Thermostat replacement tip A3 Jetta (AHU/1Z)

I don't know if this is the correct place for this kind of post but I just got done doing a thermostat replacement on my 1997 Jetta and it took me several hours. I learned a trick at the end that I feel other people should know.

So it should be a fairly straight forward job, but in the various video I watched / write ups I didn't find a clear step that explained where the oring was supposed to go. When I pulled the flange off the thermostat was still in there with the oring attached. I assembled everything with the oring behind the thermostat and it was leaking when I poured coolant back in.

The oring needs to go in front of the thermostat between it and the flange. That was the next issue. I couldn't get the oring to stay in place when reassembling and it would always slide off to one side and get pinched between the flange and the block. I tried grease to hold it but that didn't work any better. Crazy glue on the oring and then on the thermostat to the block was the answer.

TL;DR use crazy glue on the oring to thermostat contact and then clean up the mating surface for the thermostat in the block and put some crazy glue on that surface then hold the thermostat in place while it dries. Once dry the thermostat will hold itself in the block and you can put the flange back on without shifting the oring around or pinching it as you tighten everything back down.

I specified the body in the title but I'd wager this tip is applicable to essentially any other thermostat replacement.

there is a reason the flange is short, reach your finger inside and up to hold the tstat in place as you put it in. you can also do the same thing with a belt stiff rod or whatever you got, even a curved stick! works for me every time. no glue required.
grease will not work, you want to use a water soluble glue. glue stick works really well and dissolves when it gets wet.

Mongler98 said:

there is a reason the flange is short, reach your finger inside and up to hold the tstat in place as you put it in. you can also do the same thing with a belt stiff rod or whatever you got, even a curved stick! works for me every time. no glue required.
grease will not work, you want to use a water soluble glue. glue stick works really well and dissolves when it gets wet.

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Thanks for the input Mongler. I did try holding it with my finger but somehow couldn't get the oring to stay on the flange as I slid the flange into place, on the left corner it slides under the oil pan a little bit so that reduced clearance seemed to make it slide off as I lined the flange. I tried multiple times and I have relatively small hands / fingers. The grove on the flange isn't very deep and the oring isn't too tight of a fit so it slides off fairly easy. I didn't consider using wire but at last it's done and I can have heat for the coming winter.

I replaced it because I was dropping like 10c-15c coolant temp at highway and that certainly wouldn't have been fun in norther canada winter.

did you take the bolts off for the power steering and push it aside?

Mongler98 said:

did you take the bolts off for the power steering and push it aside?

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Yep power steering pump was fully unbolted and off to the side, the lines still attached just being held up by my floor jack. That dang tight gap between the corner of the tstat flange and the oil pan just made the oring slide off to the side everytime. Now that I think about it there might have been enough space for me to put the tstat in the block and then the oring on top of that then slide the flage into place but I really don't know if it would all hold into place. My main issue was that the info I had found didn't really describe how to put everything back, it was basically like poof new one is in and then bolt everything back up. I figured that the little dab of crazy glue trick might help someone avoid a good amount of time fiddling trying to line everything up as I did. Maybe I'm just an idiot but it worked well for me. Hoping it helps someone else looking up how to replace the tstat.