Just brought home a 2000 ranger 4x4 ext. cab with the 3.0 for my son last night. The motor was just put in and it has 62,000 on it. Drove it for 3 hrs. last night with no problems, temp guage was around 1/4 from the bottom. Was cleaning it out today and noticed the guage was just under half while it was running / idleing. Went back out a little while ago, 4-5 hours since it ran last, 3 Deg. F outside and the guage was still at half, stuck. I looked under the hood and the 2 temp. sensors were not plugged in all the way. The plugs were just sitting on the sensors but not reading anything. Pushed those down and connected them, then started the truck. Came out and the guage was almost to the top, hot side. The radiator had pressure but it was cold. Truck had ran for 10-15 minutes. The rad cap is new also. He showed me a receipt for the cap, anti freeze and thermostat which was just put in with the motor. I then grabbed the hose from the thermostat and it was cool. The intake at the thermostat was hardly warm. I am wondering if the thermostat's in backwards or possibly bad? There doesnt seem to be any flow. There is pressure in the hose, its just not warm. That or possibly air pockets in the system?
It doesnt seem to be using water, level is still full and there is no antifreeze smell in the exhaust. Just a little coolant seeping at the T-stat gasket.

 

I think there is still an issue with the thermostat, but I came out this morning for work and noticed that when I turned the key on the temp gauge only goes down to just below half with the truck cold. So last night when it registered hot, it was probably in the normal range since the needle does not go to the cool position. Does anyone know which sensor controls the gauge? There's one in the intake and one in the thermostat housing.