Detroit 12.7 issues, cracked exhaust center sections

Throw parts? Why don't you listen to advise. Pressure test intake and look for leaks. The info has been presented, go do it and report back.

 

Definitely sounds like a boost leak. Pressurize the CAC to 30psi and if it loses 5psi or more in 30 seconds it's a fail. Then pressurize the intake to test the gaskets an the hose to the compressor.

 

Usually the first thing a trucker b####es about is low power. I didn't see that mentioned in the OP, just poor fuel economy. Sounds more like overfuelling to me.

If the induction and exhaust system check out I'd seriously consider there being an issue with fuel delivery. Maybe even a dyno run and cut out each injector to see if one is pouring in excessive fuel.

I'm not much up on Detroits but is it possible to put "hot" injector calibration codes in with a stock ECM program?

 

The trim codes on a Series 60 only have a small effect and only do so at idle.

A boost leak doesn't have to be bad to cause low fuel economy or black smoke and high EGTs but it can be pretty bad and still not have a noticeable lack of power. Usually it'll be slower to respond but the popularity of the 171702 proves most people don't notice that

 

It’s probably leaking all the way down the hot side. I’ve had 2 do that, after only 4-5 yrs. They leaked so bad, wouldn’t hold air 5 seconds. No telling how long they were leaking New Av-tekk is supposed to last much longer. We shall see.

 

That 1in. rubber is a special line that should have a steel liner inside of it to keep from popping with all the heat from intake some parts changer put a water hose on my 12.7 and it was leaking so bad that went I went to det. eng. parts shop after they tested my intake and and I heard it hiss like a cottonmouth water ratter snake and I replaced it with the right line so try that line 1st??