I was driving on the highway for two hours or so and I looked down and saw my temp gauge spiked in the red. I pulled over and popped the cap losing about three gallons of water/steam. I replaced the water and the temp came down. I got back on the road and it stayed that in a normal range so I kept driving. Every now and then the gauge would quickly spike but then go back to normal. To me it seems like air in the line. It is too fast a spike to be a thermostat issue I think. Two years ago I flushed my coolant but I never turned on my rear heater during the flush. I didn’t realize my mistake until after I turned the heater on. I did have Toyo coolant in the truck when I flushed it but changed to Prestone during the change. Is this possibly the result of the two coolants mixing? Could something be plugged? I am going to flush again and delete the heater all together but I would like to know if I should try something extra to clean out the system if i gummed it up.