Hope this is not too dumb a question, but how are we poor souls driving 240's with no tachs suppose to set warm motor idle speed?. I suppose it should be somewhere around 750 rpm but what on earth do we have to measure it by? You can't pull up to a Duramax at a stop light and set your idle speed knob until your motor sounds vaguely like his at idle, that's apples and oranges. Was the proper idle RPM set at the factory and then we adjusted our "idle speed knobs" from there depending on conditions? I just recently set mine to what I might consider a fast idle, in my mind and ear I think it's running at 1400RPM(going purely on sound) but this adjustment makes for smoother shifts(no lag time between motor and tranny) coming up to speed. What does everyone else do?

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1981 240d
Newport,RI

Great question..I think I will sit back and watch the comments fly in. I too am perplexed with the idle speed on my 300D 1981. Help guys!

Dakota

W123 300D 1981 China Blue 196K

I set the idle adjustment screw/bolt on the engine side of the IP to the lowest idle speed the engine seems able to handle {comfortably}, then with the idle control knob in the cabin set fully clockwise {low idle position} I adjust the cable pretty much to where there is no slack. Then that allows the cable to pretty much have max travel from that low idle position ccw to a higher idle speed for starting and AC season.
I don't know if this is close to the MB recommended way, but it works best for me....

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Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John

I recently raised my idle because while it was good enough in warmer temps, it was too low in cold temps to keep the engine going after I started it unless I helped it along for 30-40 seconds. I am guessing it was around 600rpm....I raised it a bit (no tach)....and its much smoother than before and idles on its own even when started at 0F....great improvement. Its a bit lower (sounds slower) than on my dad's SD...which is set to the max of 850. So maybe 750-800ish is where I have mine, and I like it at that speed.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

About a year ago I bought one of those tiny tachs specifically made for diesels. I wanted to see what my idle rpms were on my 84 euro 240. It showed 715 rpm. According to Mercedes, the idle should be at 750+or -50rpm. When the ac is on, rpms drop. I then used my idle screw adjustment to raise it back to the 715rpm.

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Bob Jenkins