Chrysler Repair: no heat, runs cold/thermostat replacement 3.3/3.5L, engine thermostat, heat dissipation

Question
We have an 01 town and country van.  When sitting still the heat works, but when you are driving , no heat.  Temp. gauge goes normal when sitting, when driving, temp. gauge goes to cold.

Answer
Hi Holly,
What you describe suggests that the engine thermostat that should maintain the operating temperature of the engine (which should stay around mid-scale on the gauge), by increasing/decreasing the rate of coolant circulation in response to the heat generated by the engine in comparison to the heat dissipation by the radiator, is not doing its job. Probably the thermostat is stuck open and thus allowing too much heat energy to be dissipated by the radiator when in motion, so the engine cools down along with the temperature of the coolant so you get no heat in the cabin as well.
The thermostat is an inexpensive part and fairly easy to replace. Yo could do it yourself with a few tools and a drain pan to collect the coolant and then put it back in. It also may be the case if you don't know the history of the van that a former owner removed the thermostat. That you will find when you open its housing to replace it. You may find no thermostat! Or you may find one that is open rather than closed as it should be when cold. You can test it by putting it a pan of heating water on the stove and measure the temperature at which it begins to open. It should have a temperature engraved in the metal, which is the point when it should begin to open. If it opens at a lower temperature than that, it is no good.
Roland

Hi Holly,
OK its a 3.3L engine(same as the 3.5L as used in the minivans). You will want to go to the lower corner of the radiator on the driver's side where you will see a small hose attached to a draincock (valve)*. Twist the  tab on the valve counter-clockwise to release a couple of quarts of coolant into a pan for reuse. Then close the valve tight.
Then go to the top front of the engine where you will see a fat hose that comes from the radiator attached to a hemisperical housing inside of which is the thermostat. Loosen and remove the two bolts on the housing to open it up. Remove the thermostat and the gasket around the outside edge of the housing/hole. Note how the thermostat is installed (spring side down). Clean both the sealing surfaces against which the gasket seals the housing. Dip a new gasket in water and place it on the surface of the housing; install the new thermostat in the hole. Place the housing and gasket (making sure it is centered on the housing and hole) back over the thermostat so as to mate the housing properly with the hole in the engine and that the gasket is also symetrically in place. Replace the bolts and tighten to about 20 foot-pounds (the same as for a spark plug). Refill the radiator and then the overflow bottle with the previously collected coolant.
That is all there is to it. A shop will probably charge you up to 3/4 hour of labor to do this if you wish to have someone do it. But ask before authorizing so you don't get ripped-off.
Roland
*If you have the trailer towing package the draincock may instead be on the passenger lower corner of the radiator.