I had a similar situation with my '88 Grand Caravan a number of years ago. It was caused by the cooling fins on the radiator being corroded away. Running the front or rear heater always brought the temperature down very quickly.
Also check for anything that is obstructing air flow through the radiator such as a bug collection, and look for holes in the core support that allow air to bypass the radiator on the sides. Most cars also have a rubber seal under the front of the hood to force air to go through the radiator. Be sure that is in place if there is supposed to be one.
Another unusual problem is caused on some engines when the thermostat doesn't have a tiny bleed hole. The hot coolant reaches the temperature sensor for the dash gauge long before it reaches the thermostat. By the time the thermostat opens, the gauge has gone unusually high, then, when the coolant starts to circulate, the cold coolant comes in from the radiator and the gauge goes down and the thermostat closes. This cycle repeats a number of times until the system stabilizes. This also happened on my Caravan but not with a new thermostat. It started acting up many years after it was installed. Drilling a tiny hole in it solved the problem. That allows the hot coolant to get to the thermostat faster.
Saturday, June 12th, 2010 AT 8:08 PM