I know this thread is a bit old.
I hope the car is running fine now. I had a '75 300D with a block heater. I found that I could start the car reliably in temperatures down to about zero. Below that, it was a problem without the heater. I replaced glow plugs, batteries (I once had a 1050 cold cranking amp battey from a Lincoln Continental Diesel) to try to speed up the crank to help.
The starter finally gave out one day and I had one shipped to me from a salvage yard in Texas. Boy, what a difference! I never knew that the engine was supposed to crank that fast! I never had a problem starting after that.
'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)