On mine 2.4L 97 Camry, I accidentally broke the sender temperature gauge sensor and the Fast Idle control sensor while replacing the ignition wires, (I hit into the sensor that controls the fast idle with my elbow). The other sensor (a sender) I broke when I tried to remove the wire cap connector so that I could get my socket wrench on the fast idle sensor. I replaced both sensors. The temperature gauge sensor ( Toyota part# 83420-20040 Gage ASSY, Water) was replaced with a Japan built Camry part as required because the generic does not work on this Japan built car (as I found out after trying a generic). The senders wire’s connector cap was broken, so I had to soldier on a connector on to the sender wire; the new connector slides over the center connector of the temperate sender. There is no more cap that goes over the temperature sender connector. The reading I get from the temperate gauge is solid, even if I giggle the wire. The temperature sender seems to have no effect on the ECM, it is strictly for the temperature gauge. I think that the ECM gets it temperature information from the fast idle sensor as this sensor will cause a check engine light when it is bad. The fast idle sensor is a generic from Autozone. If I start the car up, it goes into fast Idle as it should; the rpms come down into the normal range during warm up. The temperate gauge reads slightly below mid-point as it should once it is warmed up (about 5 minutes). Since I replace the temperature sender, (last 2 weeks) at sustained highway speeds (50+), the temperature gauge will climb near the red zone. When this happens, if I stop the car and just accelerate it in park, it will go just about back to normal the temperate within 10 seconds and stay there for a few minutes while I drive again. Also, if I let the car just idle for a few minutes it will go just about back to normal in about 1 minute. There is no gargling in the overflow bottle when I stop the car after it appears to go near the red zone. The fans will eventually come on when the radiator is hot.
In the morning when the car is cold it takes a while before the temperature needle creeps up pass the ½ point of the gauge. It actually appears to work. Once the car is about 25 minutes running, the overheating situation appears to happen and almost always at sustained (50+) speed. The radiator is new and was replaced a month ago before I broke the sensors. The water pump and thermostat are almost 2 years old. The radiator fluid is new, not mixed as the Toyota parts person told that Camry for the 90’s are not to use mixed 50/50. I did have any overheat issue before I replaced the Ignition wires and broker the sensor. The Camry does not act like an overheated car, that is, it does not ping, knock, or not able to accelerate smoothly when the gauge is near the red. When idling, both fans go on and off with and without the A/C as it should
Is it possible that I introduced an air bubble into the system? How do you get it out? I also, read from some thread here that there is a bleeder in the back, do you know where this is specifically. Could not having a cap on the temperature sender cause this problem?

 

Where were these two sensors located, how many wires did each have going to them and what were the colors?

Did both sensors plumb into the coolant system?

You did NOT have overheating issues before replacing the parts?

There is no air bleed valve on the coolant system. Squeeze the upper and lower radiator hoses to push out as much air as possible then add more coolant. After the engine has warmed up then cooled down, add more coolant if required. Make sure the coolant overflow reservoir is properly filled with coolant.