Anyone With a DD15 and boost gauge. I need to verify something.

Variable geometry turbo? If so the vanes stick and they'll get stuck in the closed position. I've seen it on duramax pickups and 6.7 cummins. Just my first guess.

 

No you do not need back pressure to drive the turbo. The turbo creates back pressure between engine and turbo but after turbo back pressure IS NOT NEEDED. Back pressure after the turbo limits how well your turbo can work. If there is an excess of back pressure after turbo it wouldn't be able to spin. Think of it as a water pump, if you block the outlet side of a water pump the pump will slow down or stop spinning because the water can't go anywhere. It would build back pressure and again the pump would slow or stop.

 

What's the temperature like where you're at? It's cold where I live and my Mack started spooling the turbo the same as yours does. It does it at idle too, to keep itself warmed up it puts a load on itself

 

Assuming that was even your problem... on the 07.5 and 08 6.7 cummins you had to remove the turbo and physically clean the exhaust side of the turbo. The vanes that are driven by an actuator would get gummed up with soot and a good brake clean bath would do the trick.

It was ridiculous to have to remove the turbo and disassemble it on a regular basis so cummins had a "cleaning port" added to the turbo on later years so you could simply spray the vanes clean without removing anything.

A good scan tool or laptop would tell you if your turbo actuator is traveling fully and returning to 0 or if exhaust backpressure is high due to numerous reasons such as others stated.

When those turbo vanes would first begin to get gummed up it would show itself by a brief delay in the exhaust brake hanging on slightly after you returned to throttle. You would typically (assuming you were smart enough to notice) release the throttle and get back in it a few times engaging and disengaging the exhaust brake and it would free up the vanes in the turbo. That would work a few times but it would be imperative to get the turbo cleaned asap because it can build up a ton of cylinder pressure and heat by trying to pull a hill with your exhaust brake partially or fully on.

I don't know what your actual problem is but that turbo issue came to mind when reading your post. You really should hook up a scanner to it, it'll save you a ton of time.