I use a simple coolant thermometer available at most auto parts stores for 5 bucks. You could need a new thermostat. Some people use a 170 degree thermostat in Summer.I have a 95 explorer with 160,000 miles and it runs fantastic, but i have read on many posts that the temp gauge should be in the middle. And mine is under that doesnt go up that high on the gauge. How can i tell for sure if its running cold? Thank you.