okay, I have owned the truck since winter and all has been good, temps stable. About 3 months ago I changed the clutch fan as it was very noisey, put in brand new one. Temps still great. now in the summer the truck is overheating, but only when pulling a trailer. I have pulled both a small 6 X 10 enclosed trailer and a small horse trailer (empty) and both times the gage went till over 1/8 of the way from the top mark, while nte normal driving temp has normally been right at the middle. The first time was last week, I pulled a trailer for 45 minutes to a ranch, and when I got ther it did it and after stopping it boiled over , when cool it took 2 gallons in the radiator. On the way home it ran fine and after getting home the radiator was still full, the next day we pulled the other trailer and again it got hot, it did not loose any water from the radiator, just bubbled a bit after stopping. So I took it to the shop, they changed the thermostat the radator cap and tightened the lower radiator hose that was loose. Again temp was fine with no trailer, so today I hooked up the trailer and drove on the freeway then a windy road and again it still got hot, and after stopping it bubbled for 2 or 3 minutes. I plan on trying a new radiator. Any other ideas?

 

Ya most likly a radiator. Sometimes you can have a pinhole size leak and never see it. Thats what just happend to me this summer. I went camping first time is summer. and the temp when high. So I took the rad into a local shop that can pressurize test and weld it up. They found itand now its great. Make sure they installed the correct cap. I bought one from autozone, there computer said it was correct, but it was just a little to short, it wasnt sealing. I had to add another rubber gasket.
There are seveal things you can try to keep temps down while towing. Like 180 deg. thermo (normal is 190), You can get a 2 core rad if yours is 1 core. You can get eletric fans to help you. A Tranny cooler mounted behind the rad is always a good Idea too.