this black plastic octopus of an intake manifold? If so, what do I need to do to remove this? Looks daunting...:confused:

had surge in reverse condition when cold too on '94 E320 Coupe (96.4k)

will be picking up car from factory authorized service tomorrow, here are the items replaced: neutral safety switch, stop lamp switch, main wiring harness (and one other thing I can not recall right now)

hope this helps
-fad

Hmmm...the wiring harness was replaced under warranty back at 85K, so I hope I can rule that one out.

I wonder what the rest of the parts add up to...I don't want to start thowing parts at the problem, but at least I have a starting point...

from the RO:

1) performed elec test and found stop lamp swith signal to EA module intermittently inop due to high voltage drop at switch contacts (3.2V)= replaced stop lam switch, cleared DTC & rechecked stop lamp operation OK

2) performed elec test, tested circuit 50 PWR OK (12.4V), tested trans gear position switch and found intermittently inop due to fad internal resistor (10kohms)= replaced safety neutral b/up swith, cleared DTC & rechecked switch, operation OK

3) surge in reverse: verified pblm. DTC checked and retrieved DTC 80/96/182/224. checked vacuum hose/intake for vacumm leak, none found, prepared elec test & verified all input/outputs were OK at HFM (pwr 12.4V/Gnd .05V) module except intermittent high TN signal. tested ISC voltage supply OK (12.3V) tested ISC actuator and found intermittently No CTP SW signal due to bad switch contacts inside ISC actuator, replaced ISC actuator, cleared DTC, rechecked and verified normal idle speed resumed and no MIL or DTC reset.

hope this helps
-fad