I have a ford fiesta xr2i turbo it run erratic and the plugs are a black sooty color iv changed the head and gaskets its had new plugs and leads new injectors new ecu had compression test that's ok new maf sensor had all breather pipe cleaned and after all that it still runs erratic iv pulled the plug leads off one by one number 1 dont make any differences could you give me any advice to what to do check next iv had so called mechanic look at it they dnt no what it could be