throttle body cleaning

A wild guess is about $300.

What leads you to think that you need the TB cleaned?

Induction cleaning sounds a bit like some mechanic is trying to make their house payment at your expense. I would say 'no thanks' and install a new air cleaner for $10.

Interesting: the local shop charged $160, but I'm still not totally convinced the price was really appropriate. Maybe it was.

I had it done recently to eliminate this tan foam/residue I was seeing at the top of my oil dipstick level markers. Seems to have worked, but I didn't like how the shop was pressuring me to think about doing it every year (telling me scary stories about how entire engines were destroyed because the throttle body essentially exploded).

The idea behind this suggestion was to remove carbon deposits, etc., on this 98.5 A4Q 2.8 car.

I've also had the suggestion made to switch from 15W40 dino oil to synthetic (either Mobil 1 or Amsoil), and the car now has 107k miles, another consideration to decide on. Bad idea? I do use quite a bit of oil now, and it seems to be burning up, not leaking anywhere, so I wonder if the car would run better on synthetic.

Like I said, 'just a wild' guess. $160 sounds fair if it was needed at all.

Finding tan residue / foam on the dipstick or in the valve cover when you look in the oil filler cap is more indicative of a moisture problem where water is mixing with your oil. One way this can happen is if you make a number of short trips in town without giving the engine a chance to warm up and get rid of water caused by condensation inside the engine. Worst case is when this is due to a blown head gasket which allows you engine antifreeze to mix with the oil. If you are noticing loss of Antifreeze and oil I would be concerned with this. It can be checked by inspecting the spark plugs for signs of abnormal colored deposits and be performing a compression / leak-down test.

I have spent a lot of time on Audi forums and working on them myself and have never heard of anything being cause be a dirty TB other than erratic running and idling of the engine. Your 2.8 is a conventional cable operated TB which is quite resistant to TB issues. Starting in '02 with the B6 platform the operation of the TB is controlled but DBW (drive by wire) which means that the foot pedal sends a signal to the servo motor that operates the TB after the ECU considers whether your requested throttle position is proper in relation to engine operating conditions. If the ECU agrees the TB is opened accordingly. This system is efficient but somewhat more prone to conditions such a icing that can inhibit movement of the TB plate. This should not be an issue with your DBW version.

My son has the same car as you do and it has 130K miles without ever having a TB cleaned or intake manifold cleaned.

Switching from 15W40 dino oil to a synthetic will noticeably increase oil consumption and some people claim that the change can lead to engine oil leaks. Synthetic is used in the 1.8 turbo due to higher operating temps and coking / sludge problems but it is not needed with the NA 2.8 engine.

You mechanic may have good intentions but if I were you I would look for another independent shop that is recommended by other Audi owners in your area.

TonyJ '04 1.8TQ6M stage 1

[lots of very good advice, deleted, but accepted, with gratitude] Thanks, Tony! I'll pass on the synthetic oil and just keep changing mine every 3-5k miles, as I have been.