1987 4.0 Renix: About 20% of the time after starting, engine idle speed goes to about 2500 RPM. Tapping the accelerator pedal has no effect. If you put in in gear, Jeep will go about 35 MPH by itself, like the cruise control is on. It eventually settles down on its own.

It does it even if cruise is turned off at the switch. It does it hot or cold. Throttle return spring under hood snaps back firmly with no binding. Cruise control cables are not tight or binding the throttle. I do not hear any vacuum leaks.

Ideas?

 

Start by cleaning your TPS (Throttle Positioning Sensor) with throttle body cleaner. Pull the sensor off the throttle body to clean it. Also while your at it clean the throttle body. Seafoam makes a sprayable cleaner that works really well.