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91 is reverse rotation. 85 is standard rotation. Make sure it matches your set up
warped head or blow head gasket would cause it to overheat. if it ran out of coolant and over heated it might have did more damage to it. my cummins had an overheating problem but it was blowing coolant out of the overflow tank. never replaced anything trying to troubleshoot it. unhooked the turbo and drove it without any boost and it was gutless and i was on it easy but it didnt overheat. replaced the head gasket and no more problems. take the rad cap off and start the truck, do you notice bubbles in the coolant when the engine is running? let it warm up with the cap still off and look for bubbles. normally the coolant will have some bubbles due to the flow but you might notice more than normal amount of bubbles. also a bad rad cap will cause it to overheat
From your description you have a Diesel. Sorry to sound ignorant, but is there a way to test compression on a Diesel?
So a combustion leak test will show you if any hydrocarbons i.e. diesel fuel or byproducts of oxidized fuel are in your cooling system. Since you flushed yours so many times, it should show up plainly if indeed your heads/gaskets are the issue. What is Bizarre is that you run normal on the freeway...Earlier in your thread, Mike mentioned the two years are reverse direction from each other. Did you check to see that your fan belt driven components are turning clockwise? If the combustion test is negative, I would look at the long shot that your water pump is going the wrong way. If it is, it would make sense that your getting hot from a reverse flow of hot water, not up through the cooler block into the hotter head surfaces. More likely the heads are the issue. Can't see GM make such a vulnerable possibility, but wait....
Better have a spare vehicle just in case...dead truck + mad wife=deadly combo