1988 300 SEL with 111,000 miles. Car starts up fine. Idles ok first few minutes until it gets warm. When warm it runs great at speed. When I come to a stop it doesn't die (well every once in a while I suppose it does), but the RPM's oscillate and I can feel the car rhythmically shudder. A large shudder when I first come to a stop, and then ever smaller oscillations until it reaches a fine rumble that you feel every second or two. Graphically it would look like this:

Ignore the zeros and focus on the dashes. I couldn't figure out a better way to do this.

000000-
000000---
000000-----000000000000-
000000-------000000000----00000000--
000000---------000000-------000000----000000-
STOP00----------000----------000--------000----
TIME00-----------0-------------0----------0------- etc.
000000----------0000----------000-------000-----
000000---------000000-------000000----000000-
000000-------000000000----00000000--
000000-----000000000000-
000000---
000000-


I have been searching for the source for quite a while. I've read all the posts many times over. Everyone seems to have a different solution. Here is what I've done so far:

new OVP
New vacuum lines
cleaned idle speed control valve
cleaned throttle valve switch
replaced voltage regulator
New cap, rotor and wires
non-resistor spark plugs
new motor and tranny mounts
new o2 sensor
new fuel pump
new fuel pump relay
new potentiometer
new air filter
new thermostat
two bottles of techron injector cleaner

A lot of these made sense to me. Some didn't, but I either needed for other reasons (ie stopped running when fuel pump failed and blew relay) or I figured were probably due on a 19 year old car. I get why, for example the vacuum lines would cause a warm rough idle as I assume it takes a while to build up vacuum. The idle valve makes sense, as it's probably bypassed until the engine is warm. But one thing that is mentioned by many as a cause of warm rough idle is bad or dirty injetors. This is next on my list. I don't understand why bad injectors would cause a warm rough idle. Shouldn't it be equally rough cold? Even when cold the gas still goes through the injectors correct?
If I pull the injectors out to have them tested, I'm probably going to just replace them. Would injectors be your next step? Does it make sense to any of you why bad injectors would cause a rough warm idle, or am I just reading speculations and assuming them to be evidence based? As an aside, if you have any other thoughts as to what could be causing my rough warm idle, I'd love to hear them. For any who have made it to the end of this post, thanks.

__________________
Eric Zacharias

'88 300 SEL
'64 Ford Falcon
'99 Infiniti QX4

After cleaning the throttle valve, did you check if it closes the circiut in idle position?

Also, not sure about your car, but does it have a working throttle microswitch?

Where is the throttle microswitch? How would I go about testing it? I am going to check the idle valve today to see if it is closing the circuit.

__________________
Eric Zacharias

'88 300 SEL
'64 Ford Falcon
'99 Infiniti QX4

Not idle valve, I mean the switch at the throttle valve. It has 3 positions, idle, open, wide open. The microswitch on the other hand is located against the throttle link. the link activates the microswitch when the throttle cable pulls it.

http://img.eautopartscatalog.com/live/C701412167VMO.JPG