sooty and black plugs mean she is running rich, check your lambda duty cycle should be around 50% (high is rich low is lean)
Running to rich is not good for gas mileage or your catalytic converter.
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)