Water Temp Gauge

[TheSeeker]

Hello All, I am currently having an issue with my water temperature gauge. It is an aftermarket sun pro unit mounted under the dash, it is mechanical. The sender goes to the thermostat housing.

My issue is that this morning, I noticed while driving on the freeway that the temp said a little over 120 degrees. When I got off the freeway and was idling at the red light, it was was still around 120, and when I hit the throttle it went down to 100 degrees before bouncing back up.

I parked and shut off the motor and popped the hood, felt the radiator hoses and heater hoses and checked the fluid level in the overflow bucket all seemed ok, no leaks, worn parts or cold hoses.

My question is as follows, Does this sound like the gauge/sender is bad and needs replaced or is there another issue in the cooling system ( stuck open thermostat, water pump failing, etc) I should look into first. I have no idea how old the gauge and sender are, they were on the truck when I received it from my dad. I would just like to get on the right track. Ive lived in phoenix my whole life never had a vehicle run under 130 degrees lol.

Engine is 250 six, manual trans.