First, can we get a little more information from you?
1) What engine is in the vehicle, 3.1L?
2) Does the temperature guage show the engine is overheating?
3) Has the vehicle had any recent repairs (related or not to the problem now)?
When you replaced the thermostat, did you open the air bleeders to vent air pockets out of the engine (not running)? Also, what was the condition of the coolant previously? Mileage of the vehicle?
Ok, There is atleast one bleeder that has to be opened to let the air escape from the engine. One is on the front left of the engine above and behind the water pump on top of the bypass tube. There may or may not also be one on the thermostat housing. As for the coolant; water only, boils at 212 degrees F. The normal operating temperature of the engine is about 220 degrees F (give or take a few degrees). It is absolutely necessary to use a 50/50 mix of antifreeze and water. I would suggest these corrections first, along with a new coolant reservoir cap to start with. There could be bigger issues, but this is the least costly and most common at this point.
I hope this information helps. Reply here if you need further assistance with this issue. Thanks you for posting with us on JA.com
As long as the system is sealed good and everything is correct so far, you may have a head problem allowing engine compression to enter the cooling system.