Water temperature...
 

...again, because I don't know what it used to run at, but I could have sworn it was pegged at 90degrees on the guage regardless of speed and air temperature.

Seems to be 10degrees lower at the moment - all the time.

Is this normal? Faulty t-stat?

Thanks,
Zip
2002 S8 FE

Sounds like faulty thermostat.. but if it never moves up beyond that, then likely sender.

If its stuck fully open then it would not reach opererating temprature as it needs to open further than 90 C to cope with very hot days.

Faulty temperature sender is common - So common you're local Audi garage will have them on shelf in stock. They're about 30 quid and 10 minutes tops to fit....

Its called a 'dual temperature sender' as it has two independent sensors, one is used for the Engine ECU and one goes straight to the instrument cluster....

I can dig you out a part number, but from memory the Audi parts guys know it as the 'green one' +++

So what temperature(s) should I be seeing...
 

...is a fairly constant 90degrees about right?

Zip

Mine has been rock solid 90 since i bought it no matter what I do.

Yup, in fact its documented somewhere that if its 90 degrees +/- 10 degrees it still shows 90 degress...... I guess it looks pretty 'straight up'..