sounds like you have a bad cooling fan. if you only have an overheat problem in traffic then it is most likely that you dont have a fan that works. the fan should come on when your engine temperature reaches to 221F then shuts off when your car cools down to about 190 to 201F. so when your on the highway there is no need for a fan since you have just the ram air to cool down the car. check you fan let the car run until the temp gauge is about 3/4 up. the fan should kick on. if you have a scan tool then look at the data stream and wait until 221F. then is when the computer of your car turns on the electrica fan. if your fan doesnt come on then you may see coolant come out of the coolant cap. this happens due to high heat and high pressures inside your coolant system. try and tap the cooling fan motor with a hammer if the fan comes on with a tap of the hammer then you have a bad cooling fan motor. job cost about 250 dollars to fix. sometime cheaper depends on who does the work. thanks for using just answer let me know if this helps. i am here to help thanks again Jon
ASE Certified Technician
fully certified gm tech and hybrid trained