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 and returns to the engine. This controls the temperature of the engine. While the turbo on your Volkswagen TDI is oil cooled, I avoid using stop leak products because they can gum up and clog the heater core (you won't get cabin heat) or turbo water coolant lines.
If you are experiencing coolant temperature malfunction without engine overheating or other symptoms, it is likely that the coolant temperature sensor is the problem. They are more failure prone than thermostats. If you have a bad sensor, the coolant needle on the instrument cluster will be low and will bobble around a bit. I had this happen once and replacing the coolant temperature sensor fixed the problem. A couple of years later, my coolant temperature was reading low, just within the next hashmark down from 190 degrees, so I thought my new coolant temperature sensor already went bad. Replacing it again didn't fix the problem which confirmed a bad thermostat. I replaced the thermostat and now the gauge pegs right to 190 and holds steady there. When the coolant temperature sensor went bad, it bobbled and varied more. When the thermostat went bad, it was more consistent, staying in a position that was about one hashmark low. Thermostat pictures, tips, and this paragraph above from frugality, thank you!
Here are some tips on replacing the coolant temperature sensor. Coolant temperature sensor is shown here because it's too small for a separate writeup and it could mimic a stuck thermostat. Replacement is not suggested unless it's bad.
Symptoms of a bad coolant temp sensor are the water temperature gauge not
rising or an ECU fault for coolant temperature sensor. They should have 2
outputs, one for the cluster and one for the ECU. If the sensor is bad,
the engine could think that it's warm and will give too short glow plug duration,
which could cause hard starts in cold weather. If you think you're having a glow plug problem that is solved by
unplugging the cooalnt temp sensor, it's probably bad. You can verify this
by reading it through VCDS engine measuring blocks, group 7. Here is a
screenshot of a normal warmed up engine. On a cold engine, these
temperatures should show ambient temperature.
To replace the sensor, look on the driver's side cylinder head. This view is on an engine out of the car, looking at it from the left side of the car. Remove the clip and pull the sensor out. Quickly push the new one in and you shouldn't lose too much coolant. If you don't have G12 handy you can just top off the reservoir with distilled water.
Make sure the engine is cold or else the coolant system will be hot and under pressure and spray out and scald you.
If you have a square plug you use the blue sensor, VW# 078 919 501 b (078919501b),
if you have a round plug (green or black plastic), you use a green sensor, 059 919 501 a (059919501a).
thermostat for ALH and BEW engine VW# 044 121 113
o-ring VW# 038 121 119 b (size 50 x 4)
thermostat flange adapter for ALH engine VW# 038 121 121
thermostat flange adapter for BEW engine VW# 038 121 121 d
50/50 mix of G12 (pink) coolant and distilled water, see 1000q: coolant flush for more details, don't mix G12 with generic green coolant
10mm socket for engine cover nuts
T-25 drivers or bits for splash shield-belly pan
6mm socket for lower radiator hose clamp at flange
5mm hex bit (allen bit) on a 6" ratchet extension for flange bolts
spring hose clamp pliers (optional but suggested)
This is a comparison between the OE thermostat ($66 at my local dealer) and
an aftermarket thermostat ($19). The aftermarket thermostat will not
attach to the flange (pictured below). The pointy end is not big enough to catch
on the 2 pins for installation. I recommend going with the original VW thermostat.
aftermarket one comes with a new O-ring. Tthe VW one did not so purchase an
Remove the engine cover (3x 10mm nuts). You can also retrofit pop off sockets so that you can just pull it off. See 1000q: engine pop cover mod for more details. Remove the coolant reservoir cap to let air into the coolant system.
Raise the car, rest it securely on jack stands or wood blocks, and make sure the car is safe and secure before getting underneath it!
Drain the engine coolant by loosening the radiator drain at the lower corner
of the radiator. You can use a hose to divert it into a bucket. This
will only drain the radiator. You can use compressed air at the coolant
reservoir or remove one of the oil cooler water
hoses to drain the engine and it will save you some mess when you remove the
thermostat, see 1000q: coolant flush and bleed for more details.
Remove the hose clamp from the lower radiator hose at the thermostat flange. This is just underneath the fuel injection pump.
Remove the 2 bolts holding the thermostat flange to the block, 5mm allen bit on about a 6-inch ratchet extension.
Cover the alternator with foil or a plastic bag to avoid coolant spilling on it.
Remove the thermostat flange. The thermostat is underneath it and will probably be stuck to the engine. The Bentley service manual says to rotate the flange 90 degrees counterclockwise but this is impossible given the diamond-shaped opening in the bracket that surrounds the flange. The only thing to do is to pull it straight back, which will damage the flange. You could try using a metal hook to pull on the thermostat which will also pull the flange out.
Here is how the themostat sits in the flange and what can happen if you pull
straight back. To aid in installation, the thermostat holds onto 2 pins that are molded into the flange.
o-ring and use a new one during installation. If it's broken, loop a wire
through around the metal tip of the thermostat and through the flange to hold it
in place during installation. When it's installed, loosen the loop and
pull it out. Tip by cattlerepairman.
Clean the mating surface of the thermostat. Do not use gasket maker on
the seal. Lubricate the new o-ring with coolant.
Reinstall the bolts and torque to 15 Nm (11 ft-lb.). Don't overtighten them or else it can crack the plastic flange.
Reattach the lower radiator hose and tighten its clamp. Double-check that the radiator drain valve is closed.
Start the engine and check for leaks. Watch the expansion tank and add coolant if the coolant level drops. If there are no leaks, the rest of installation is the reverse of removal.
Do you have more tips on how to replace the thermostat in your Volkswagen TDI? Please post it in the VW TDI discussion and repair forum linked at the top and here: myturbodiesel.com TDI forum.