Have you changed the coolant? Have you tried a new cap? Is the computer capable of doing a cylinder contribution test? If the temp gun confirms it's actually getting hot, all the injectors are working equally well, and you've got fresh, properly mixed coolant and a working cap that holds pressure, then look at the water pump.
In the Duramax world, you can look at balance rates. It tells you what they're doing at idle. To do this it needs to be at 170+ degrees coolant temp AND you have to know the fuel rate. Stock "should" be 8mm³. Sometimes, when tuned and especially with larger injectors, they're around 9mm³. Whatever balance rates displayed are a difference from the fuel rate. The tolerances are +/- 4 in park or neutral and +/- 6 in gear.
If a cylinder's balance rate is high (+) then it's flowing less. A low balance rate (-) shows that it is dialing back fuel which may indicate too much fuel and would be indicative of a leaking nozzle. There's also the issue of ball and seat wear on the return side of things leading to low rail pressure and/or possible hard starting. Especially warm starts.
Aside from that, you can kill individual cylinders with a scanner to see if it changes anything. Really, unless there are hard starting issues, white smoke at idle, or fuel in the oil (and low oil pressure, even at higher RPM) an LB7 is usually good to go.
I keep actual rail pressure, desired rail pressure, and coolant temp on the main screen of my CTS2 monitor. Along with separate boost and EGT gauges, they keep me in the know if anything is going wrong.
@77GreenMachine How fast were you headed up the mountain when it was getting hot? Sometimes slowing down or downshifting is the answer, as much as it sucks. Higher RPM will lower EGTs and usually coolant temps, but will raise oil temps. Lower RPM, high boost/engine load, and "lugging it" will raise EGT. It's basically the same at as your old 6.0 and the 8.1...let it eat! Spinning it up won't hurt it
Oh! Is the fill point the highest point in the coolant system? I know on my truck's highest point in the coolant system is actually the upper coolant hose to the turbo. When I drain the cooling system, I fill it up, bleed it at the thermostat housing, and then pull the upper turbo coolant hose off to get the last bit of air out of it.
TBH, it sounds like a water pump definitely wouldn't hurt. Although, they're a cast iron impeller and don't usually present any flow issues.
@shawn I'd call it a degas, if it's setup like the pickups. No cap on the radiator, but a separate bottle where you fill it that acts as a reservoir and overflow with a cap that's rated to somewhere around 15 PSI.
I guess you're also supposed to keep children warm or whatever...
