Carbon buildup is a common issue with direct-injected engines, and the Audi V8 and V10 engines are no exceptions. Because the injectors spray directly into the combustion chamber, there isn't a fuel mist traveling down the intake ports to keep the intake valves clean. Oil vapors are constantly being introduced to the intake system by the positive crankcase valve (PCV), and after enough time, these oil droplets will start collecting on the intake-port walls and intake valves, eventually choking the airflow into the engine to the extent that efficiency and performance are drastically reduced. The resulting carbon buildup can be cleaned by removing the intake manifold and blasting the intake ports with walnut shells, but this is a time-consuming and potentially expensive job, due to the engine location.
Carbon buildup leads to another common issue, because the built-up gunk may interfere with the proper function of the tumble-generation flaps in the intake manifold. These flaps have their own reliability downsides, however, because their plastic actuating levers could fail and trigger limp mode from the ECU, or the screws which locate the flaps on their shafts could come loose, fall down the intake ports, and proceed to destroy the engine.
There are two solutions here, and both of them are best applied when performing a carbon clean: You could simply "de-flap" the engine by removing the flaps and their shafts completely, block off the shaft bushings with plastic plugs, and never worry about them again. The only real downside to this approach is that the idle quality will be slightly less smooth, but this operation may also cause problems when testing the emissions-control system. Ask a knowledgeable mechanic if this will be the case in your state.
The correct way to fix the tumble-flap issue is by dismantling the flap system, replacing the actuating levers with metal parts, and re-fastening the tumble-flap screws using heavy-duty thread-locking compound. Either way, this operation should be added to the to-do list when the first carbon clean is performed.
Excessive oil consumption is another issue with both the V8 and V10 engines, but are often caused by defective PCV valves. However, these are very high-revving engines which usually drink some oil between services, especially when subjected to hard driving. If the oil consumption exceeds a quart every 600 miles and the PCV valve is working as it should, this would, unfortunately, require an engine rebuild to remedy.
Fuel-injector failure is another risk with these high-revving, high-compression engines. Because the fuel injector's tip is exposed to the combustion chamber's extreme temperatures and loads of vibration, simple metal fatigue could eventually result in the injector tip shattering, sending metal debris into the combustion chamber and leading to total engine failure. Once again, attend to this issue with every carbon clean operation.
Ignition coils are a frequent failure point on high-revving engines, but are easy enough to rectify. However, if one coil gives out, the other won't be far behind, so rather replace them all as a set when the time comes.
Mileage: PCV valves can let go any time from 50,000 miles, and carbon buildup will become noticeable around that time as well. Fuel injectors could fail from around 75,000 miles, and ignition coils have been known to fail from 45,000 miles.
Cost: PCV valves cost around $200 for the V8 and roughly $138 for aftermarket replacements, excluding labor charges for fitment. Carbon cleaning will cost up to $1,400 due to the engine location, and replacement injectors will set you back about $200 each at the dealership. De-flapping the intake system will cost about $40 in plastic blanking plugs. OEM coils for both engines cost about $50 each.
How to spot: Leaking PCV valves will cause reduced performance and rough running, as will sticking tumble flaps, and fuel-injector failure will likely first manifest as a misfire that new spark plugs and coil can't remedy. Carbon buildup will cause reduced engine performance and increased fuel consumption. Coil-pack failure will make itself known by misfires and an illuminated Check Engine Light (CEL).