n14 stc

I have a n14 stc in my dump truck I redid a few winters back I have not used it much hauled some loads with in just from the local pit all off hwy . I did not know the history of the truck as the owner died and I picked it up from his son. I was told it was just rebuilt and looks like that is what had happened found out when I drove it home was the first few miles on the new engine. it runs well good power but it smokes blue after it is warmed up when you left off the throttle or idle I does not always do it but 90% of the time it does. Just not sure were to start I feel l it is a bad injector but I am not up to speed on the stc of these engines it is a 91 330hp

n14 stc