Chosen Solution
I have a 08 Chevy trailblazer when I’m in park my rpms will rev up and then go back down.
If it is sudden like it is jolting, you may have a faulty throttle position sensor (TPS). Most of the time it will show a check engine light if the tps has fully failed, however if it is just beginning to fail it may not show a check engine light. I would bring it to a mechanic and have them test it. hope this helps!
