My 1998 VW Jetta TDI has 214,000 miles on it. Gradually it started to produce heavier and heavier black smoke. I had the intake manifold removed and it was full of oily carbonized soot. After cleaning the car quit smoking and mileage improved. However the engine hesitates and smokes a little more with the MAF sensor plugged in. Unplugging the MAF sensor allows the car to run clean with no hesitation. However with or without the MAF the car has minimal power. Generally it is still fine for around town but acceleration on the freeway is slower than it should be. I can still drive at 70 or 80 just fine. Haven't tried any faster. I replaced the MAF, but no change in behavior, so I have left the sensor unplugged. Air filter is fine. I'm thinking wastegate valve, EGR valve, or turbo charger could need cleaning or replacement. This car has always been a good running vehicle with great fuel mileage. Any thoughts or other ideas.