So here is what I have figured out (I know NOTHING about cars btw). The thermostat I had in there was replaced in December. I changed it anyway. My water pump seems to be fine because once I changed the thermostat I could finally feel water flowing through the top radiator hose (before, the water wasn't flowing and the bottom hose was cold and the top one was hot). My fans do not turn on unless I turn on my AC or heater. My heater ran cold when I first turned it on after changing the thermostat, but eventually warmed up again.
I suspect that the problem is the fan relay, but it's a $30 part, and I don't want to waste the money if I am just going to have to take it to my mechanic anyway.

Does anyone have any suggestions as to what the problem might be?
I added the back story to my problem at the bottom just in case.

I was hoping that someone could help me with my car problem. I have a 2000 Camry with about 180,000 miles on it. Just a few days ago I drove a long stretch up a pretty steep pass. I always check my temp gauge since I have had over heating issues in the past with another car. My car was fine, but when I stopped at a store I noticed that my car was getting hot. When I got back to my car, it cooled down and didn't get hot again. The next day I drove it 300 miles and still it was fine. It wasn't until the following day when I was driving the 300 miles home that I noticed my car getting hot again when I got stuck in traffic. I turned on my heater which seemed to help, but later on, my heater started blowing out cold air. I turned it off since my car was staying cool, and when I turned it on again, the air was hot. By the time I got home, my temp gauge was still fluctuating, and the heater started blowing cold again. The next day, my car did fine, even stopping at lights and in 85 degree weather. The only time that my car has over heated before is when my thermostat and heater core went out last December. They have both since been replaced.

 

4 cylinder or V6 engine?

The issue could be the cooling fans. The fans are there to pull air through the radiator at low road speeds (stop and go) and when the car is not moving. At higher road speeds the air flow should be adequate not to need the fans.

There is a coolant temp switch controlling fan operation mounted on the lower plastic radiator tank. This switch and fail. You can check its operation by unplugging the harness plug to the switch with the ign switch on, the fans should come on.

The V6 engine has 2nd switch mounted on the coolant intake manifold attached to the head.


A few reasons for varying gauge temps and cold heater when the car is driving at higher speeds (fans would normally be off) are low coolant, air in coolant system, issue with heater control valve controlling water flow through heater core, if the car has an automatic climate system sometimes the heater/AC air flow doors can malfunction, issue with water pump impeller.

Next time the heater blows cold air check the heater hoses, they should both feel hot.

 

does the fan has something to do with the performance of the car while running?

 

It is a 4 cylinder. The heater hasn't blown cold again. But now I am running through a lot of coolant and there was some white smoke coming from the hood this morning, but it was fine this afternoon when I drove home after topping off the coolant and running the heater. Unfortunately, I won't be able to check the sensor until tomorrow when I can get some help. But thank you for that advice. I will see if it works and let you know.

 

When checking for coolant leaks look at the upper and lower plastic radiator tanks, they can crack and leak. Look at the plastic air shield on below the radiator for signs of coolant.

Also check the coolant bypass hose on the coolant exit manifold of the head.

 

Did you check the procedure that toyomoho suggested. if still you have any issues let us know.

 

Sorry for the late reply. I tried everything. We changed all of the relays and the sensors. And it still wasn't working. There didn't seem to be any leaks. So I took it to the mechanic and he changed another relay and the sensor on the bottom of the radiator. The fans worked for a bit. But then the car started over heating again, and the fans stopped working again. My mechanic said it was because the head gasket was warped now, and something about the pressure being off. I kept insisting that this doesn't explain why the fans wouldn't work. There is a language barrier and I was being left out of the conversation due to my gender. But in the end, he couldn't figure out how to fix it aside from rebuilding an engine. My car is too old, and the body isn't great so engine isn't worth rebuilding. But thank you for all of your input.