M116 3.5 hunting idle
As Steve states, the hunting idle during warm-up was a problem with the D-Jetronic engines, particularly the M116 3.5 in the 280SE3.5, 280SEL3.5, 300SEL3.5, 350SE, 350SEL, 350SLC and 350SL (our 350SL here in Oz was a 3.5, not a 4.5 like the North American deliveries).
My 350SLC suffered the problem. Replacing the auxiliary air valve did not completely fix the problem. With age, numerous other slight vacuum leaks probably combine to raise the cold idle speed to the fuel cut-off speed, resulting in the hunting. I considered the possibility of a temperature switch that disabled the closed throttle fuel cut-off until the engine reaches normal temperature (this would also prevent the hunting that occurs when decelerating at low speed with a cold engine). The easier option that I used was to place a restriction in the hose ahead of the auxiliary air valve. With some experimentation the correct size restriction can be chosen that will result in an acceptable cold idle speed around 1000rpm without an unacceptably low hot idle speed. My M116 has worked fine with this mod for many years now.
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.