My 1991 Nissan Pathfinder has been having problems. When its running, the exhaust smells strong and rich, black smoke sometimes is expelled from the muffler, but not always. It also sputters and burns a ton of gas. The brain box, air filter, fuel filter, water pump, and battery have been replaced. The fuel system has also been flushed. Other than a tune up, what can I do to fix this problem? Is there some kind of sensor that may be causing this? Any information would greatly be appreciated.