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,

by in Potwin, KS on February 09, 2014
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1 answer 2 comments
ANSWER by , February 09, 2014
Probably a bad thermostat. When it opens it stays open and not maintaining the temperature as it is supposed to do.
COMMENT by , February 09, 2014
Changed the thermostat,made no difference. The warm/ cold cycle continues
COMMENT by , February 09, 2014
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.