2000 Volvo S40 shaking and black exhaust

Prior to this morning my car was fine. I started it up & drove to work, my gas light was on but didn't stop at that time. When I was leaving on lunch after starting my car the check engine light came on, it felt like it was going to run out of gas but after giving it some gas and some sputtering it seemed to run fine. I noticed dark exhaust coming out of the pipe. I went to the gas station, put a bottle of gas treatment and 6 gallons of gas in. I made several other stops and each time I restarted my car it would sputter, if I slowed to a stop it would feel like it was going to stall. I continued to see the dark smoke coming out of the exhaust pipe. Check engine light has stayed on. Help? Not too many people around here work on Volvos and I don't want to have to pay the dealership if I don't have to.