DIY to change thermostat on a '98 A4 1.8T
My car's temp would rarely reach the center and I knew something was not right. I changed the sending unit and that still didn't fix it so it had to be the thermostat. After looking at some tutorials (which were apparently wrong for my car), and searching on a variety of forums, I tackled it with instructions from a B6, even though I have a B5. In any case, here's a brief tutorial and hopefully it will help others down the road who have a B5 since I was unable to find any instructions for a B5.
Mine is a '98 and there may or may not be differences throughout the years so that is my disclaimer.
Anyhow, this is a simple 30 min job. I took 3 hours b/c I made the mistake of using instructions for a B6 A4.
For the B5's, you DO NOT need to take off the throttle body or the alternator. Actually you will be working under the car the whole time.
Sorry, back to the instructions:
1) Put the front end of your car on ramps or jacks with jack stands.
2) Remove the engine belly pan
3) Remove the driver's bumper trim piece. Just pull on the left side of it and the entire piece will come out. You may have a small tab on the trim piece that you can pull to make it easier, I didn't.
4) Now behind this panel you just removed, you will now see a red circular plastic fastener with a + on it. This is your radiator drain plug. Get a bucket/container to catch all of the coolant that will drain out. First, open the coolant reservoir cap then unscrew the plastic drain plug and catch all the coolant that will come out.
5) Now get some safety glasses and get under the car (I'm not sure how dangerous coolant is but I can't imagine it's good for your eyes if you get any in it). You will see the thermostat housing directly above the oil pan bolt. The housing is held on by two 10mm bolts. Remove those. No need to remove the metal bracket that may be in the way. Just have a small socket extension to get around it (that's what I did).
6) Before you pull down on the thermostat housing, get the bucket/container again b/c as soon as you pull it down, the other half of the coolant from the engine block will drain out.
7) Remove the thermostat from the engine block. It may be stuck on b/c the O ring has melted on. Use a screwdriver to pry it off and the thermostat will fall right out.
8) Install the new O ring onto the housing and then the thermostat on top of that and reattach to the engine block with the two 10mm bolts you removed earlier.
9) Fill with new G12 coolant in 50/50 or appropriate ratio, turn car on and turn heater on max. Let run and refill as necessary until the fluid in the overflow reservoir does not drop. I used about 5 liters of 50/50 coolant/water.
10) Once car has reached proper operating temp and refilling coolant is no longer needed, double check that the housing is not leaking. If it is then your O-ring may not be centered properly.
11) Replace engine belly pan and give yourself a pat on the back.
12) Dispose of old coolant properly.