dieseling

I have a 1983 dodge ram 150 1/2 ton with a 318 engine, it drives perfectly until stop and turn off the car when it starts "dieseling" for sometimes up to 10 or 12 seconds, any suggestions I can do to solve this?

sounds like the timing might be in need of an adjustment, although i do have a few other ?'s. 1) Is the truck really hot right before you shut if off? 2) Is the idle on the truck higher than its supposed to be? Check those things and see if that solves your problem.