I have a 2002 Audi A4 1.8T with 85,000 miles. Recently my car started overheating, I replaced the water pump, thermostat, and timing belt. The mechanic also ran tests to make sure there were no issues with the head gasket, those tests came back saying the head gasket was a-ok. Yet, my car is still overheating but only on days when the outside temperature creeps above 90 and I am in stop and go traffic. I have to re-fill the coolant tank about once a week depending on how much I drive. I find no coolant puddles under my car when it is parked, so I do not believe it is leaking. What else could be causing my car to overheat?
Since the mechanic found no issues with the cylinder head or the gasket I would replace the radiator cap. It’s probably releasing small amounts of coolant as you drive. This would explain the reason to refill the overflow tank on a weekly basis.
Is the fan running when the car heats up? Has the fan shroud been removed? What is the condition of the radiator? Look down in it and see if the tubes are filled with corrosion. Pull off the cap(cool) and check to make sure the coolant is full. There may be air in the system.