If ever your Jetta run hot, and you can’t seem to find the problem. Look for the following faults
1. Top radiator hose hot but bottom hose cold
2. Car is temperature is normal when you drive slowly
3. Water is boiling out of the radiator bottle when you drive fast.
Before you check the thermostat and for a leaking gasket, check first the water pump. On these cars, the water pump impeller tends to become loose from the drive shaft. What happens is there is no circulation of water in the engine and radiator.