First it is diesel fuel not gas because it has not been refined like unleaded gas.drain the tank first then fill the tank with gas. Change out the fuel filter also.yomay need to add stp gas treatment to help clean the engine out. Try this and see if it clears up. It should not have done much or any damage to the motor. Except maybe the fuel injectors,fuel filter and fuel pump. If you did not run the car a long time after filling up with the diesel fuel. Hope this helps.