Jetta, Golf, New Beetle thermostat removal and replacement

difficulty: 2/5
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Introduction

This article shows how to replace the thermostat on an a4 chassis mk4 VW TDI.

Most pictures, tips, and the paragraph below from frugality, thank you! 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.

Parts

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
catch pan
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
torque wrench
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. The aftermarket one comes with a new O-ring. Tthe VW one did not so purchase an o-ring separately.

Procedure

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. Note the 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.