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This topic recently came up in the Audi TT Forum UK for the 3.2 VR6 BUB/CBRA engines so I thought I would share our findings since can affect other VAG engines as well.
While problems with the SAI system are usually attributed to one of the major components (e.g. air pump, Combi-valve, solenoids or air/vacuum lines) carbon build up can be more problematic for the larger V6 engines, specifically the 3.2 FSI AVS and 3.0 TFSI. The problem has to do with the accumulation of carbon inside the SAI ports (air passages) which are located inside the head. Heavy accumulation of carbon deposits can prevent air getting through the head and out to the CAT. While this won't affect engine performance, it can throw a DTC anytime the SAI system runs during a cold start.
Thankfully, the 3.2 VR6 BUB and CBRA engines are not prone to this issue and the problem can be usually be resolved with one of the recommended repairs in the Ross Tech Wiki pages linked here and here.
Fault codes P0491 & P0492 -
The solution for carbon deposit removal, as proposed by VAG in their TSBs, is to use a special set of hoses which are connected to a high pressure washer system. These hoses are inserted into the SAI ports and the high pressure water simply blasts away the carbon deposits. Here's a good Youtube video that discuss the carbon build up problem and demonstrates how these special hoses are used to clean them out. I have also linked the TSBs referenced in the video here and here.
The image below is the 3.2 VR6 BUB head viewed from the rear (exhaust side) and shows where the Combi-valve connects to the head (1), and the associated SAI ports for Bank1 and Bank2 (2). Similar ports are present on other engines and is where the carbon buildup can accumulate.
So if you find the air pump, Combi-valve, solenoids, air and vacuum lines are okay, and you are still getting these fault codes, this might be the problem.
While problems with the SAI system are usually attributed to one of the major components (e.g. air pump, Combi-valve, solenoids or air/vacuum lines) carbon build up can be more problematic for the larger V6 engines, specifically the 3.2 FSI AVS and 3.0 TFSI. The problem has to do with the accumulation of carbon inside the SAI ports (air passages) which are located inside the head. Heavy accumulation of carbon deposits can prevent air getting through the head and out to the CAT. While this won't affect engine performance, it can throw a DTC anytime the SAI system runs during a cold start.
Thankfully, the 3.2 VR6 BUB and CBRA engines are not prone to this issue and the problem can be usually be resolved with one of the recommended repairs in the Ross Tech Wiki pages linked here and here.
Fault codes P0491 & P0492 -
The solution for carbon deposit removal, as proposed by VAG in their TSBs, is to use a special set of hoses which are connected to a high pressure washer system. These hoses are inserted into the SAI ports and the high pressure water simply blasts away the carbon deposits. Here's a good Youtube video that discuss the carbon build up problem and demonstrates how these special hoses are used to clean them out. I have also linked the TSBs referenced in the video here and here.
The image below is the 3.2 VR6 BUB head viewed from the rear (exhaust side) and shows where the Combi-valve connects to the head (1), and the associated SAI ports for Bank1 and Bank2 (2). Similar ports are present on other engines and is where the carbon buildup can accumulate.
So if you find the air pump, Combi-valve, solenoids, air and vacuum lines are okay, and you are still getting these fault codes, this might be the problem.
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