Stanadyne is the only product I have used successfully in industrial diesels subjected to days of cold weather idling. Stanadyne also is a large mfr of pumps and injectors. If an additive can help, which I kinda doubt, it's where I would go. I know zero about Duramax injectors. In older injection systems, after days of idling you would get carbon buildup at the tips, destroying the spray pattern so the fuel would dribble from the tip instead of atomize causing incomplete combustion. 50 years in heavy equipment repair.
