91 toyota 4runner 22re manual 4 cyclinder. Having Problems with my thermostat gauge fluctuating up and down. Also, when i go to the mountains or im on a steep incline for a long period of time changing from first and second frequently engine will start to get hot. Changed thermostat yesterday but still having the problem. Any suggestions on things I could check? Or maybe you know the problem? Thank you

 

Try cleaning the sensor in between cylinders 2 and 3. It gets a hard water build up on it. I just use a brass tooth brush. It seems to help some to clean the ground wire by the drivers kick panel next to the fuse box. Remove the gauge cluster assembly and tighten all the screws in the back as they tend to loosen up over time. Here is a thread you can look at as well>>> https://www.yotatech.com/forums/f116...ge-fix-271152/

 

Yeah definitely don't rule out the gauge working properly. But explain more about how it fluctuates? If it's not the gauge:

Personally I'd do a coolant system overhaul, since the thermostat didn't do it. Flush, if not replace, the radiator. Or pull it and have it cleaned and tested. New coolant. New fan clutch and belt. Feel the water pump pulley once the belt is off it and see if there's any play.

Sounds more like restriction in the cooling system.