I did a HEUI troubleshooting thread, based on a 466E, but the system is very similar to the 6.0 HEUI , most of the pressures are close and the procedures should be very similar.
Low power usually means low HPOP either via pump weakness or electronic signal.
thats what I was thinking, but what i noticed a little while ago is that when pressing the primer with the fuel pressure gauge still attached, I noticed the relief valve is leaking a thin steady stream as soon as the system gets to 10-15psi, once I prime the system up to 40 - 45 psi, which is the highest it'll let me prime, there's a more steady/thicker stream of diesel coming out of the regulator.. I dont believe this is correct operation of the relief valve is it?
anyone know the correct name for relief valve or better yet a part # mines is a 2002, it's located at the back of the head/fuel rail
edit: I guess I should clarify I checked the relief valve with the return hose off, and engine of pressurizing system from primer, and then started engine and checked relief valve while running also, pressure never got above 50 psi, but valve was open and returning diesel