Starting with a cold engine. Remove the Radiator cap. Watch the water in the top of the radiator. It will move when the thermostat opens. It will spit steam at you if you have an air pocket that stopped the pump from working. So look out. Most thermostats have a hole in them to prevent this, but some do not. The "gurgle" you hear is water boiling. More often than I would care to remember, air in the system has caused overheat issues. If the pump is bad, no water will move. If there is air in the system the pump won't move it. If the pump weeps or leaks, (see 52Carl's post) it won't keep hot water moving like it should. With the belt off, wiggle the fan front to back. If the bearing is bad you will feel it. Just for your information, steam is 1800 times bigger than the water it comes from. Steam means No more water left in there. The thermostat can be drilled(cheap fix). On a lot of these old trucks, the radiator pressure is low, if not 0. Steam will cause them to spit like crazy. Worst case, you have a bad head gasket(bad machine shop), warped head(bad machine shop), or cracked head(bad machine shop). Start with the easy stuff. Don't panic and figure the bad stuff until the easy stuff is checked.
