McRoth,

Diesels are relatively simple machines, and provided the essentials are there for starting they do well in nearly any temperature. From the description you have given, there seems to be a temperature threshold that affects the car's willingness to start.

You have addressed the first suspect, the battery. Next the items that are affected by temperature, low temperatures that is, are fuel flow, starting rpm, and possibly compression.

Fuel gets thicker and more prone to parafin precipitation as the temperature drops. In the teens the stuff can clog the fuel lines and not start, or even shut you down while driving. Consequently, good fuel, with a conditioner to keep the parafin in solution (or up to 15% gasoline) at the lower temperatures. Also, if you have a borderline performing filter, the lower temperatures will make it perform worse, also potentially leading to fuel delivery problems making starting harder.

Oil also gets thicker as the temperature drops, leading to higher pumping loads to get the oil circulation going, the load on the starter gets higher. This added load can slow the starter down, to the point where the heat of compression, which the engine relies on to burn the fuel, gets absorbed by the cold block as fast as the heat is generated, leaving the conditions inside the engine unsuitable for burning Diesel fuel leading to a no-start event.

Your valves are typically adjusted "cold" or at ambient temperatures on a warm day if you have the choice when doing the work. If you adjusted them on the tight side of the tolerance, they may be too tight when it is cold out. I adjust them a little loose as I live in a cold winter climate (my daughter in Upstate New York is using the car, also a 1982 240D) and it works fine that way.

You mentioned you have a new starter, and this is an area, if you check the posts on the subject, where you must be sure you buy from a decent supplier. Fastlane is great, but if you get their starter, get the one for the later 300 D, SD motors as they are noted as HD vs. the standard starter for the 240D. The higher rated starter fits right in, no problem. The issue here is how the rebuilders do the rebuilding. I have written on this issue before, but starters are rated for peak duty, not continuous duty, while cranking a Diesel for 30 to 90 seconds starts to look like continuous duty to the starter. It will overheat and damage the insulation, leading to internal problems with the wiring and its insulation. Most rebuilders do not address this, they just replace the bolt on parts. Bosch rebuilds do a better job, and replace old, worn wiring on the rotor and stator. This is a long winded way of suggesting your starter motor may be less healthy than you think and it could be contributing to the problem of slow starting rpm.

Some of your solutions are to look as noted above, for leaks in the fuel system, change to a synthetic oil (I highly recommend Mobil Delvac 1, 5W-40 but other Diesel specific synthetics are likely fine and lower cost - the Mobil 1 standard oils are not acceptable for your car as they are not Diesel rated - you would need their 15W-50 and possibly a 0W-40), for lower pumping losses. You should also check your vavle clearances and adjust them to the loose end of the tolerance.

I assume the car runs fine otherwise, so I doubt you have a really significant internal engine issue. On my 1982 240D I had some difficulty starting and it turned out to be broken engine mounts. I know this is controversial on this site but I am convinced half or more of the starting torque went into tossing the engine around in the engine compartment, leaving substantially less to rotate the crankshaft. It also led to a mechanically damaged starter. I replaced the motor mounts and the starter and now the car starts, admittedly with some protesting, at single digit Fahrenheit temps, not plugged in. It also enjoys an annual fuel filter change (about 20,000 miles) and runs on Delvac 1.

There is typically no single fix to this issue as any one can sometimes be overcome but together they can amount to too much of a hurdle. Hope this helps, 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)