Okay, let me interrupt again with another comment of value. A lot of original thermostats have a tiny bleed hole with a little dangle hanging in it. That is to allow the hot coolant to migrate over to the thermostat so it will open. On some engines the thermostat is a good distance from where the heat is being generated, and the temperature sensor for the gauge is closer to that heat source. Two clues are the thermostat will open if you allow the engine to run long enough at the excessively high temperature, and it will usually open if you run the engine then stop it when it gets too hot, and do that multiple times.
The more common symptom is the gauge goes higher than normal, then suddenly drops to too cold, then repeats four or five times before stabilizing. The high temperature is detected by the gauge's temperature sensor, but it takes about another twenty seconds for it to reach the thermostat, causing it to open. When it opens, coolant starts to circulate, and the cold coolant from the radiator rushes in and causes the thermostat to close again. This happened on my 1988 Grand Caravan, but the funny thing is it did not start doing that until over a year after the thermostat was replaced. The fix was to drill a 1/16" bleed hole in the thermostat.
Tuesday, January 31st, 2017 AT 2:49 PM