Over the past year, I rebuilt the top end of my 6.0 in my 2008 E450, replacing everything that needed it. Once I had it all back together, I took it out for a test drive. It shifts through the gears fine when I barely touch the throttle, but it only builds up to 8 psi of boost at most. Since then, I’ve tested the VGT solenoid, cleaned and rebuilt the turbo, and checked the IPR and HPO system. All the sensors are new. As I mentioned, it shifts well with light throttle, but when I need to downshift for passing or something, it downshifts and my boost goes negative. I have to ease off the throttle to stabilize it and allow it to shift. I’ve checked for boost leaks none found at 25 psi and exhaust leaks, and my intercooler is the all-metal version, so no leaks there. I’m at a loss and really don’t want to replace the turbo. Does anyone have any ideas or advice? I have several videos showing the symptoms if anyone is interested in helping.
Inspect the nipple on the intake manifold for any blockage in the hose leading to the MAP sensor. If it’s obstructed, the MAP sensor won’t detect pressure buildup, resulting in no additional fuel being commanded. A less common issue could be a clogged EBP sensor tube, which typically causes significant lag but still allows the system to reach full boost.
Hey, thanks for getting back to me. 
I have already tackled all the basics, replaced the EBP sensor and tube, changed the MAP sensor hose, and cleaned the nipple. Now, I’m at a loss and not sure where to turn next. It could be a computer issue, a turbo problem, or something else I have not considered yet.
Did you check to make sure the vane control shaft was intact when you cleaned the turbo? It shouldn’t rotate 360 degrees and have brakes on both sides.
When I had it apart, I can recall moving it back and forth, and I’m fairly certain 90% that it would not turn 360 degrees. At that moment, I did have the actuator and plunger out.
What symptoms, if it were broken, would I experience?
It does not cycle when it first starts up. Must it?
If it’s malfunctioning, it’s typically stuck in one position, often resulting in no boost, though nothing is guaranteed in the automotive world, I have encountered some bizarre issues that defy logic. What isn’t cycling? The turbo? It should, once the engine warms up, as it continuously runs monitors and cycles under certain conditions. Have you checked the MAF readings? Also, verify your fuel pressure, it should remain above 50 psi while driving. Low fuel pressure can lead to reduced power and boost.
The vanes move freely since I can cycle them using FORSCAN. Simply put, I lack BOOST🤦🏼♂️.
Some trucks that I have observed start up while cold and then cycle. That occurred during a few of my previous examinations. By the way, the boost issue existed beforehand.
There is constant fuel pressure more than 60 psi. I am without power, so I am unable to receive a true WOT. It will not pull under WOT as a result. However, regardless of the test I was performing, my fuel pressure never fell below 60.
WOT stands for wide open throttle, meaning you have the pedal fully pressed down. During a relative compression test, you hold the pedal at WOT while cranking the engine which should not be running. If the fuel pressure stays above 50 psi while you’re flooring it, regardless of the boost level, then the fuel system is likely not the issue. If everything else checks out, the problem might just be a new turbo is needed.
I should have explained, haha.
I meant that I am unable to get it to react as though I’m giving it WOT.
Which brings me to my second query, the turbo comment. Given that I have already rebuilt it without replacing the VGT solenoid, what could possibly go wrong with a turbo to prevent it from producing boost?
Are you sure the catalytic converter isn’t clogged?
I am. I have the honeycomb catalyst on the back of my truck. 