The car is overheating. I changed the thermostat, but I am still having issues.
There are still a few more things to check beside the thermostat. The cooling fan, water pump, and radiator all need to be checked. If the engine is overheating while traveling at a constant speed above 20mph, you can scratch off the cooling fan. White smoke out of the tailpipe, or water in the oil are indicators a potential issue with the head gasket. If you would like this repaired, consider having an expert mechanic, like one from YourMechanic, diagnose why your car is overheating.