We ought to make a "W123 Diesel Cold Weather Starting" thread and keep it as a sticky since there are a lot of W123's out there and every Fall and Winter this topic becomes a "FAQ."

These old engines will run in Winter provided they are getting decent compression, have a starting rpm that ensures the heat of compression is maximized, functioning glow plugs, and fuel delivery. Sounds simple but there are a number of systems and subsystems that have to work together to achieve all that, and poor starting can be any permutation of weaknesses in these systems that reaches a threshold on a certain day......

So, the issue of compression was brought up, and if your car has been treated reasonably well in terms of oil changes with proper grades of oil, the rings and cylinders will likely be ok. They have cast iron heads and are not as susceptible to overheating to cause head gasket problems, but, compared to newer Diesels, have a tedious requirement to adjust valves. Poor valve adjustment (they close with wear) and you won't be breathing right, and you will waste battery and starter oomph to turn the engine over.

Proper oil for winter is also critical. These older cars love Mobil Delvac 1, or mine did/do, but any decent Diesel rated oil for winter service will be helpful. Valve adjustments are critical. Good battery is critical. Healthy starter is critical. Glow plugs are critical. A "solid" fuel delivery system ("solid" meaning without any air from essentially the in-line filter to the injectors) and injector nozzles that produce a mist and not a spurt of globular, congealed Diesel fuel are critical.

I am not sure but I thought 300D's had the idle speed adjusting knob on the dash like a 240D. Maybe not on the turbos. I never owned one so I don't really know. If you have it, turn it "up" when the engine is cold and it does the same as you holding your foot on the pedal a bit. When the engine warms up and the idle speed is too high, you turn it down. The turbo may have something more sophisticated and I cannot offer any advice if it does.

Good luck. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)