difficulty: 2/5
back to 1000q: B5.5 VW Passat TDI "how to"
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Introduction
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 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.
Parts
thermostat (87-102oC)
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)
G12 coolant (see 1000q: Passat coolant flush
and bleed for more details, do not mix G12 with generic green coolant!)
10mm sockets - wrench
pliers/thin screwdriver
spring hose clamp pliers (optional but suggested)
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.
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 electricty.
Here is a picture with the alternator and bolts removed.
Remove the flange. You may have to twist the flange to break the seal
and your flange may have pegs inside that 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
the seal. Insert the new thermostat in the same up-down orientation.
Make sure it's seated and refill with G12 coolant. Check for any leaks with a test drive.