You are correct in that there is no idle adjustment screw for that engine. Every thing is done electronically through the computer and related sensors.
What you are probably experiencing is due to a bad throttle position sensor. When this sensor fails, it can allow the computer to think the throttle is depressed which in turn makes the engine rev up very high. This is a quite common thing on those engines so I'd start there for sure.
GM Dealer Technician For 18+ Years In the automotive industry for 20+
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