it run hot
Need more antifreeze or need a new thermostat,
To add above: Water pump, head gasket, head itself. Easy tell tho. 1. Is there water on the oil dipstick? 2. Is the bastard smoking and if so what color and what does it smell like? If neither one, great, it will be soon if not fixed now. If not 1 and 2 then I suggest antifreeze, or new thermostat like Mr. Buck said. Also would check the water pump for leaking. Most have a weap hole that tends to leak as the seals inside the pump go out. If you have no water/antifreeze, it did just disappear. Check hoses and radiator for cracks. Easy to do this when engine is running at optimal temp. System is pressurized so it will spray out assuming there is something to spray. Another way to check if it is the thermostat or water pump is to open the radiator cap WHEN COLD ONLY!!! Leaving it off and then cranking the truck. As the engine heats up, it will suck down the radiator fluid. Takes about 5-10 min depending on model, ect. from cold start. Antifreeze should swirl or draw down. If it spits it out, you have a cracked head or blown head gasket. If i doesn't do anything, prolly thermostat, temp sensor, or water pump. The ladder are the cheaper fixes. The head is well.... big $$$. Good luck. I believe I've covered it all.
P.S. When checking the hoses/radiator for spray/leaks, radiator cap has to be on to create pressure.
Looking for a Used Sierra 1500 in your area?
CarGurus has 64,400 nationwide Sierra 1500 listings starting at $1,500.
Search GMC Sierra 1500 Questions
GMC Sierra 1500 Experts