Are the fans turning on? If they are not, the engine will not overheat at highway speeds when there's sufficient natural air flow.
December, 11, 2012 AT 5:33 PM
Yes the fans are turning on and also we have replaced the reservoir cap for the radiator but it continues to run hot.
December, 12, 2012 AT 1:30 AM
Sorry for the delay.
Next, feel if the entire radiator is getting hot. If there's cool spots on it, the flushing didn't completely clear the blockage. That blockage is a common result of GM's use of the red Dex-Cool antifreeze. We call that stuff "Dex-Mud". To verify that, run the heater on one of the higher speeds. That will act like a small radiator and reduce the overheating. Also see if the cooling fins on the radiator crumble off easily. It's a little too soon to have that problem but if they do, they won't effectively dissipate heat to the air.
The next thing to consider is a leaking head gasket. Combustion gas leaking into the cooling system will pool under the thermostat and prevent it from opening. Thermostats must be hit with hot liquid to open. Your mechanic can do a "sniffer" test to check for a leaking head gasket. He will draw air from the radiator through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present that liquid will turn bright yellow. That test is a little harder to do on GM vehicles that have no separate radiator cap, but it can still be done.
February, 17, 2013 AT 7:21 PM
I have just recently had the same problem.
The GM mechanic changed my thermostat as well as flushed the radiator, after doing all that and $550.00 later, the car still runs hot and was overheating even while driving. I took my vehicle to a friend of mine who works at NTB, and they found that the main cooling fan was not coming on at all. Also per the GM Mechanics my friend tested the heads because the GM guys said I had a leaking head casket, and friend found nothing wrong with the heads. So he replaced the fan and relay switch (and whatever else he needed to do) and again after all that and another $550.00, the car will drive fine and not over heat until you stop and the car will idle low somewhere around 500 rmp and again will over heat. They have had the car for 3 days trying to figure out what is wrong. The only thing I can find online is that the throttle body is not working properly and will cause the car to idle rough, however I can't seem to find anything about the over heating.
I am looking for some ideas as to what could be the problem. And also if I find anything out I will post.