Time to replace turbo?

as for the turbo part. i would check your air to air first. the easy way is to cut some wood to fit into the rubber tubes. on one side put an air pressure gauge. on the other side put a female air chuck. than make a double male air it up some unhook the air to air side of the air line and watch and see if the pressure drops. also some soapy water to spray it down with and look for bubbles. a leaky air to air will cause lost boost pressure since the guage is on the intake, lost fuel mpg and power lost if its bad enough. also can cause higher egt ) exhaust gas temp) which can take more power and mpg away

 

I'm presuming you have a ISX? Common areas of boost leak besides air cooler is at the blue o-rings that are inside the clamps on both sides of air cooler and the small hose on the driver side, back of intake line, with that hose that goes down to the compressor. That hose leaks a lot and has been updated to a hard metal line because of how often it leaks.

 

Yep, like the others said, you need to pressure test your air to air system for leaks. Turbos really do not have a set replacement interval. This is something you would replace at overhaul time. With a million on it now for sure the rings and liners are getting tired. This may explain some boost, power and fuel economy loss. EGR valves do start to leak as well over the course of time. The valve may not be making a good seal and boost pressure is leaking out into the exhaust - pressure test will confirm this. As for your engine running on after key off, the problem is still with the shut down solenoid and shut-off disc. If you only had the actual solenoid replaced, you need to remove it again and replace the disc and spring that is behind it. The disc will wear out and not make a proper seal to fully cut off fuel from the engine.