I have a 2002 Jeep Liberty Sport, 3.7L that continues to overheat. The problem only seems to occur when I drive for 20 minutes or more, and above 65mph. It does not overheat when I drive in town at 35-40 mph, or for short distances. When it does start to overheat, the temp instantly goes down when I turn the heat on full-blast. I have replaced the thermostat, the water pump, had a pressure test done, and had a full flush. However, it is still overheating! All of the mechanics that have worked on it seem to be playing guessing games, and I'm tired of spending time and money trying to correct the problem when I still have no answers as to what the actual problem is! HELP!!

I have replaced lots of radiators in jeeps, they seem to clog up easily. To check remove the top hose from the engine and remove the bottom hose from which ever side is easiest. Use a garden hose at the top hose and turn it on at least 1 to 2 full turns. the water should run threw the radiator with no problems, not backing up at the top hose. BUT before you do this check the front of the radiator/condenser to make sure it is clean, free of leafs and bugs. The problem here is air flow threw the radiator. This can be cleaned with a pressure washer.

I had a similar problem with my 2006 Dodge Dakota cooling system. After the same tests and replacements you mentioned, the dealership replaced the radiator with a new radiator and the cooling problems disappeared. Apparently there was a defect internal to the radiator that did not let the radiator provide maximum cooling at higher temperatures. Hope this helps.