Low quality fuel can cause this condition if contaminants are present, such as too much alcohol, which lowers combustion temperature. When the fuel doesn't burn hot enough, it leaves a sooty coating on the inside of the engine, which can accrue over time. 

The engine in your vehicle is meant to reach a specific running temperature as you drive. You may notice, for example, that the engine works just a little harder in the cooler months of the year. This is partly because it needs to use more fuel to get up to that special temperature where everything just works better. Short trip driving in general can cause this as well, as the internal parts of the engine don't get hot enough to effectively burn off that extra fuel.

Fuel additives can also cause this to happen. Using third party fuel additives that get emptied into your fuel tank, or some already in the fuel mix, which are promised to clean your fuel system as you drive, are not specially designed with your vehicle's components in mind. They're more of a generic product that can have a range of effects.

As with any service concern, the first step is to speak with one of our service experts if you feel this is an issue. A service technician will then inspect your vehicle thoroughly to determine if this is indeed the issue, and the extent of such a situation as well. Some cases of carbon build-up can be cleaned using a special cleaning formula, which should resolve most concerns, especially when caught early on. More severe cases require some more in-depth engine repairs to physically clean the hardened material from major engine parts.

Always be sure to stick with top quality fuel for your Audi product, and ask our Service or Parts staff about our range of approved additives made specifically for your vehicle that can minimize or even prevent this from occurring.