Antifreeze ts boiling back into reservoir. Possibly not circulating properly. Thermostat problem? Temperature fluctuates with changing load on engine. Approaches the overheating "red zone" with an extended mountain climb.When going down the mountain, temp. gauge drops to below normal operating level. Fluctuation seems odd.

Pam are you sure it's boiling? If the Antifreeze is boiling I'd think you'd see steam coming out of your reservoir as the water expands into the reservoir, but you didn't say you see anything like that.
Of course engines get hot, that's why we have the cooling systems but it doesn't sound like it's "boiling" to me with no signs of visible steam unless your antifreeze level is really low?

If it's really boiling, and you antifreeze level is good, then your reservoir should be getting filled up with expanded hot water that goes into it to catch it when it expands.

If the water isn't going into the reservoir, then maybe your antifreeze level in the radiator is too low and you need to add antifreeze?
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Best to fill the radiator with the engine cooled down. If you ever have to open the radiator when it's hot, NEVER open it with your bare hand as you could get scolded with boiling water. Instead use a big thick towel to remove the radiator cap off a hot engine. Again, if you can, just wait until the engine cools then open the radiator cap.

With the engine cool, you can fill the radiator with a 50/50 antifreeze mix. The 50/50 mix come premixed 50 percent water, and 50 percent antifreeze. So you can throw that stuff straight into the radiator.

Non mixed antifreeze needs to be mixed 50/50. Some people use 50 percent distilled water when they mix it because distilled water doesn't have the minerals in it that may get stuck in your radiator. That's being a bit picky about it though. Tap water works fine in a pinch. The other 50 percent is antifreeze. You can mix it right in the radiator too, put in a measured amount of the antifreeze, followed by the same measured amount of water. The water pump will mix it all together when you run the engine.

Put the cap back on and run the engine and if/when the water "boils" it should go into your reservoir. If it does boil in go in the reservoir then yes...your engine is getting hot and the antifreeze is boiling.

When the antifreeze boils, it's probably because your thermostat is stuck closed. That means the thermostat isn't letting water circulate through the radiator to get cooled. Thermostats are cheap...no more than $30 I'd say for most cars. Changing them is pretty easy too as they are usually right at the top of the engine. A mechanic may charge $100 to change it....but it's an easy job on most vehicles requiring just a couple sockets/socket wrench.

Its normal for the car to have antifreeze going to the overflow tank if thats what your asking.

Behind the tempearture switch is an arm that is telescopic and raises and lowers the temperature. If this arm is either broken off or loose, it can create an inability to change the temperature. It can also be a problem with the heating unit in the ac in the car.

is the water pump working and circulating the antifreeze. if it is working, I would flush out the system and replace the thermostat before refilling system. you can buy radiator flush at any auto parts store. you can back flush system through radiator. just follow directions on back of flush bottle. have a good one!!

First make sure there is a good 50/50 mix of antifreeze and water and that it is filled properly. Second, it sounds as if the thermostat may be stuck. If it's overheating, the temp sending unit controlling the fan is going to tell it to stay on. Change the thermostat and fill it with fresh 50/50 mix of water/antifreeze. Run the engine to normal operating temperature with the heater running and the radiator cap off. Ensure there is good circulation once it reaches normal operating temp. If there appears to be no circulation you could possibly have a bad water pump as well. This should take care of your problem.

Is the coolant level full? Sounds like air pockets, as a temperature sensor can't read air temperature. If it looks good, you might have a circulation problem, due to plugged radiator, or heater core, or possibly a worn out water pump. You can also "feel" around the thermostat to see if it's opening properly.

sounds like thermostat

Check the temp control sending unit, and circulation in the radiator, change the antifreeze, and check the thermostat by placing in boiling water or replace.

HI. It could possibly be the the thermostat is stuck shut. If thats the case when the vehicle warms up it will probably overheat all the time. The best way to tell is buy a new one and change it. They are usualy around $10.00 at any autoparts store. If you change it and it still overheats, it could be the waterpump.

One of the common things that gets missed at times is the radiator is down in cooling antifreeze. The car will act as normal during the warm up cycle and then the temperature guage will rise and then it will slowly come back down as the thermostat opens and/or the electric cooling fans come on. In actuality the engine is overheating, but it is not boiling over. The level of abtifreeze is at just the right level of lowness that you experience this phenomenon. After the engine cools off , (DO NOT REMOVE WHEN HOT - YOU CAN GET SCALDED WITH BOILING WATER/ANTIFREEZE)), remove the radiator cap and check the level within the radiator and also the antifreeze reservoir level. You can buy a 50-50 mix of antifreeze and water that is ready to put into the radiator. If the level in both the radiator and reservoir are correct, you may have a bad thermostat or some other problem.
Bob - 2003 Nissan Sentra SE-R SPEC V