VW MK4 VR6 2.8L 12v/24v
Poor or faulty air mass sensors can cause numerous problems without even setting a check engine light. Typically owners do not notice the decrease in performance driveability or fuel economy till the problem gets really noticeable. Air mass sensor usually die a slow death so most customers notice a huge improvement in performance smoothness & fuel economy when replacing their faulty air mass sensor with a new one. It is not uncommon to have to replace a air mass sensor every year especially on cars that have been modified.
Replacing air mass sensors is one know solution for "lean" or "rich" engine codes
BOSCH part number: 63 135