my maro' wont shut off

sometimes when i shut her down she likes to keep running or diesel as i have been told...it's like a spit and sputter and it is really embarassing...some times it might last as long as a minute...its an 86 carbed z28...it starts fine and runs good.. but i think it might idle a little high and there is an engine light on...the guy who i bought it off of said that the ecm may be bad or something to do with the timing...but im told if it runs ok the ecm should be good....im so confused...does ne body have a clue to what this could be???

thread is a repost