ISL9 residual coolant pressure on cool engine

[garyMADP4314]

I have had no cooling problems on my 2012 ISL9 to date (bought used Jan 17). Newmar Mountain Aire with Freightliner chassis and side radiator. In late June I drove it in pretty hot Central Texas weather to a Cummins shop to have a genset problem fixed, 125 miles round trip. Coolant temperature remained normal. Three weeks later I was prepping for a weekend trip and opened the coolant expansion tank cap and was surprised by remaining ambient temperature coolant pressure, and coolant spewing all over the engine compartment and ground below. After the spewing the level in the expansion tank seemed OK, so I assumed it was somehow overfilled (although I check level regularly). Midway through the trip I noticed the engine running hot (temperature approaching 240oF, but no warning light) during the 90 mile trip in ~95oF ambient and a 1000 ft elevation increase, towing a 5500 lb Ram crew cab. I slowed from 65 to 60 and the temp dropped to around 200. At the park, after the engine cooled I checked the coolant again - no residual pressure at the expansion tank but the level was low. I added almost 1 gallon of distilled water to return the level to normal. On the return trip the temperatures remained closer to normal, but we traveled earlier in the day, had a 1000 ft downhill over the course of the trip, and I drove slower at 62 mph. I thought all was good, but I just checked the coolant for an upcoming trip (I have a fogged sight glass, thus the manual checks) and encountered residual coolant pressure again. Something seems amiss, but it is apparently intermittent. Any ideas?

[BFlinn181]

Your radiator cap is perhaps a 14 psi cap, so it will hold that pressure. Anything over that psi, and the coolant should go into the expansion tank. I've had some cars that have a thick walled plastic tank that can hold pressure and have a tight lid, other vehicles that have a cap with a vent hole and the tank walls didn't look like they could handle much pressure. I'd consult Cummins and see what they say about your observations.

I drive by the temperature gauge, if I'm overheating, I downshift a gear. Usually only happens on a grade, but I had it happen once when I was driving on a level road into a 40 mph headwind.