look on audiworld.com in the forums (link below)
the 2.7 Bi Turbo is a very popular engine used in the A6 and S4s of that generation. Check the audiworld.com forums - they might be able to help you sort it out.
If the timing belt has not been replaced I suggest you do it soon. If the timing belt goes, the engine could suffer bent valves and a broken piston. Usually Audi recommends replacing both the water pump and the belt at the same time. I did mine on my 2000 A6 4.2 at around 80K or 90K. Unfortunately that car got totaled with only 115K on the clock.
They really are great cars, but the maintenance can be a bitch. You could look for an independent shop which might be cheaper than the dealer but make sure they are used to working on them because they ARE complicated. How is the car's condition? Does it still look good, do you enjoy driving it? Those are thing to consider as well. If the car is beat to hell and you really don't care one way or another about it, it might be best to cut your losses and get out of it and into something else. Only you can decide that.
If it's a Tiptronic, don't be too eager to change the transmission fluid. It is a lifetime fluid and unless something is wrong, it's best to leave it. When I asked my Audi dealer service advisor about changing the fluid on my 2006 A6 4.2, he said they wouldn't do it unless it was having problems and strongly advised me against taking it to another shop to do it (basically he said they wouldn't want to see the car again). I have dealt with this guy for about 12 years and trust him completely. That car got sold at 110+k miles and is still going strong as far as I know. And it had about 40 or 50 track days on it.
Also too keep in mind its a 15+ year old car with 200k miles on it. If you and the previous owners were good about the recommended maintenance (most are not), that shouldn't be a problem but if not, who knows?
I always get frustrated with people who buy cars that REQUIRE (I know most don't, but all Audi's do AFIK) premium fuel and always insist that it runs just fine on regular and the knock sensors take care of it. That may be true, but running it that way causes excessive heat which is not good for the engine over time, AND you are not getting the full performance (and enjoyment) of the car. My motto is, "if you can't afford the
proper gas and the maintenance, don't by the car."
Good luck with it. They are fun to drive and if still in good shape, still catch my eye on the road when I see one.
A6 / S6 (C5 Platform) Discussion - Discussion forum for the C5 Audi A6 and S6 produced from 1998-2004
www.audiworld.com