we have a 94 buick roadmaster and when we turn it on the side with spark plugs 1,3,5 and 7 throws out black carbon looking stuff once it warms up it stops throwing it out we have changed out 4 fuel injectors,the fuel pump, the fuel pressure regulator and all the spark plugs also the service engine light is on and it won't show any codes and we can't get it to turn off.anybody know what this could be? any and all help is greatly appreicated thanks