Thermostats on Volkswagen’s 1.8T engine are not known to fail often. But if they do, you will be unlikely to find a DIY guide that says you can replace it without removing the alternator.
When I got my 2002 SEAT Leon, the temperature gauge was acting a bit funny, bouncing back and forth between about 70-90º when warm. I initially replaced the coolant temperature sensor, which just plugs into one of the radiator hoses, but nothing changed; in fact, the gauge eventually stopped moving at all. As temperatures outside dropped, I noticed I wasn’t getting much heat inside. So it was evident that the thermostat was stuck open. Thankfully, it’s a cheap repair, as you can pick up a Febi OE-quality replacement thermostat on eBay for £7.
Begin by jacking the car up in front, as this will help minimise coolant loss when you remove the thermostat. Next, take off the bracket attached to the intake manifold, held in place by two bolts (indicated by the arrows below) removable with a 5mm hex key, and use something like a bungee cord to keep it pulled back. You’ll need this out of the way to access the thermostat housing. You will also find it helpful to remove the dipstick, which just clips on to the oil pan and should pull right off (although remember to put it back on before you actually remove the thermostat, as coolant will flow out freely).
The thermostat housing is held in place with two 10mm bolts. Neither are very tight. The top bolt is easily accessible with a ratchet extension and a flex joint. The bottom bolt, however, is far more tricky to access. The best way to do it, as pictured below, is to go between the runners on the intake manifold. You will need two long extensions and your flex joint to reach it. I used a 3/8”-drive, as the only flex joints I have are for my 3/8”- and 1/2”-drive ratchets. If you have a 1/4”-drive flex joint, you’ll find that more useful – the 3/8”, being bigger, was a very tight fit.
If you have smaller hands, you should be able to get your hand underneath the manifold and spin the bolt out with your fingers. I don’t, and thus it took a lot of patience and the use of a small 10mm spanner to back it out all the way. Again, if you’re using a 1/4”-drive flex joint, you should be able to back it all the way out with the ratchet. Then just pull the thermostat housing out the way (it will drip a bit), and you’ll have exposed the thermostat.
At this point, you’ll want to put the dipstick back on, and then simply pull the rubber o-ring off, and take the thermostat out. Make sure you have a drain pan underneath the car, because a good amount of coolant will flow out. Once it’s finished draining, put the new thermostat in, fit the new seal that comes with the thermostat, and reattach the housing. After everything is back in place, top up the coolant following usual procedure.
Now, I’ll be honest – you may find it more trouble than its worth to do it without removing the alternator. But if you want to defy those who say it cannot be done, now you can.