My 6.0 runs hot also pulling medium to long grades. I have to drop at least 1 gear and keep the revs up just to keep it below 225º pulling a 6% grade on a 85-90º day. It's frustrating because the engine has plenty of power but the cooling system doesn't seem to be able to handle it. I sure would like to be able to solve this problem. I have cleaned the outside of the radiator, drained and flushed the radiator, replaced the cooling fan clutch, replaced the thermostat and changed the radiator cap. I even bought an external water sprayer for the radiator but didn't install it because I couldn't figure out where to put the tank. I figured I would just tap into the existing MH water supply instead but haven't got around to it yet. The temperature gauge matches the ultra gauge that I have. I wonder however if the sending unit (if it has one) for the gauge could be wrong which would probably affect the readout on my ultra gauge as well. The fan clutch doesn't turn on until 225-230º and that just seems too hot to me. After it's on I can keep it between 210-225º. I read somewhere that the cheaper fan clutches turn on at a lower temperature and I am considering trying this but it takes quite a while to take everything apart just to get to the fan clutch. By searching the internet I was able to find others with a 6.0 with this problem when towing. We tow a 10' cargo trailer as well. I have posted on several forums but have yet been able to figure it out. If you find something that helps please post it here so I can try it also.
Seeing your above pic that's right about where mine ran pulling the same grade this spring with the outside temp about 65-70º. I have tried to push the engine harder before to see if it would continue to rise and ended up just one mark below the red. So now I just baby it up the hill. On the grapevine going south I know I could pull it at 55mph but have to bring it all the way down to 30MPH to keep it below 225º on an 80-85º day.
