Temperature gauge rising and falling

[EverettC]

My 1996 manual ford ranger 4x4 supercab is having a strange problem with the temp gauge. When I first start my car it takes a long time to get past C then when I start driving it will reach operating temp at 1/2 the temp gauge, but then the temp will suddenly drop to around 1/4-1/5 of the gauge and then it will rise and drop over and over. This happens when driving and when idling. I’ve done some research and it seems like I would just need to replace the sender and the sensor. Does anyone else have this issue? And what did you do to solve it?

 

[Shran]

I chased this problem on my '94 and it was ultimately a partially plugged radiator. I think what would happen is a big pocket of very hot coolant would make its way past the obstruction, spike the temp gauge, and then it would sit in the radiator for too long, causing it to cool much more than it normally would (thus the constant up/down.) Or it was getting hot enough to boil and there were pockets of steam getting caught... anyway I tried multiple thermostats, three water pumps, hoses, two fan clutches, flushed several times and the last thing was just to replace the radiator.

I saved that for last because it was new when I bought the truck but the engine had an incredible amount of dirt, rust scale and crap in the cooling system. Every time I would flush it, the bottom of the radiator would always be full of rusty mud and I'm sure it just got permanently plugged. It's fine now...but man...I have seen very few cooling systems that were that bad.