hard start

hello , I hope some of you guys can help steer me right ! picked up an m998 runs out very well but hard start and kicking lotta white smoke followed by big puff of black smoke / then clears and runs great / easy start when warm .
Assumption of course plugs / guy we bought from said he just changed em , we changed them again anyway ! we changed the pcb even though in terms of wait lite etc it seemed to be working /we changed coolant temp sender sensor still same issue of hard start and white then puff black smoke / we even changed fuel filter although seems the fact that its belching white smoke (unburned fuel )and then black smoke (burned fuel)would indicate fuel not an issue ! been scouring site and came across grounds as a possible issue going to try that tonite ! any other thoughts would be much appreciated

The white smoke means glow plugs. Check everything again.

What’s hard to start mean? Is it temperature dependent?

Scroll down the list of today’s posts. HMMWV glow plug gospel....last few pages. Similar condition described.

Plugs for sure but also make sure you have clean fuel (that isnt old)..
Bad diesel will make all sorts of issues for ya...

Check that the glow plug harness connection to the plugs is not feeding 24v constant when the ignition switch is in the run position.
It can do this even if your wait to start light is cycling normally. If it is, that's your "smart" box. Workaround is to put in a manual glow plug controller.
search my posts if you need help.

If the plugs aren’t working and it’s cold...it will not fire up. He may have a bad plug or two. Don’t get him running down a rabbit hole. The truck will smoke, it’s ok. Read the TM, understand how the vehicle functions. It’s not a new Duramax....

And after you've done all this go to the G503 site and read Steve's excellent post about starting issues here: https://forums.g503.com/viewtopic.php?f=63&t=230913