That is called, "Dieseling"". The cylinders are so full of carbon that the carbon gets hot and keeps the cylinders firing from the red hot carbon. Try a fuel system cleaner. BG44K is the best to use. You will have to go to a car dealership parts counter to get it. It costs around $20 a can and you should get 2 cans. The vehicle may act funny until you go both through cans. Cleaners that you get at parts stores really don't do much and definitely won't be able to remove heavy deposits. So, it is worth the money and trouble to get BG44K.