Sounds like its still running

The engine is running on it 'dieseling'. More than likely the timing has been knocked out of adjustment. You didn't state mileage, but I suspect the timing gear has sheared a few teeth and caused the engine to lose time. Yes, it's as bad as it sounds. There are two gears, one on the cam one on the crankshaft connected by a chain. OEM uses nylon on the top gear to make it quiet. Buy a timing set with a metal upper gear have a.mechanic install it. While you're at it, if you haven't replaced it, do the water pump and use only Red Dexron coolant.