Whew! Traveling at elevations from 6-9000 feet. The antifreeze hit over 230 at times. I had to pull over, downshift and really slow down. What finally worked was to lift the bed up about 5 inches and wedge it with wooden blocks. After that, the Cummins was cooler. I plan to see my mechanic when we return home. I've already cleaned the radiators a couple of times. Can high altitude cause this issue? Thanks.

Absolutely high altitude will effect engine cooling!

Any time I am over 5000 ft. I have to slow or shift back to 5th to keep engine RPMs up and higher airflow thru the radiator. If outside air temp is above 90F, I will see 200F engine temp and if 90F at 5000-8000Ft I have seen 220F engine temp. only once I had to stop and that was with 110 outside and engine temp warning came on (225F)

Trans temperature never above 175-180F.

High altitude the turbo has to work harder, aft turbo temps climb EGT goes up. Cooling effect of thin air is greatly reduced!

Unless there is coolant loss, never shut the engine down, just stop quickly, take the load off and raise the RPM to about 1500-1800 until temps come down and stabilize.

The air is much less dense at those altitudes, and so is less effective in cooling. Density also decreases with higher temps and humidity.

Some folks recommend a product called Red Line Water Wetter. Your engine is supposed to run 30 F cooler. Have no personal experience with it.

Tried the water wetter. Not sure it helped. How about flushing the cooling system and putting in new antifreezes? Can anyone recommend a good brand of radiator cleaner?

Richard, I suggest rodding and cleaning the radiator or replacing with a larger one. I am sure you will get more input here.

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Did I suggest in your other post 9you have duplicates) to turn on the cab heat, to try and dump some of the engine heat in the cabin?