Here's something else, my car does the same crap (99.5 A4 B5). If its at standstill, the temp gauge climbs to the middle, but as soon as I hit the highway, the needle drops to "C". Now, my heat has been getting less over the last two years. This year when I turned on the heat I get nothing but cold air after 10-15 seconds. The hoses going into the firewall are always hot but no heat whatsoever!! I've taken it to different mechanics and they all give me a different answer. One mechanic even told me to start a systematic replacement of parts starting with the thermostat. I don't want to replace parts that are still working. Seems the heater core is working fine but I'm not sure about the thermostat. The thermostat in the car is the original one that came with the car (I bought it brand new). Any chance that I may be looking at an expensive repair here ? I don't wanna ditch the car even though it has just clocked 308,000 miles and still runs great! The dealer is asking for a $120 diagnostic that will pinpoint the problem. I'm pissed because its frikkin cold outside and I'm driving with no heat!!
HELP !!
Thanks !!

Whoa whoa whoa! How the hell do you still have the same thermostat at 308k!?!? Have you never had the timing belt replaced? If you've had the timing belt done, there's slim to none chances that you're on the original thermostat. If by some odd miracle you are, that's definitely your problem and you should start with replacing that before getting into heater core replacements and such.

^^ Agree..

That's the exact behavior you should expect to see from a thermostat that's stuck open. Change it, put in new coolant, burp the cooling system, and you're golden.

Hahahahaha. How come I always get the same response when I say that anybody?? I financed my car when I bought in '99 because I was planning on keeping it for a long time. For one, I drive a lot (mostly highway). Audi always sent me a reminder for the scheduled maintenance. Amazingly, after 2 years of ownership, their estimation of the mileage on the car was within 500 miles of its actual mileage. I never missed a single scheduled maintenance under warranty and even after. The timing belt & water pump have been replaced religiously after every 60K miles. The service records don't indicate that the thermostat ever having been replaced. I would know because I have a copy of every single one of them and I pored over them a couple of months ago. The dealer says that the only other car they have that still shows up for service over 300K is a V6 A6 with 360K.

Did you have to do the same with yours ? (including no heat?) I pray that it fixes mine. Thank God for the heated seats coz that car would be sitting in a garage or a junkyard right now.

Haha, well you get the same response because it's the most common practice to replace the timing belt, water pump and thermostat all at the same time. Have you tried calling them or going in there to ask if they replace the thermostat as well with the timing belt but just dont document it? Cuz it seems really unlikely that at that high of mileage they wouldn't have at least recommended having it changed at least once, if not already done it themselves without noting it.

But as said, if it is in fact still the original one, that's your most likely culprit for your erratic gauge and lack of heat. The cost of labor on it will be about on par with a timing belt job, since like I said before, they usually replace the thermostat at the same time as the belt and for good reason; its the same amount of teardown to get to it. The cost of the thermostat itself is relatively cheap, but the labor getting it installed is what is going to cost the most. If you're getting close to another timing belt job, I would recommend having your mechanic include a thermostat with the belt and water pump.

^^ At 308k, he just had his 5th timing belt job....

Yep, that's exactly what I did - 100% fix.

And for the love of god stop taking your car to stupid mechanics, and spend your money with more intellegence and buy a full repair kit, like this http://www.ecstuning.com/Audi-B5_A4-...Belt_Kit/which saves you a boat load of money right out of the gate. I wouldn't be suprised if you have been driving around with a broken thermostat for a 100k, from what i have read here.

P.S, that link is for a 30V 2.8L, you might have a 12V, since you didn't tell us your model year.