I went camping this weekend and towed an 8k camper 3 hrs through the mountains with no issues. Check coolant level before I left. Coming home, I climbed a mountain 2 hrs into the return trip, EGTs and coolant temp were fine going up the hill. Came over the top and started down the hill and 5 miles of down hill later the check gauges light comes on, coolant temp 245. Coolant blown out the overflow and all over everything. I shut the truck down and coasted to a stop. Inspected everything while waiting on coolant to arrive. Couldn't fine anything wrong. An hour and a half later when coolant arrived, I put 3 gallons of coolant in to fill the radiator and overflow tank. Started it up and brought it back up to temp, no leaks, CEL or smoke. No signs of coolant in oil or oil in coolant. Drove the rest of the way home no issues. Still no leaks. Pulled the Allen behind the thermostat, no trapped air. Anybody have any idea what would cause this or anything I should replace for a bit of insurance against this happening again? 03 Cummins manual trans, pretty much stock, 169k miles, coolant flushed at 120k never had any overheat issues before

 

I would first, replace the radiator cap with a BSN cap of the correct rated pressure. I'm not saying you have the wrong cap, but possibly one that is no longer working correctly. Secondly, I would use a radiator pressure testing tool to pressure up the system and see if you can see any fluid leaking anywhere. Don't forget to check the very short hose under the turbo as it is known to leak. If you see leakage, fix it then retest to ensure you have all the leaks repaired. If you don't see any leakage, but the pressure won't hold, I would suspect an internal leak (unlikely since you said the oil was ok).

Next oil change, I would send a sample to Blackstone Labs. They can tell an awful lot from used oil. BTW, I enjoyed my '03 till about 298k miles when I upgraded.
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