I have a 99 V6 4.0 Ranger and a couple days ago my Temp Guage started acting up. I would start my truck and it would warm up to normal (Mid temp) and then my heater in the car would go cold, then I would see my engine temp gauge go down to cold, then go almost to the red, then back to Normal. After this the heat would go back on.and everything would be fine for the rest of the ride. I have just replaced my thermostat and this morning it did it again. This happens everytime I first turn on my engine, from sitting for a while.

Any suggestions on what is wrong?

 

possible air in the system, try and bleed it or if possible use a vacuum filler

 

i'd bet 50$ on it being air in the system, or low coolant... my mustang would be like that, would warm up regular speed, get a little heat, and you could start to feel it in the vents... then it would blow ice cold, and my temp would go at a ridiculious rate, up to over heating, for about 2 minutes, then the heat would come back on, and the needle drop to normal almost instantly