Cat c15 splutter question

Hi I have a 2005/6 cat c15 in a kenworth w900l.
Had a platinum engine rebuild dec 2017.

Just lately the last 2 or 3 times I've sat the truck for a day or more, when I come to start it she starts no problem but the 1st time I put my foot down it spluttters with a big puff of smoke and then drives fine...doesnt do it again.

She doesnt do this during regular daily use on and off either......only when she's been sat.

Is it likely to be a leaking injector or something else?

It's still under rebuild warranty so no problem taking it back just wondered what peoples thoughts might be before I take it back.

Many thanks

 

Ok thanks yea totally get what you mean.......it runs perfectly rest of the time