In my years of doing this I have noticed that when 6.0l high pressure pumps start to go bad or fail all together it is hard to nail them down definitively as the root cause! You literally have to deadhead the pump to see if its bad or going bad and as all of you know this is a pain in the butt!!
Some of the issues that come to mind is a long crank then starts and runs sluggish, or starts fine but when driving at around 45mph then smashing gas pedal it feels like your towing a boat it just wont go!! the funny part is it wont even set a low icp code until it just wont start at all. That doesn't help does it!! So this is what I found and im going to share it with you guys!
The big thing to remember is that the high pressure pump separates the high oil system from the low! The I.P.R. is the exhaust for the high pressure oil system. So with this said one only needs to remember that when you air check the high pressure oil system and you close the I.P.R. valve manually with wires or you use I.D.S. or some other means that there should be no audible hissing air heard at all. So lets say you do this and have no air leak what do you do then? YOU REMOVE OIL FILTER CAP!!
EVERY failed or weak high pressure pump I have ever had does this because I have checked every pump that was bad. If they are weak they bubble less and you need to remove oil filter cap to hear it. When they are really bad you literally can feel the oil filter housing vibrating and gurgling.
I am posting a video of this for you to help and don't worry about applying to much shop air to the oil system because I don't think your shop air is over 3k psi LOL!!
IMG_15931_zpseb43c215.mp4 video by jugernaut2 | Photobucket