I have a 6.5 I just bought I got rid of my powerstroke. I looked at the truck in 20* weather it started first time great it ran great has a new DCM. Well it got cold out again and know the truck slow starts it seems like it cranks slow and it starts but it bellows black smoke out when starting the clears up after started. Truck runs great has no blow by when at operating temperature. The guy I bought it from put a new starter in and batteries are newer. I'm new to the 6.5 motor I don't know anything about it.

 

Have batteries load tested and clean all grounds especially the one on the rear passenger side head and if all that checks out you may want to drop the starter and clean between it and the engine block I have seen dirt and grease cause slow start problems as well.

 

As stated above, load test batteries, clean grounds, make sure batteries have a decent CCA rating also. In addition, do the battery terminal mod (search for it on dieselplace.com, it's free/cheap), check your battery cables and replace with larger gauge as necessary, check glow plugs (replace with Bosch Duraterm if necessary), look over your glow plug harness and make sure it's in good shape (PTWS sells a very nice one if yours needs replacing). While your in there, make sure the starter has the bracket on the front of it.

Good luck!

 

Sorry to get back so late ok thanks for the info.