First you need to install a thermostat, which has nothing to do with how hot or cold it is where you live <img src="/forums/images/graemlins/cheers.gif" alt="" />
You will also get better gas milage with the thermostat installed.

Since your overheating at speeds above 50mph, I suspect a few things. Since you have lots of air flowing at 50mph you should not overheat. I suspect you have a clogged catalytic coverter, crushed exhaust pipe or some type of exhaust restriction. Second, you might also be running lean because of the missing thermostat, running lean causes excessive engine heat.

My final thought is your fan clutch could be bad. Since you don't have a thermostat it will take a long time for your engine to warm up. After your engine is warmed up, have a friend turn the truck off while you watch the fan. It should stop almost right away. If it continues to spin then it's bad.

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