Greeting from Mikey at Just Answer!
Make sure the car has a proper 50/50 mixture of coolant and water. Use an antifreeze tester.
The cooling system on the 3.5 has an odd thermostat design that requires hot water returning from the heater core to open the thermostat. A clogged heater core can prevent the thermostat from opening and cause the car to overheat.
Flush heater core separately and recheck the car. If the problem improves but comes back after flushing the heater core then replace the heater core.
I have seen car huslers that drill a 1/8 hole in the thermostat body to improve thermostat operation. This is a desparation move and may have a negative impact on the cooling system by making the engine run to cold.
A bad head gasket can also cause the car to overheat. The best method to check for a combustion leak is to use a block tester, also known as a combustion leak tester, to determine if you have exhaust gases in your cooling system. A combustion test kit can be found at your local NAPA auto parts store. The part number is(NNN) NNN-NNNN The price for this test kit is $50.00. Exhaust gases in your cooling system can suggest a head gasket leak, a cracked block, or a warped head, etc. To do the test, add the blue detector fluid to the (block-tester) plastic container according to the directions, and place it onto the radiator filler neck. The squeeze bulb is placed on top of the reservoir and squeezed repeatedly squeezing the bulb will draw air from the radiator through the test fluid. Block tester fluid is normally blue. Exhaust gases in the cooling system will change the color of the fluid to yellow, indicating a combustion leak. If the fluid remains blue, exhaust gases were not present during the test. The vehicle should be started and gotten hot with radiator cap off and coolant level 3-inches below full before performing the test.
If fluid turns yellow, remove cylinder heads, have them pressure tested and resurfaced if needed, them put it back together with all new gaskets. Head gasket failure on the 3.5 engine is a commom problem. Confirm with block test.
Edited by Mikey on 10/2/2009 at 12:28 AM EST