Idle Mystery on M117 engine

I have been having an intermittent idle problem; it jumps up and down between 600 and 2000 RPM. It will also stall, sometimes even coasting to a stop. It was so bad it would stall after running only one mile. I changed the fuel pump relay (with rebuilt) and it seemed to go away but when it really warms up (about 90-95 C) it will act up again. When it is warming up, and warms up, it runs perfectly and idles so smooth and quiet that I have to look at the tach to be sure it’s running. In city traffic the temp goes to 90-95 C and the problem appears. If I get on the highway or just go without a stop, and the temp drops to about 80-85 C, it goes away. Seems to be linked to the temperature. Any ideas?

Steve
1985 SL500 (Euro) (With Lambda system)

[Benzmac]

I have had to replace many of the idle speed control units and the actuators. When the actuators fail, they pull way too much current trying to get the idle down. This will burn out the driver in the control unit. Most of the time, you will end up replacing both.

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Benzmac:
Donnie Drummonds
300E
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM
OWNER OF MB AUTOWERKS .COM

[Q]

Sounds like the problem I was having with my 116 motor. My mechanic gave me two options. A new idle speed control module for $300 or a manual idle control valve for $25. I had spent plenty at the time on my car, so the $25 sounded pretty good. I took a while to tweak it out to a good balance between air conditioning on in drive versus a/c off in park, but it is fine. I have read that the solder joints on the circuit board can cause the problem I have resoldered all of the joints, but haven't reinstalled the module yet to see if that did it. Thankfully, I remembered to ask my mechanic for the electric idle control valve back before I left.