1998 Venture with 127K miles. New plugs right after successfull intake manifold gasket repair at 118K. Has run well ever since, till today.
I pulled into a parking space, put it in park, and then the engine revved up to about 1200 rpm. Shut it off and turned back on. Still revved. Shut it off and went shopping. Came back out 15 mins later when my youngest acted up in Target, sat at idle while my wife finished shopping for another 25 minutes. It sounded fine. (Watched a lady get a ticket for parking in a fire lane. Classic.

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As I was leaving I noticed the "Service Engine Soon" Light was on. Not flashing. Just on.
So then I drove another 5 highway miles to the grocery and left it idling while I went in to pick a few items. My wife shut off after 5 more minutes of higher revs (1200ish).
The basic question to all is, what is the cause? How to fix?
Bigger question and discussion topic-
I bought some STP injector cleaner and fed it to the tank. Figured I would start with the cheap fix.
The bottle claims "Made with Jet Fuel" which would make the random buyer think it is nitro or something. I know Jet Fuel is one step below Kerosene, but higher BTU per Gallon. "Carrier for active ingredients" is in small letters on the bottle.
The cleaner even smells like kero.
Has anyone used Kero as a cleaner or treatment? Say, adding a quart to a tank of gas? Are the "active ingredients" really what does the work, or is it the Jet Fuel?