I recently purchased a 2005 A8L and am trying to figure out if I have an unusually low idle when the A/C is not running.

After the engine gets up to normal operating temperature, when I come to a complete stop with the transmission in "D", the idle will come to a rest at approximately 780 rpm, but if I am stopped and pressing the brake for more than a few seconds, the idle will drop to just under 600 rpms, and I begin to feel a slight vibration in the car due to the low idle speed.

I recently noticed that when this is happening, if I put the car in Neutral, and remove my foot from the brake, and then press the throttle to increase the rpm's the idle will setlle back to around 780 - and the car will idle smoothly

I don't know if this is related to the braking system, or if is an engine issue. Either way, it's annoying.

I brought this issue up to my dealer, and of course they told me that it is operating "normally".

Since I don't have another A8 to compare, would anyone please provide the average idle rpm's of their car without the A/C running, and at normal operating temperature?

Am I crazy, or is this normal?

Thanks in advance for any assistance, and sorry for the long post.

I had the same problem and it was the coil. After the coil was replaced the car idlled normally when in "D" and at a stop. Good luck.

I had a similar problem with my D2. It turned out to be deposits stopping minor movement of the throtle body butterfly valve when almost shut (at idle in other words). Once cleaned the idle was perfect again. So the valve was not quite positioning as the ECU wanted it to be. Sometimes it felt like it was going to stall. Sounds a bit more serious than your symptoms though.