I spent the whole day messing with this thing today. I installed a rebuild kit on the STC valve, which consists of new o-rings for the spool valve cartridge and a new spring for the spool. The valve itself is dead simple. Just a spool with fuel pressure on one side and a spring on the other. When fuel pressure reaches 23 psi, the spool opens and allows oil to flow to the injectors. I tested the shift point on the bench and it is perfect.
I did some testing of the STC system before removing it, but it's really hard to interpret the results because the only place to check for oil pressure to the injectors is after a check valve, so the pressure goes up, but it obviously never comes down.
In my testing I capped off the STC oil line to the injectors and it made no difference in how the truck runs. So, its kind of inconclusive at the point.
My hunch is that either there is a blockage in the STC manifold to the injectors, or there is an issue with the injectors.
More to come.