Exhaust pressure- CM871

[Brewster64]

I bought a used 08 and the truck has some serious gremlins. I've done the EGR tuneup, changed some sensors and plugs, and had a few repairs done.. here's the thing, the truck has been very inconsistant. Fuel economy is all over the map ranging from 5.2 to 7.8mpg, load and terrain don't seem to matter whatsoever. Driveability is all over too. Engine doesnt idle great either, surges if you bump it up. Truck will at times pull well at 38 - 40 PSI man press and shift and drive fine, other times it'll pull to almost 50 psi but isn't running shifting worth a damn. In both these instances it may or may not be having an overheating issue. At times both oil and water temps will spike fast when pulling, I can pull it back with the fan. Other times it does fine and just climbs slowly to about 220 oil temp and 210 water temp.(Normal) At other times truck seems to to derate where it will only boost up to 28 PSI, BUT it runs, drives and shifts great, with no heating issues at all. The other day i noticed a small amount of soot on steel line that goes from exhaust manifold to thermostat housing, so I tried to remove line out on the road but the manifold side was not coming out. I ordered a replacement line at my local cummins shop so I could feel free to break it when I got home.Got home, and attempted to remove line,a lot of peneterating oil and heat but still wouldn't budge, just skipping by on flange wrench.I removed the top of the line so I could slide a box end onto it and hmmm, I can wiggle the bottom flange about 1/16" in and out of the bottom fitting that wont budge. Finally removed bottom fitting(And it was super tight all the way out) to find that someone had crossthreaded it and it never had sucked the flange tight.( Read major pressure loss) I repaired the threads , replaced the line, I had already replaced the exhaust pressure sensor. I was originally going to remove line to make sure it wasn't soot plugged, it was clean. I haven't run with it yet, but the truck now idles smooth and a quick bobtail around seems like its running correct, I may have found my red flag! Lets hope so. Remember it's not always software, don't overlook the hardware..