The thermostat used on the 2001 Volkswagen Passat is a standard mechanical thermostat equipped with a jiggle valve and a compression spring. Once the engine reaches 180 degrees, the thermostat increases the flow rate of radiator fluid through the engine and the radiator. As the fluid passes through the radiator, the fluid cools, thus lowering the operating temperature of the engine. This process works continuously until the thermostat corrodes or becomes blocked with coolant sludge. When this happens, replacement of the thermostat is the most logical option for repair.
Place the front of the vehicle off the ground with ramps or jack stands. Set the emergency brake or place a wheel chock behind the rear tire for safety.
Place a drain pan under the lower radiator hose that is capable of holding 5 gallons of fluid.
Remove the radiator band clamp with pliers and pull the hose off the radiator. Fluid flows from the hose and the radiator; catch it all with the drain pan.
Follow the lower hose to the water pump. The thermostat housing bolts onto the water pump. Remove the hose from the thermostat housing with the pliers. Inspect the hose for dry rot. If you locate any, discard the hose and use a new one.
Remove the two bolts that secure the thermostat housing to the water pump with a 10mm socket.
Pull the housing, gasket and thermostat off the water pump. Discard the gasket and thermostat.
Clean the surfaces of the water pump and the thermostat housing to ensure a proper seal.
Place the spring end of the thermostat into the housing and then place the gasket onto the base of the thermostat housing.
Place the thermostat housing against the water pump and secure it with both 10mm bolts.
Slide the radiator hose onto the end of the thermostat housing and on the inlet tube of the lower radiator. Secure both ends with the hose clamps.
Open the radiator cap and drain all of the radiator fluid into the radiator. Close the cap when finished.