How do I know when to change the thermostat or know when it isn't working??

I flushed my radiator about three times until water was clear and then refilled with half and half, coolant/distilled water. I did this because it hadn't been done since I bought the truck ('95 Ford Ranger, 107K miles) from a neighbor two years before, plus the fact that the truck's coolant temp gauge needle was going past the middle mark but not all the way into %26quot;H.%26quot; Afterwards, it was a little better, but it still happened once in a while for long trips on the highway. Just changed the air filter w/ a K%26amp;N filter and strangely things seemed to improve, but the needle still moves a little past the middle. I don't know what else to do. So, I'm wondering about the thermostat...How do I know when to change the thermostat or know when it isn't working??If the engine takes too long to heat up then it probably is a bad thermostat. Watch the temp Gage and see if the temperature is in the proper range when the temperature gets up to the proper operating temp then the thermostat will open up letting the water circulate through the engine.You can tell when this happens as you may be able to hear the thermostat click and followed by a sudden and temporary drop in temperature as the cooler water from the radiator starts going through the engine and the warmer water from the engine goes to the radiator to be cooled by the fan also you may sense a sudden and temporary drop in engine speed when the fan kicks in .How do I know when to change the thermostat or know when it isn't working??thermostats go out often. they are easy enough to change and cheap. is your fan working? with it warming up your fan could have shorted out a while ago and it's just now affecting you temp gage. if it is okay. just change the thermostat. if that doesn't work pay for the diagnosticHow do I know when to change the thermostat or know when it isn't working??No idea how your gage is calibrated, but, most of the time a %26quot;stat%26quot; will stick open, not shut. That will cause too low of a temp and no heat from the heater. Too hot (in or real close to)the red and that is more apt to be from some other cause, fan, air flow etc.How do I know when to change the thermostat or know when it isn't working??When your vehicle starts to over heat,check thermostart and when it starts to operate below normal temperaturesHow do I know when to change the thermostat or know when it isn't working??First off what engine do you have? the thermostat should be changed every 2 years. You should have the cooling system pressure checked for leaks.