I started having problems with my 98 Deville w Northstar about 2 weeks ago. First it was hard to crank but it cranked and I had no codes. then about a week and a half ago it stalled on me and started emitting dark "smoke" out of the exhaust just once still no codes. Then a few says later it was still doing all of the above but then it threw the following codes: P300 Current- Engine Misfire detected, P1380 Current- Misfire Detected-Rough Road Data Not Available, P1599 History-Engine Stall, P0152 Current- Heated O2 Sensor Circuit High Voltage Bank 2 Sensor 1, And I also had one for the O2 Sensor Bank 2, Sensor 2. So I talked to my mechanic about changing out the O2 sensors and he told me it could all be caused by a faulty or clogged fuel filter. So I changed out the fuel filter myself and drove it for the rest of the afternoon and for awhile the codes went away. Then after a few hours the codes came back but it was only the misfire code. So I changed out the spark plugs and drove it again and it drove fine for a bit then threw out just the misfire and the O2 sensor code for the front sensor Bank 2 Sensor 1. I noticed when I took out the spark plugs that there was a black sooty residue on the plugs similar to the soot on the inside of a chimney. Today I changed out the front O2 sensor and drove it again and I am still getting the hesitation and stalling only now when I idle it is throttling itself. I also noticed the same sooty residue on the O2 sensor probe tip when I took it out. The code that is showing now is the misfire code. Any ideas would be greatly appreciated.