I have a friend with an 03 dually when his truck sits sometimes only 20 mins it's hard to start he has replaced his lift pump thinking it may have been just going out, but it helped a little he says the truck has around 137k on it but once it starts it runs fine and strong anyone with any ideas

 

Sounds like injectors to me, best pull them and have them tested. Check the limiting valve also could be leaking.

 

Any codes?

https://www.dieseltruckresource.com/...s-t224833.html