So it's an '04, 1.8 Turbo.
I had an issue a few months back with the fans running all the time. After trial and error, I'd replaced the Engine temperature sensor, then the thermostat. Problem solved.
Now, I have an issue with it overheating, but there seems to be no rhyme or reason to it. symptoms are
1) will remain at 90 degrees (the norm) for approx 20 mins
2) once it begins passing 90, 1 of 3 things happens:
a) will continue to climb, giving a coolant warning (until I shut it down)
b) will rise to a certain point, say 3/4 then start dropping to 90 again
c) reach a random point, again lets say 3/4, and then remain there
3) coolant resevoir stays at ideal levels (between hash-marks on resevoir)
4) fans never run upon overheat

I've checked the lines for flow (upper lines run hot, lower runs cold), no leaks.
A/c works, heat works
I got nothing, as coolant systems are a mystery to me.

 

Sounds like a water pump issue to me. Did you replace the water pump when you did your thermostat?

 

Possibly a defective thermostat. Are you running AC? Does the electric fan kick on when AC is on? Did you bleed/burb the system? Flush it out also? Pump could be cavitating I suppose.

 

Did you replace just the upper temperature sensor or the lower one near the thermostat also? I would work on bleed/burp again.

What's your mileage? Close to a timing belt/ waterpump interval? Waterpump would be the next consideration but you say you have good heat at all times so if you have heat the waterpump has to be pretty strong.

 

I replaced the upper tmeperature sensor. Ya all signs seem to point to burping the system. Last flush was '10 when I returned from overseas.
mileage is about 236K, and I am close to needing a belt change, possibly a new tensioner......But as I said, it belts out the heat, AC workks, Fans DO NOT run when it overheats, So I'm inclined to think it's not (or just hope lol) it's not eh waterpump

 

I'll be doing that as soon as my work schedule allows, and I'll keep you posted for your own knowledge. Thanks guys

 

The fact that the upper radiator hose is hot and the lower is cold indicates that there really isn't flow through the coolant system. Also, you indicated that the gauge will move up and then back down once again. I would seriously suspect the water pump is on its way out and is probably freewheeling on the shaft from time to time and then working again for a short interval. It's only a matter of time before it totally gives out if that's the case.

Try bleeding the system as this is the cheapest method. Then if it were me, I would begin to contemplate verifying that the pump is working correctly.

 

ok so here's the latest. Sorry for the delay; being military has drawbacks sometimes when they send you everywhere.
So, I got a second opinion, after a shop replaced the coolant temperature sensor AND the cooling fan motor. That was needed, but didn't fix the problem. They said my headgasket was shot.
The second opinion revealed "No it's not the head gasket, it's the rad is plugged and the vacuum (?) lines at the back had disintegrated (the ones that snake under the intake manifold). I also need a new belt tensioner.
So after returning home finally (ordered parts while i was gone) I have delved into replacing the tensioner and the rad myself. I'll let the "second opinion shop" handle the vacuum hoses. I just dont want that headache.
***OH also: i attempted burping the system and there was not air in it***
Thanks for all the guidance guys

 

It could be a clogged radiator if crap from a hose broke off and got lodged in there.

Luckily for you, you now know how to bleed a coolant system on an Audi because you'll need to do that again after you replace your radiator.