MP7 boost pop

[Midwest Farmer]

I’ve looked all over and can’t find anyone with the same problem as me.  I’ve got a 2010 cxu with the MP7 with around 320k miles.  I bought it used from CIT.  Had some serious boost leaks and several  sensors needing replaced. It now builds 48 lbs of boost but will randomly ”pop off”, it blows a puff of black smoke out the exhaust and drops a few pounds of boost according to the gauge.  I made a device to pressurize the system and check for boost leaks and also temporarily blocked off the discharge valve to eliminate it.  I found and fixed an o ring leaking on the elbow coming off the turbo and the small hose at the EGR cooler.  It has no codes and no mechanic has been able to solve it. Also, it does black smoke accelerating under load... which it shouldn’t... thanks for any help.

Edited March 28, 2020 by Midwest Farmer
Added information

[Mackpro]

Way to much boost . Probably turbo actuator is stuck in the full boost position. A guy around here has been doing crappy emission deletes. He’s using a “Export” engine file made for a eXport MP with no EGR and a non-VGT turbo.  Then locking the actuator in the full boost position. On the MP8 it kinda works OK up to about 42-45 psi of boost . But the MP8 I worked on started back firing through the intake/air filter at about 46-48 psi of boost . 

[Midwest Farmer]

The next thing on my list was to attempt an actuator calibration.  Is this something that could be freed up, or does it likely require replacement?

[Mackpro]

Mack/Volvo makes a gauge that checks the turbo stroke with the actuator removed. 90% of the time the turbo has to be replaced. We have seen actuators go bad or not calibrated but it usually throws a code.  We did have a turbo not doing the full  stroke  and had our DPF cleaning guy come to the shop with some special machine that shot hydrogen ( or something similar) into the intake/air cleaner and cleaned the carbon out of the engine and turbo. I don’t know what this service is called but I saw it work in this case and we didn’t have to change the turbo. 

[Midwest Farmer]

Sometimes the truck will do it with lower boost, maybe around 30-35psi.  It seems like it’s worse when the truck is fully warmed up.

[Midwest Farmer]

After all the trouble, it ended up being the boost pressure sensor on top of the intake just behind the intercooler.  I had replaced the sensor on the rear of the intake near the firewall, but it was the other sensor.  Hope this helps someone else in the future.