Abnormal ECT pattern

[dnf12908]

I just got back from a 6200 km. trip. It was 90+ degrees the whole time. I was towing a 26' TT. The first day my coolant temps were all over the place, going right off the top of the gauge putting the truck in limp mode seemingly on level ground.

Then it would cool right down and be good for a while, then it would happen again. I bought a new thermostat, changed it out in the campsite the next morning and all was good as far as the temps stabilizing, but the overall temps still seemed too high. Cruising at 65 mph, the temp would be around 217. Pulling a slight hill without backing off would send the temps to 230+.

The last day of the trip, this situation got worse. Pulling a hill, my Insight would show 245+! Of course I backed off and cranked up the heater, and that cooled it enough to get over the hill. Most of the time it would cool down quickly once I broke over the top of the hill. The odd time the temp would stay up for quite a while. The fan would come on at 230 but not stay on all the time as the temp climbed. As a side question, is the fan governed by ECT or EOT? My deltas are within the 15 degree spec.

Does this all sound like a water pump failure? Or could it possibly still be a faulty thermostat, even though I just replaced it?

What are the chances of this being a head gasket issue? I don't have any typical symptoms (puking, white smoke, loss of coolant, moisture in tailpipe, etc)

I've got a water pump coming tomorrow but I'd like to be sure what the issue is before I dive into that. Any help would be greatly appreciated.