How many k's?
Last air filter replacement?
Last fuel filter replacement?
I'd be changing the filters as a couple of easy ones to rule out and can't hurt given that you probably don't have the historical knowledge of were the current stuff has been prior to you taking over.
What do you get from a compression test?
If this stuff doesn't fit with your skill set &/or available tool library, then get your machine checked out by your local diesel specialist.
Delving in deeper, if your injectors do need work, you'll probably want to be sure that they function correctly. The test doesn't appear radically complex:
but as with most of these things it's tools, wherewithal, knowledge and time.
The 5L / 5L-E engine is in that "old school" category, so it shouldn't take too much to spot the issue. My 5L just chugs along and has had the benefit of regular servicing over its lifetime. It's a solid piece of kit.
Whatever is the issue in your case, hopefully it's not an expensive fix.