Van continues to run when turned off

I have a 93 chevy astro 4.3 liter engine with 147000 miles. Recently it was serviced for a new fuel pump and gas filter. A week after being that when I turn the van off and take the key out it continues to run. For me to get it to stop running I have to disconnect a plug (2 wires) going to the distributor. One thing I know that it's not is run-on. Also the oil gague goes low to red when at a stop most of the time, but when pressing the gas peddal goes normal. Can you please help? The van runs perfect other than this.