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Why does the cooling system slowly heat up then rapidly drops down to cold ? on 1993 Ford Aerostar

With the engine running the temp indicator slowly rises to about the o in the word normal then drops down to cold. This cycle continues throughout the entire trip. Rarely get hot enough to warm the car,

Probably a bad thermostat. When it opens it stays open and not maintaining the temperature as it is supposed to do.
Changed the thermostat,made no difference. The warm/ cold cycle continues
Had a similar case here on RepairPal not long ago, we went back and forth believing his thermostat was new and wasn't the problem. How else will the coolant by-pass the thermostat, how do you introduce cold water to the system and by-pass the thermostat. If you purchased a lessor expensive thermostat, it might work well for every day driving in the spring and summer, but apparently it's not working to build heat. Purchase a better grade/or line of thermostat, more expensive. Later the next day he posted to me , he now has hot air and he will be able to blow dry his hair on the way to church.
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