Sorry I have posted this elsewhere on the Mk4 TDI Section but I think this is more appropriate::

I have a Mk4 Golf TDI which has done about 60,000 miles. Running on fully synthetic oil which has been changed every 5000 miles throughout it’s life and it performs well. The only major repair was the turbo being replaced during it’s first twelve months. Last year before it’s MOT the exhaust was blowing a lot of black smoke and fitting a new MAF cured that instantly.. The MOT is about two months away in 2110 and it has started blowing black smoke again. Although I initially suspected the MAF, which is guaranteed by VW for two years, I checked the EGR settings using my Vag-Com. It was 12233 and so I bumped it up to 33768., Te result – the black smoke reduced to almost nothing. The reading for the MAF is 550 (where the range on screen is given as between 240-340). According to a piece in the TDI Club the MAF reading should be of the order of 480 plus or minus 30 – anything out of that range should be considered as faulty. The local VMW main dealer tell me that in order to have the MAF replaced under warranty it needs to be tested by them on the vehicle. If it is faulty it will be replaced free of charge (parts and labour). If it is not faulty I would have to pay for the diagnosis. Chicken and egg situation. The bottom line is would it pass the MOT….. But I would be grateful if anyone has any practical experience using Vag Com of how critical the MAF reading is and what latitude there is before it would be considered faulty.