The thermostat on your Volkswagen helps the engine keep operating at a predetermined temperature for best fuel economy and engine performance. However, the thermostat mechanism wears down after months of use and subsequently fails. If stuck in the closed position, the thermostat will not allow coolant to circulate, causing the engine to heat up dangerously; if stuck in the open position, the engine will never reach proper operating temperature, causing fuel consumption and poor engine performance. In either case, the thermostat must be replaced.
Remove the cap from the coolant reservoir.
Raise the front of your Volkswagen with a floor jack and support it on two jack stands.
Detach the insulation tray from the bottom of the radiator using a ratchet and socket, if your particular model is equipped with one.
Place a catch pan underneath your Volkswagen radiator.
Open the radiator drain valve and drain the coolant. Then close the drain valve.
Follow the lower radiator hose toward the engine and loosen the hose clamp at the flange attached to the engine. Use a screwdriver, slip joint pliers or ratchet and socket, depending on the type of clamp used on your particular model.
Remove the two mounting screws from the radiator hose flange using a ratchet and socket.
Detach the flange from the engine, O-ring and thermostat.
Set the new thermostat in place. Make sure the spring points toward the engine.
Place a new thermostat O-ring.
Set the thermostat flange in place and start the two mounting bolts by hand. Then tighten the bolts using the ratchet and socket.
Attach the lower radiator hose to the flange and tighten the hose clamp using the screwdriver, slip joint pliers or ratchet and socket.
Lower the vehicle.
Fill the coolant reservoir with 40 per cent antifreeze and 60 per cent water up to the upper line or the MAX mark on the reservoir tank.
Replace the cap on the coolant reservoir.
Start the engine and run it at 2000rpm for 3 minutes. Then let it idle until the cooling fan comes on and turn off the engine.
Open the reservoir cap carefully using a shop rag and release the steam. Add more coolant mixture to bring the level up to the upper line or MAX mark again. As you operate the vehicle, keep checking the coolant level at the reservoir and add more until the level stays at the upper line or MAX mark with the engine cool.