How can you tell if the thermostat is stuck open? I get very little heat when the truck comes up to temp. The guage looks like it is around 180-190 degrees.

 

In the past, when my stat was stuck open, the main symptom would be that the truck wouldn't warm up past (about) 130*-140*Try feeling the top and bottom rad hoses. Top should be really warm-hot and the bottom should be luke warm-warm. If your getting very little heat, your heater core is most likely plugged, assuming it's not by passed.

 

180 to 190 is normal. The only effective way I know to test a tstat is to pull it out ( a pita on these trucks). Take a look at it and it should be closed. Drop it into near boiling water and it should open up, and then start to close when you pull it out of the hot water. If in doubt, change it out. Pretty cheap insurance, especially if it has been in there for a while.

 

I'd just be lazy: Remove the radiator cap from the cold engine and run it. If the thermostat is stuck open you'll see coolant flow right away. If you don't, it's not stuck open.

If you're curious to get a ballpark approximation of the temperature at which it's opening, let the thing run until you see coolant flow, and stick an instant read thermometer in there. Add thirty degrees (Fahrenheit) to whatever you measure, and that's about the operating temperature.

You might have a clogged heater core. The hot ticket there is to backflush the thing, throw chemicals at it, backflush it some more, then blow out the tap water with compressed air before reconnecting and topping up the coolant. A couple of 5/8" garden hose ends should just about fit into the heater hoses, though you might need a couple of wraps of electrical tape to get a nice tight seal.