Truck will start when cold but not when hot?

 

high pressure oil leak....must see 450-500 psi at the icp to fire the injectors...when the oil is cold it is thick enough to make enough pressure but when it warms up it thins out and wont make 450psi any more

i would start with a icp air pressure test then if you hear air under one of the valve covers its probably an injector oring

but that is only one of the many places you can have a high pressure oil leak

 

What year?

A ScangaugeII is a handy tool to verify that the HPO pressure is too low to start.