I have a 1980 El Camino that has a rebuilt 350 engine from a 1979 Malibu with about 1500 miles on the engine. The engine starts and runs great but it is starting to diesel (keeps sputtering) when I shut it off. What are some possible causes? When I got the car the previous owner only ran supreme gas and I am not. I don't know if it may be a low octane problem or not.