Sure would be helpful to know which engine you have. Does this problem occur only for about the first 15 minutes after starting the engine, or does it only happen in slow city driving?
If you have a V-6 engine, the 3.0L Mitsubishi engine in particular, there is a very small bleed hole in the thermostat. The problem occurs if you have an aftermarket thermostat installed that doesn't have that bleed hole.
After startup, as the engine warms up, hot coolant reaches the sensor for the temperature gauge before it reaches the thermostat. By the time the hot coolant reaches the thermostat causing it to open, the gauge has already gone real high. When the thermostat opens, cold coolant rushes in from the radiator, the temperature gauge goes down, and the thermostat closes again. Then the process starts all over. After doing this a few times, the temperature fluctuations smooth out and the gauge will stay in the normal range. This happened to my Caravan, but the funny thing is, it didn't start until a few years after that thermostat was installed. After living with it for two years, the fix was to drill a 1/16" hole in it and reinstall it. That bleed hole lets hot coolant reach the thermostat at the same time as it reaches the sending unit.
If the gauge never goes too high when you're driving on the highway, but it does in the city, you'll need to check whether the electric radiator fan is turning on. The fan relay is no longer the simple, reliable device used since the 1970s. To prevent the silly complaint that people could hear the fan run, they now use an extremely trouble-prone electronic module that slowly ramps up the fan speed. That electronic relay is bolted to the radiator core support on the driver's side, about halfway down. It must be bolted securely. If it is loose, it will get hot enough to burn your hands.
The fan motor could also be defective, but the relay is much more common.
The V-6 thermostats are located on the passenger side of the engine. The upper radiator hose attaches to the housing.
Saturday, January 16th, 2010 AT 5:21 AM