2002 gmc van overheatig

There are many different temperature ranges of thermostats. The old one may have been a 165 degree thermostat. The new one may be a 195 degree one. You will then notice that the gauge will run higher than it used to.

Sounds like a thermostat and/ or the system is low on coolant.
Its probably not going to reach 200 degrees when the outside air temp is below freezing, but you should get plenty of heat if the system is working correctly.

More than likely sounds like your heater core is bad or clogged, easy thing to do is have it backed flushed, also you may need to burp you cooling system it may have a air pocket due to the radiator change

Your temp sensor has nothing to do with your heater. The temp sensor needs coolant in order to work properly. So first check the coolant level because if it is low your heater will act the way you describe. Once the coolant gets low enough the heater will blow cold. Another possibility is the radiator cap. After the engine has warmed up it expands when it cools down it contracts. As it is contracting if the cap is weak air can be drawn into the system and cause air pockets to form and will cause the same problem

Have the system flushed again and replace thew thermostat.

Heater blowing cold air until I get on the interstate, then it works; however, when I stop at lights it starts blowing cold air again. The temperature gauge will be in the middle but the car will still be cold until the car starts moving.

Since warm air eventually comes out when the vehicle is in motion, you may have an air mix door that is stuck in fresh or is somehow blocking warm air from the heater core.

To see if the air mix control motor is bad, you need to retrieve your Diagnostic trouble codes (DTC's) from the heater control panel. To run the diagnostic function, do the following:
1. Turn the ignition switch ON (II).
2. Turn the fan switch OFF.
3. Press the recirc control button to select Recirc (recirc light on.)
4. Press and hold the recirc button to select Fresh (recirc light off) and continue to hold the button down until the recirc light comes on for two seconds and then goes off.
5. If the system is okay, the recirc light stays off.
6. If there is a problem, the recirc light blinks the DTC to indicate the trouble. One blink=a problem in the air mix control motor circuit. Two blinks=problem in the mode control motor circuit
Three blinks=problem in evaporator temp sensor circuit.
7. Turn the ignition switch off to cancel the self diagnosis function. Run the diagnostic again after repairs are complete.

Note: the air mix control motor has a rod which connects to a plastic link. The link controls the mix doors and also connects to the heater control water valve via a cable. The air mix motor may check okay, but if the link is broken then you will still have problems. the motor is on the bottom of the heater unit in the front passenger's foot well.

check coolant level ,must be low ,chck for coolant loss

there is a valve on the water pump and one on the thermostat housing open those up one at a time until you get a little bit of antifreeze come out then tighten them back up. All you will be doing is bleeding the air out of the cooling system. my car was doing the same thing after I changed my thermostat.

on GM vehicles they still use clutch fans and when sitting at an idle for a while the fan is not pulling enough air through the radiator and condensor so therefor the a/c starts to put out warmer air and the coolant temp. will rise a little bit. You could try replaceing the fan clucth and see if that helps any.