Doesn't anybody check compression anymore?
I have seen a lot of replys to posts with hard start,slow start situations on vechiles with 90K miles or better and no one hardly ever says anything about compression.
Now 90K miles is a lot of revoloutions on that engine, a lot of wear so things ain't gonna work quite as well or as fast as they used too. The 6.0 haas so many issues with driveability we sometimes over look the basics.
You can have the fanciest computer and injectors you want, but without good compression you will get no combustion period. As common as all the other problems are they only deal with fuel delivery, only compression will ignite the air/fuel mixture and give you combustion.
So make a compression check one of your first diagnotic procedures. After all it is not that hard to pull a glow plug and screw in the adapter to do a compression test.