hi all I recently bought a 59 plate Mazda CX-7 diesel when I accelerate I get a black puff of smoke.the car has been back to the place I bought it several timers and they said there is nothing showing wrong rac done a diagnostic test and all was clear. Does anyone have any suggestions?also when on the motorway in high gear and on a slight incline the car starts to shudder feels like the wheels are fighting against each other but clears after a few minutes.and only seems to do it on sight inclines.feel like the 4wd is not right.can anyone help cheers