I have a fiesta 1.4 TDCI. When I bought it there where no pins in the MAF sensor and the fault was shown as the MAF sensor. The car starts and runs ok, it's a bit sluggish and it only gets around 50 mpg on a run at 70 mph. I fitted a new MAF sensor hopeing to get a bit more power and better MPG. The power is about the same but the MPG went right down and black smoke is coming from the exhaust, so much that I pulled over and disconnected the MAF again and the car is back to running normal but with low MPG. Any thoughts as to what the problem may be?