First thought was rotted / crumbling cooling fins on the radiator, but since you already replaced it, feel the radiator and heater hoses when it overheats. They should be too hot to hold onto for very long, but not hot enough to burn you. Also notice how hot the air is that's blowing out of the heater. If the temperature gauge reads hot, the heater air is cool, and the radiator hoses are cool, suspect a plugged heater core, a loose water pump impeller, or a leaking head gasket.
Some cooling systems require hot coolant to flow through the heater core to reach the thermostat. A way around this is to use a thermostat with a bleed hole in it or to drill a 1/16" hole in it. That will get the hot coolant to flow over to the thermostat. Sometimes you can remove just the upper hose from the thermostat housing, then reach in to prop it open as a test.
Broken water pump impellers are common on Volkswagens, not on Jeeps. You would see no flow through the radiator and the hoses and heater will be cool if the impeller is loose.
A leaking head gasket will introduce air into the cooling system. Thermostats do not respond to hot air. They must be hit with hot liquid. Your mechanic will use a special tool to check for a head gasket leak. Air is drawn through a glass tube with two chambers of dark blue liquid. If combustion gases are present, the liquid will turn bright yellow.
Wednesday, March 10th, 2010 AT 5:09 PM