My wife and I made a 3hr trip to the coast this weekend and while driving the windy road to the destination, which is up/down pretty big hills all around ranging from 15mph-50mph for about 55miles. When we started getting into the hills I noticed that the temp gauge was just a tad above the bottom/low line. It barely moved up at all throughout the whole drive. The next day it did the same thing.... shorter drive but same result. We drove home and again same thing. Although once we hit the freeway, flat land steady 70mph, it warmed up to normal temps and stayed there. This is the first time Ive seen this happen with the truck. I also did notice that while climbing hills the truck felt weak... laggy.. which kind of made me wonder if the fan clutch was sticking. Temps would actually go down or stay the same while climbing.... of course while climbing I was giving more throttle and at low speed which would have meant the fan, if stuck, would be drawing a lot more air over the radiator.
During this I did try the heater... it was warm but not super hot like I know it can be. Also the trans temp still reached operating temps according to the gauge.
FWIW I recently had fuel in my coolant and did the injector cup replacements... replaced all the cups and radiator/heater core hoses and flushed the system 4 times. Someone told me to replace the t-stat and WP but I didnt because they were not leaking. Could the fuel damaged the t-stat? I checked the coolant when we got to the coast and it was normal, no fuel/oil.
The ambient temps during these drives were 50-70*.
My gut feeling is I have both issues... interm stuck fan clutch and stuck open thermostat. If the fan clutch was stuck wouldnt the t-stat still do its job and regulate temps?
Sorry for the long post..... any info is appreciated.