From the TDIFAQ:
Avoiding the Need for Intake Manifold Cleaning - Recalibrating the EGR System - this vehicle is equipped with a CCV (crankcase vent) system and an EGR (exhaust gas recirculation) system. Oily fumes from the CCV system can combine with carbon particles from the EGR system to form a black sticky tar in the intake system. This eventually starts restricting the amount of airflow into the engine, resulting in gradual power loss as the vehicle ages and the intake clogging becomes worse. In rare cases a MIL ("check engine" light) may be tripped, with an intake manifold pressure control code set, but normally this does not happen and no codes are set. Operating conditions, fuel quality, oil quality, driving habits, and general condition of the engine all have some effect on how long the intake system survives before clogging up. It can be prevented and this recalibration is highly recommended for all TDI owners.
A VW specific scan tool such as VAG-COM is required for this procedure. With the engine warmed up and running at idle in neutral with all major accessories such as air conditioning turned off, connect the scan tool cable and start the scan tool software. Select engine control module. "Login" using 12233 as the access code. Select "adaptation" and go to adaptation block 3. Give the accelerator pedal a quick "blip" to ensure that the EGR remains turned on for the next minute while you perform the following steps. Note that the display shows a default adaptation value of 32768 and shows approximately 250 +/- 20 mg/stroke of air intake volume. (If it does not show this, you are either on the wrong screen, or you have an older ECU that differs slightly - see next paragraph.) If all is well, enter 33768 as the new adaptation value, and select "test". Note that the displayed air intake volume changes, usually to about 370 mg/stroke. The specification limit is 370 mg/stroke, so if you want to remain within OEM shop manual specifications for emission control reasons, you might have to enter a number slightly smaller than 33768. (If you don't care about road-legal NOx emission limits then leave it at 33768. [Depending on the usage cycle, NOx at this setting can be increased by as much as 40%]) If all is well and you have a seting that you are happy with which results in an intake air volume of 370 mg/stroke or just a hair less, enter "save". Now your intake manifold either will never clog, or will take so long that the engine will wear out first.
Note for '96 Passat and possibly other older ECU variations: The adaptation numbers are in a different range, and you won't be able to get feedback on what you are doing on the same screen. The EGR adaptation number for a '96 Passat with the original "BK" ECU is 188 rather than 33768 and you'll have to go back to "measuring blocks" group 3 to see the effect before "saving" the new setting.
As for warranty, no one has mentioned any problems nor should there be. This adjustment is also completely reversable should one be concerned.
Most provinces and states that do emissions testing on diesels, just check opacity, so this will not be a factor when testing.
