2007 Detroit 14 ltr. Issue.

38 years trucking, I bought a clean Western Star, 2007, 14 ltr Detroit with variable turbo.
Power is low for half he day, then picks up a bit. Exhaust temperatures never exceed 698 degrees.on the hardest of pulls
( I haul 5 axle, 140,000 lb )
Idling, coolant temperature rises, fan comes on, pulls temp down and it repeats. Outside temps, 60-65 degrees, still cool out.
Every filter ,oil etc changed, no difference.
Detroit shop ( that's all they work on) installed a new turbo last week, did a full tune up, still no change.
With coolant temps always climbing, and exhaust temperatures always low, is it possible the EGR system is somehow allowing too much exhaust gasses to enter the combustion chamber ?
How could this happen, if so ?
And if any consequence , my engine brakes are almost non effective. Can this be tied together somehow?
Thanks for your support, I'll keep a close eye on this thread for any feedback anyone might have.
Thanks folks.

 

Who is Leon, where is Leon ?
The shop I use has a guy who's done a bunch of these.
He increases hp to 610, the shop has to add one tube for exhaust that otherwise will be routed into the engine via the EGR, the rest is programmed.
But , just for the heck of it, is Leon in southern Ontario, ....Canada?
That's where I am.