Chevy truck overheating -- 2004 Silverado 1/2T RWD -- 6.0 engine

is the thermostat in backwards / is there a flow of coolant through the radiator ? the shroud is important to draw the air through the radiator , , hopefully you don't have a blown head gasket

Thermostat is removed, but no, it was not in backwards. There is flow through the radiator. I pulled the heater IN hose off the pump (plugged the pump out hose-barb). Hooked a water hose up to heater-IN hose and can push water through the entire system. This is with the truck OFF. The small hose that runs from the top of the radiator to the plastic overspill tank was used as my OUT. So water went through the entire system (excluding the inlet-to-pump hose). BUT - with that hose open and draining onto the ground, there was system back-pressure enough that the overspill tank's pressure relief was also in full flow, draining. In other words, their was enough back pressure to trip the pressure relief device, which the lid on the plastic is that device I guess. Unless there is some sort of regulating/unloading relief valve at that outlet (passenger side, top of radiator) the radiator may be partially blocked. There is hot air coming off it, proving flow, but is enough water flowing? I think maybe the radiator is partially stopped up. Still unsure of this stuff. As you say, I could have a blown head gasket I suppose. Update, Shroud has been reinstalled and it makes no difference at all.

what does the exhaust look like ? look under the oil fill cap what do you see ? there's some kind of test strip you can get to see if there is water in the oil and there's always a compression test

the oil cap looks normal underneath. There is no white - no evidence of an oil/water emulsion. Also the oil level as remained normal. The exhaust appears normal. there is no smoke and no residue at the tail pipe. the hose that runs from the top of the radiator over to the plastic reserve tank: At that outlet, is there some sort of restrictive device, like a spring loaded valve that will open if the pressure comes up high enough - but otherwise is closed and allows no water to pass?

Also the issue of the heater is odd. At normal water hose pressure I can blow water right through the core, with the engine off - key removed. Yet I am getting cold air; no heat at all, with the truck running at about 300 degrees F. I am still running with the thermostat removed: Make me wonder if the mechanical thermostat provides some back pressure that helps push water through the core. I guess there is a solinoid valve that opens, allowing hot water to circulate through the core when the controls call for hot air. But if no water is trying to travel that route, then it wouldn't matter if it were open or not. The thermostat being partially obstructive so water can be pushed to the core just doesn't sound reasonable to me though. I can't find any technical information (you know how the internet has gotten).

from what I understand , older vehicles had a valve in the hose that would open to give heat but now it always flows and a vent door opens to give heat , I guess the cores clog from stagnate water , the rad cap is the valve that feeds/taps the over flow , if you have that thermostat out get it in a pan of hot water and get a common kitchen thermometer and see when it opens up , it should have a thermostat for normal operating , did you use a rag to plug the port for cleaning gasket surfaces and left it in there when you did the water pump ?

did you use a rag to plug the port for cleaning gasket surfaces and left it in there when you did the water pump ? No. It was all done quickly and there wasn't any stuck on gasket to scrape. I pulled the hose that goes to core inlet and put a female water hose fitting - all nice and tidy so there'd be no leaks to confuse things. (I worked in industrial pressure washers most of life, so I'm always careful when chasing strange problems that may have to do with blockages or restricting devices and so on.) When I ran water through the system, I carefully blocked all escape paths. I went through the core and back out - back through the water pump, to and through the radiator and then finally out the small hose that runs back over to the plastic tank (passenger side, top of radiator, directly above the 1.5" out hose). The other end of that hose was removed from the tank, so the water could drain onto the ground. The tank hose barb was sealed with a piece of properly sized water hose, folded and secured. So the system was pressure-tight. The tank check valve was blown open the entire time. My beginning working theory was that all the water would simply drain right out the tank hose. But apparently there was back-pressure being created somewhere, either debris partially clogging the radiator, or core, or there is a pressure pop-valve there at the outlet of the radiator that runs back over to the tank.

It seems strange that I can't find actual tech info about that system on the Internet. If you know where I might aquire such information, I'd be much obliged. An official diagram from GM with all part numbers would be awesome. But I'd be happy to have anything.