The top radiator hose is above the thermostat. If the top radiator hose is hot, the thermostat is opening. The thermostat is not the problem. Perhaps the radiator is clogged up or the water pump is bad. Look elsewhere.
If your truck is overheating!!! Then check the upper radiator and lower radiator hose's , if the lower is cooler the the upper the either the thermostat is not opening or the radiator is clogged
Most of the time it is because the thermostat has gone bad and is not opening at the proper temperature. Cure: replace thermostat. If the thermostat has been replaced, flush the radiator. Buildup of residue in the bottom of the radiato0r will cause overheating, but it is maintained over heating, not temporary.
Check the water pump
Low coolant? Thermostat not opening? Radiator plugged or restricted? System airbound? Water pump not circulating coolant? Hoses collapsing? Defective radiator cap?
I don't own a 1993 Nissan Pickup Truck, stupid! check your thermostat to see if it opening
Low coolant? Radiator plugged or restricted? Water pump not circulating coolant? Thermostat not opening? Cooling fan not working? Hoses collapsing under pressure Defective radiator cap?
Thermostat needs to be replaced, it is not opening (most common). Is electric fan on radiator running normally from time to time?
Cooling fan not working? Thermostat not opening? Plugged radiator? Water pump not working? Defective radiator cap? you have to bleed the thermostat
Low coolant? Cooling fan not working? Thermostat not opening? Radiator restricted or plugged? Water pump not circulating coolant? System airbound?
There could be several symptoms. But the most common would be overheating with the water in the radiator staying cold. If the water in the radiator is hot, the thermostat is opening. If it takes a long time for the water to get hot, (heater takes a long time to warm up) the thermostat may be stuck open.
If the temp gauge reads normal and the engine is not overheating, the thermostat is doing its job. The thermostat maintains the coolant temp by opening and closing allowing the coolant to flow or not to flow. When it is closed the coolant doesn't circulate.
Thermostat not opening correctly, low coolant level, poor coolant flow (i.e. clogged radiator), head gasket, radiator cooling fan not operating correctly, if it is boiling over,posible weak radiator cap.