ok, so i searched all the posts for operating temp issues, but still need solid answers..
i have the 99' 2.8q and the past week the temp. guage goes to normal temp. then would fall to cold then go back up to normal, usually at hwy. speed. Now, tonight the temp guage hasnt dropped, but instead it was raising gradually up to a dash before hot. at 55-60mph then would gradually go down once i got into in town speed then climb again on the hwy. I belive its the Engine coolant temp sensor not sure if its blue or green (im not sure where it is exactly). or if it is the thermostat... im lost n new to the a4 but guidance would be greatlyappriciated

 

Could be either, but from your symptoms, I would put my money on a failing thermostat.

 

crap, that means more money doesnt it.. i got a volkswagon/audi dealer 30min. away should i contact them first? or is it easy to do yourself? and were is that sensor and the thermostat located? is the therm. in the water pump right? god this sucks, right b4 a trip on sat. too, go figure....

 

i would just replace the temp sensor since its such a cheap part, if that dosent fix the problem then hate to tell ya buddy your gonna have a PITA time replaceing the thermostat or haveing someone do it for you, have you changed teh T-belt yet if you havent replace teh Tstat when you do that.

 

I dont know on the 2.8, on my 1.8 the thermostat is real easy to replace, follow the bottom radaitor hose to the engine, thats where the thermo is located.
Ihave never heard of it being in the waterpump, I think you are incorrect.

 

looking at the engine on the left hand side in the back under aobut a foot of wires and hoses is the blue sensor. theres a DIY on it int he DIY section and also on audidiy.com

 

alright i looked it up on audiworld on how to change the sensor. im gonna get a new sensor in the morning and hope thats it. it just worried me that it all of the sudden started going toward hot instead of cold to normal.

 

so i just put the new sensor in and took it for a drive and the same thing still happened.also right before i got home to install the new sensor, the temp warning light dinged and flashed at me.so do i go with thermostat now? is that a pain to install? this is drivin me crazy guys....

 

if u got huge hands u will suffer lol. but i got fairly small hands and took me like 20 min

 

So im definately in the process of tearin down the front bumper, radiator support, and other things needed to get to the thermostat area. i got a friend who has the same car whose helpin me in a few days. im goin to the dealer to pick up a timing belt kit, water pump, and thermostat cuz i heard its a bad idea to get aftermarket parts so im goin with OEM. its gonna be like $400 total i guess. thank god for friends huh. i so dont know what im doin but i got a book for the car now so that helps. I hate AUDI right now.... but i still love it at the same time.