If your TDI is not reaching full engine temperature and the air coming out of the vents is not very hot even on the hottest setting, it may be time to replace your thermostat. Luckily, replacing the thermostat on an ALH is a straightforward affair.
Buy the Right Parts
To make this job easy, you should purchase a new thermostat and a new thermostat housing (IDParts has a thermostat replacement kit with both together). Make sure the thermostat and housing have locking tabs.
The locking tabs allow you to assemble and lock the thermostat and o-ring to the housing BEFORE installing it in the engine. Getting a new thermostat in without using locking tabs is EXTREMELY difficult, so we do not recommend buying thermostats that are not locking tab compatible.
You also need at least 1.5L of coolant available to top off your cooling system after the job is done.
What Tools You’ll Need
You’ll need a 10mm socket & ratchet, preferable 1/4 drive, plus an 2-3″ extension for it.
You’ll want to assemble the replacement thermostat and housing before taking your old one out. If you do this correctly and have the replacement unit ready and assembled, you can pull out the old thermostat and put in the new one within seconds and limit your coolant loss.
To attach the thermostat to the housing, first place the o-ring on the housing. The o-ring will be held in place between the housing and the thermostat. With the o-ring in place, insert the thermostat and turn 45 degrees. This will engage the locking tabs.
Ensure that the o-ring is sitting correctly around the entire circumference.
If the thermostat is installed correctly, it should be tight on the housing.
Once the new thermostat and housing is assembled, it is time to remove your existing thermostat from the engine. Thermostat replacement must be done on a COLD engine only. DO NOT ATTEMPT TO REPLACE THE THERMOSTAT ON A HOT ENGINE.
First, remove the engine cover held on by 3 10mm nuts.
The thermostat is located below the injection pump.
Remove the coolant hose and bend it upwards to prevent coolant from leaking out. The thermostat will be closed so coolant should not flow out of the engine.
(To make things easier to see, we took photos of replacing the thermostat on an engine that was not in a vehicle, but the process is exactly the same.)
There are two bolts that are on opposite sides of each other, largely vertical. Remove the two bolts. Have the replacement thermostat and housing ready – if you have a friend available, have them hold it inside the engine bay next to where the thermostat is installed.
With the two 10mm bolts removed the thermostat will likely be stuck in place. Pull on the housing, you may need to rock it back and forth to get it free. It is also possible that the housing will come off without the thermostat. If that happens simply pry the thermostat out using a screwdriver. When the thermostat does come out coolant will begin pouring out as well.
Once you get the old thermostat out, quickly push the new housing and thermostat into place. The quicker you put the new one in place the less coolant you will lose. Hold the thermostat in place with your hand while you catch the two 10mm bolts. Having an extra set of hands here is good too!Tighten the bolts to 11 ft/lbs, reattach the coolant hose. Start the car and check for leaks. Keep the car running and let it reach operating temperature while watching the coolant level. If it falls below the MIN line top off with G13 coolant.