Coolant change and flush.
A coolant flow diagram for a 1997 Jetta diesel would show coolant leaving the water pump and traveling through a 3/4 inch inside diameter hose, up the front of the engine, to the top of the engine block. There it either takes an easy path through the wide, upper radiator hose (only if the thermostat is open) or it flows directly into the engine. Hot coolant exits the driver's side of the engine head and goes to the exhaust gas recirculation (EGR) device to cool recycled smoke. This adds even more heat to the coolant. Next the coolant travels to the heater core in the dashboard. Finally the rubber coolant hose exits the dashboard and joins a metal line which returns coolant to the suction side of the water pump. Two skinny hoses direct bubbles to the top of the coolant reservoir as the system purges itself of flow-restricting air bubbles. The bottom of the coolant reservoir is T connected into the suction line to the water pump. The thermostat is in the water pump inlet and blocks incoming flow from the bottom radiator hose until the engine has reached the temperature at which the thermostat is designed to open 195 degrees Fahrenheit. A corroded thermostat can get stuck open which would prevent proper warm-up. Or, A thermostat can become stuck closed, which can cause overheating and boil over.