I'll start of by saying that this is for my uncles truck. I think it's a 1997 12 Valve manual. He's had quite a few issues lately and he has replaced the thermostat, radiator and radiator cap. He claims when the truck is just warming up it goes up to 210 degrees for quite a while, then shoots until the gauge is all the way down(I'm assuming this is the thermostat opening). Now I have read about this happening because the radiator is overkill on these things unless they're hauling something but 210 seems kind of hot for the thermostat to open. Also, he says the truck will be running from 170-190 hauling his trailer (20,000lbs) but when going up a hill it shoots up to 220 degrees and even when coasting down the other side of the hill, takes a long time to cool back down. So what do you guys think? I've heard of a bunch of stuff it could be, thermostat, clutch fan, clutch slipping not allowing the fan to kick in, water pump is weak. Let me know!

 

Try checking resistances on the sensor, could be bad. Also, did you flush the system with the cleaner to remove buildup inside the long block. Other than that maybe the water pump getting weak. If its a new radiator there shouldnt be any oil buildup on it blocking airflow. Based on slow changes in temp, it could be that its clogged or weak pump.

 

Is it stock?

 

The truck is bone stock

 

Hm. That is way, way more than a stock 12V is supposed to pull, but on a stock tune I'm a little surprised it's getting hot enough to raise the water temp that much. Generally when pulling heavy and that happens it's either overheating the automatic until it backfeeds heat into the engine (not a problem with the standard) or the EGTs are so high that the coolant can't keep up. I've never seen a stock p-pump go much over 1000EGT, and it should be able to run that temp all day long and not overheat. Towing heavy though, all bets are off. That truck is only rated for something like 14K and that's with a gooseneck. If it generally runs good and the only symptom something is wrong is that it gets hot when pulling more than it's supposed to, I'd say he's working it too hard. If there are other symptoms or drivability issues, there may be something else, but it's hard to say from here.

 

So how do I test the guage?

 

Unfortunately, the simplest way is to compare it to one that is known to be accurate. Or as Dave said, there is some electrical testing you could do on the sensor, but that would only tell you if the sensor is working not so much if the gauge is accurate. You could check it against a conventional thermometer or lazer gun style, but it would only get you in the ballpark unless you can somehow get it at the coolant where the sensor is.
It sounds like the thermostat is working. As for the fan and fan clutch, it's irrelevant at speed. If it was overheating while sitting or crawling through town it would be something to look at, but otherwise no. A weak or worn water pump could be an issue, but I'm not sure how you would diagnose that beyond that they generally start leaking when they wear out. You should be able to find something out if you send in a coolant sample to a testing lab.