I have a 1989 GMC Sierra 2500 2 wheel drive. I started my truck, drove it down the street to return a dvd and returned home. When I pulled into my driveway I put the truck in park and turned the ignition off and pulled the key out and my truck was still running at full idle I put the key back in and tried to turn it off again and nothing happened. I disconnected the battery and the truck was still running. Out of desperation and panic, I popped the center spark plug from the rotor cap with a screw driver and got a little ZAP! lol and the motor finally shut down. What's going on here, what area should I be looking at? Any help or advice would be greatly appreciated!

Thanks
Brady James

 

Probably the ignition cylinder.