Here's the run down, I changed my oil and oil filter 2 weeks ago in my 08 6.4L. I'm not sure if it's coincidental or if it has something to do with my issues but here they are. BTW I have an H&S minimax tuner: If i stomp on the throttle there is really bad turbo lag for a few seconds and my exhaust pours out heavy black smoke until it picks up and takes off. My gas mileage went from 16mpg to 10 mpg. I dropped it off at Ford and they couldn't figure out whats wrong with it so I'm not sure where to go from here. My minimax says there are no DTC's found when I run the check.... Any thoughts?
Thanks in advance!

 

Here is a TSB that may or may not help:
6.0/6.4 Fuel Economy:
Ford vehicles equiped with the 6.4 PowerStroke diesel are not rated for fuel economy by the EPA. If economy is in question, a boost test should be performed. If boost reaches 22-25 PSI in third gear, wide open throttle under load, then th engine s operating normally and no economy related repairs need to be performed. If not in specs, then the engine will need to be diagnosed--check sensor reading, VGT and EGR operation, fuel pressure and quality, crankcase pressure, intake or exhaust restriction, exhaust or CAC leaks.
Fuel economy will be affected by many factors, including: excessive idle time (one hour equals approxmately 33 miles of driving), fuel quality and blend, ambient temperature, driving habits, vehicle use, towing, loads, add-on accessories. Broadcast Message 1271, 4450, 6989

Best thing to do is take it stock and see what happens.

Also what kind of filters did you use.

 

I had a wix filer and I thought that may have been the problem so I changed it out with a Motorcraft filter and cap. At ford they said they couldn't run the test because my tuner had to be unplugged while their computers used the port to run diagnosis?! H&S says that's total user error and they should be able to figure it out...

 

You will need to put it to stock and let them access the port. personally I would do that, see how it runs then make a decision to take it in.