I have a 93 4runner 3.0 and i recently replaced the thermostat, and radiator due to overheating problems.

The temperature gauge has been acting weird, going from the middle of the gauge to the red almost instantly, then will randomly very fast settle back to normal.

Seeing as how I live in Colorado and its pretty cold frequently where I live, I have a hard time believing...

A.The temperature gauge can change from middle to red in 5 seconds and
B.My car can be overheating when its not even 40 degrees

Note: Car seems to be fine when stopping to inspect the radiator/engine, temp seems at a good level.

So how hard is it to install a new temp sending unit?

 

Mine does the exact same thing. I usually just tap on the gauges and it will slide back down to the middle. Ive been meaning to take a look at it but havent gotten around to it yet

 

pull the plug from the sender unit under the hood and ground it. Your sender is probably bad, or the connection is bad at the sender unit.