2001 Audi A8L
This car was purchased from the Clinton Auto Auction. While in general buying a high end German luxury car from an auto auction is a great way to throw away money, this particular car is almost entirely made of aluminum; body/frame, engine, transmission, rims, and majority of the suspension components. With this much aluminum if I were to scrap the car I could get back most of the money I spent on. Unfortunately for my wallet I fell in love with how wonderfully comfortable this car is for someone of my size (6'5" and not skinny).
The 2001 model year for the A8 was the last year of its first generation and was the flagship car of the Volkswagen group until they created the Phaeton in 2004. This car is unique as being the first all aluminum uni-body and the first five speed automatic in a production car.
The picture on the left shows the upper control arms which is another distinct feature of Audi's; where there are two arms instead of the "standard" one A-arm. It is the same on the bottom giving the suspension a total of eight control arms connected via ball joints to the bearing housing (steering knuckle). This allows the tire to maintain constant camber while turning keep full tire contact with the road. Coupled with the Quattro torque vectoring four wheel drive system it provides impressive handling, particularly while accelerating out of a turn.
The engine in the model I purchased is the 40 valve 4.2L V8, orignally producing 310hp @6200rpm and 302ft/lbs @ 3000rpm. It has a dual overhead cam valve train with three intake and two exhaust sodium cored valves. The extra valves allows each to be smaller weight reducing the valve train inertia. This along with other design features of the end cause it to be a extremely fast revving engine.
The intake also features a set of two "change over valves" that modify the length of the path the air travels before hitting the intake valves. These allow there to be three difference renounce rpm's that will cause the reflected pressure waves to increase intake air pressure. Autozine.org has a good description of the different approaches to this, including the on used on the 40V 4.2L here. This helps to provide a near flat torque curve, at its peak, from 3000 to 4000 rpm's.
This in a belt drive interference engine and as such buying a used one and driving it without replacing the timing belt and components risks destroying the valve train. The belt and tensioners were then the first on my list of repairs to the vehicle. while in there I also changed out water pump and thermostat, as they are driven by the timing belt and I didn't want to have to do this procedure again. If your planning on owning one of these cars and doing your own maintence it is crucial to get a copy of the service manual which is produced by Bently Publishing and available here. It comes on a CD that only runs on windows 2000 or XP. Assuming you don't happen to have a shop computer that still runs XP (I use an old laptop for this) you can run it through an XP emulation using virtual PC.
The vacuum system on the Audi A8 is rather complex, particularly due to the vacuum actuated change over valves in the intake. Many of these hoses degrade over time and a few specific ones must faster than others due to the engine heat in the area they are located.
The Fatal Flaw
The Audi A8 featured the world's first five speed automatic transmission in a production vehicle the 5HP-24A. This four wheel drive transmission features a built in center Torsen differential to mechanically transfer torque from the front to rear wheel or visa versa seamlessly depending on conditions. The transmission is used in many BMW and Jaguar applications and is still in production for its use in the Volkswagen Phaeton. The transmission itself is a very tough and durable design. The problem is the Audi owners manual which claims that the transmission fluid doesn't need to be changed for the life of the vehicle. This is true if you consider the life of the vehicle ending when the transmission needs replaced. Since a new transmission and the labor to replace it is often near or more than the resale value of these cars (which depreciates so fast because of this issue) it usually is the end of these cars.
What happens is the transmission fluid pressure regulator is in an aluminum bore and as the fluid gets small particles of steel in it from, normal operation, that eventually the filter and magnets aren't sufficient to remove, this bore begins to wear. This causes pressure spikes on one of the clutch packs putting excessive force into the thrust bearing pictured below. This wears the bearing race down until it is razor thin which causes the clutch drum to shift forward unseating an O-ring on its input shaft. The failure of this O-ring causes the transmission to start slipping; initially from first to second then also second to third and so on until whenever the transmission reaches operating temperature it goes into limp mode. This hammers it into fifth gear which is a big problem trying to start from a stop light going uphill.