The thermostat is a bimetal element which expands and contracts based on temperature. When it opens, it lets coolant into the radiator where it cools down. This controls the baseline temperature of the engine. I do not recommend using stop leak products because they can clog the heater core (you won't get cabin heat) or turbo water coolant lines. The turbos on TDI are oil cooled only. If you have a radiator leak, replace the radiator now instead of having to replace the radiator and heater core later. If you notice that the radiator fan clutch isn't working or that the fan clutch bearing is making noise, see 1000q: fan clutch and alternator removal and replacement.
If the thermostat fails, it sometimes causes overheating by failing in the closed position.
BHW TDI engine (2004-2005 Passat TDI): 044 121 113
AWM engine: 050 121 113 c
o-ring: VW# 038 121 119 b (size is 50x4)
plastic flange: VW# 038 121 121 b
G12 coolant (see 1000q: Passat coolant flush and bleed for more details, do not mix G12 with generic green coolant!)
10mm sockets - wrench
spring hose clamp pliers (optional but suggested)
CAUTION: the engine must be cold. If it's hot the system is under pressure and will spray scalding hot coolant everywhere.
Remove the engine cover (3x 10mm nuts). You can retrofit pop off sockets so that you can just pull it off. See 1000q: engine pop cover mod for more details - it's for a Jetta but it's the same idea.
Drain the engine coolant. See 1000q: Passat coolant flush and bleed for more details. The trick is that you have to bleed air out of the heater core during filling. You could also just deal with the lost coolant and top it off later.
Remove the intake piping on the driver's side. This will give you access to that side of the engine. There is a 10mm bolt holding the piping down and some hose clamps.
Remove the 2x 10mm bolts holding the plastic thermostat flange to the engine block. Follow the lower radiator coolant hose and you'll see the flange. It's behind the alternator. Below is a side view on an engine out of the car for illustration (standing on the driver's side looking at the engine).
CAUTION: you are working next to the alternator. Be careful to avoid
touching any exposed positive wire to ground with any rings, watches, necklaces,
tools, or anything else that can conduct electricity.
Here is a picture with the alternator and bolts removed.
Remove the flange. You may have to wiggle the flange to break the seal
but there isn't much clearance so the plastic fingers inside the flange will
probably break off. The flange has pegs inside that hold the thermostat in
place - below is a picture of the Jetta part showing how the fingers hold the
thermostat. Make sure
the old o-ring is removed. Wet the new o-ring with coolant during
installation. Do not use gasket maker on
There may be some tabs on the thermostat so try rotating it as you remove it
to "unlock" or unscrew the tabs.
Tighten the bolts to 11 ft-lb (the flange is plastic so don't kill it). Refill with G12 coolant. The rest of installation is the reverse of removal. Check for any leaks with a test drive.
Bleed the coolant system of air. See 1000q: coolant flush on B5 Passat for details. If you don't have heat after introducing air into the system, the heater core is either blocked or has an air bubble.