Jetta 04 has the highest water temperature recently in spring, and it is high when driving at high speed, and 120 mph exceeds 120 degrees.

The water temperature has little to do with how many miles you run. No matter how bad the water tank is, it won't be so obvious. It has little to do with carbon deposition.

It should be the defect of cylinder gasket, which must be related to high water temperature and water resistance of water tank.

Jetta (cross-flow scavenging engine) is suitable for rough driving, and it is easy to produce carbon deposit if it is too gentle.