It is doing what is called, "Dieseling". This is when there is so much built up carbon on pistons that it glows red hot and ignites fuel still in cylinder and keeps the car running. The best way to get rid of deposits is to use a fuel system cleaner that really works. BG44K is the only reallly good cleaner on the market. You will have to go to car dealership parts counter to get it and it is $20 a bottle. Runn 2 botttles as indicated by manufacturer and by the thied tank of gas after you have run the 2 cans if it is still doing it run one more can.