I had my '98 A4 V6 - same engine as yours I think - done at 73K miles for
under a grand at an independent Audi shop. That included new water pump,
tensioners and thermostat. So the $1800 quote seems a bit high even for the
dealer. Failure of the TB will almost certainly cause extensive engine
damage because of the interference design where the valves will "kiss" the
pistons if the cams stop turning. I'm assuming your quote includes the
ancillary items that should be changed along with the TB like the water
pump, etc. The reason is that failure of these other items can trigger
failure of the belt, they require almost the same extensive labor to perform
and you can be almost certain that the water pump will fail before you have
to do the TB again. To sum up, you're at the mileage where you should have
the TB, etc service done. I'd just consider having it done at an independent
shop that specializes in Audis.
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