I have a 2001 Ford explorer. My temp gauge goes from the middle level to high, and back down to middle. also, the radiator coolant transfers?

Bad sensor

If your radiator has been serviced recently, or replaced, they may have put a higher PSI rated cap on your radiator. The PSI of a radiator cap is determined upon the altitude and operating temperature of the vehicles engine. Check the PSI rating of the radiator cap, and look it up in the owners manual, or service book. OR just google it. 2nd possibility: If the thermostat has been improperly replaced with a higher temperature cap, the engine coolant that circulates via the bypass hose and the water pump, when the thermostat is closed, becomes FAR too hot, eventually the thermostat opens and circulates the VERY hot coolant through the radiator, which is why you see your temperature gauge eventually go down again. Replace the thermostat with a lower temperature one, allowing the thermostat to open and close more frequently, allowing the coolant to circulate more often, rather than vice versa with a higher temperature thermostat. To summarize: HIGH temperature rated thermostat >175 degrees Fahrenheit LOW temperature rated thermostat < 150 degrees Fahrenheit HIGH temperature means the SAME coolant is "stuck" between the head and the block, is not circulating fast enough to dissipate heat until the temperature of the thermostat reaches opening temperature, allowing the coolant to flow through the radiator (sometimes dangerous on OEM parts, due to excessive heat, and pressures in the system LOW temperature means the thermostat is constantly opening around the low operating temperature of your engine, so that is around 150 or so, if the engine is a v8, it may be around 170 degrees or so. The reservoir contains excessive radiator fluid, because as the water boils and gets hot, the pressurized coolant pushes through the radiator cap's rated PSI, and into the overflow tank. This is NORMAL. But what is NOT normal, is why it is doing it in the first place, again, check the thermostat, and if that is not the case, when the engine is COLD and has not ran for over an hour, open the radiator cap (ONLY WHEN COOL) and squeeze the fat hose that goes to the lower side of the radiator, look into the hole on the radiator, and if the fluid moves up and down easily, then there are no restrictions in the flow. One more possibility, is that the water jackets around the headgasket(s) is clogged, restricting some water flow. I recommend a radiator flush, to remove debris from the cooling system. I am going to assume that this vehicle has NOT had a radiator service such as a fluid flush, since manufacturer date. Most people think that they don't have to flush or replace the coolant, but that is false. Dirt and oil, particulates, and metal particles, and gasket residue, rubber from deteriorating hoses, etc etc etc, can clog the jackets. BE CAREFUL WITH THIS, as the cooling system is best when you put a decent amount of work into it, as it determines FUTURE life of your vehicle. ANY car will last a very long time if you properly maintain the cooling system, and the oiling system.

the transfer of the coolant in normal, the tank is called," the expansion tank"coolant goes back and forth from there to the rad, the coolant sensor may be bad, or the t/stat is slow to operate. replace the t/stat, and go from there

any leaks? top off the fluid in the radiator and tank. turn it on and watch for leaks. check the hoses. if all looks good, you may need the radiator replaced.