2009 prostar isx 600k miles manual trans

What RPM is it at all the time?

 

So let me get this straight. You have a NASCAR engine in that thing running 9000 rpm and while in neutral it goes 20 mph?

 

LOL, I figured. Some Cummins can have idle speed adjustment turn on and changed by your cruise control set/resume switch. Sounds like your idle speed is simply set to 900. Should be 600-700. See if you can toggle it back down.

 

Cruise/PTO is different. This actually idle adjustment and all you have to do is hit Resume or set over and over until the RPM comes down to what you want. Engine should be running with park brake applied and cruise turned OFF. I can't remember if resume or set decreases the speed or not so you'll just to play with it and see. Hopefully this works.

 

Is there or was there ever a PTO installed?

If so it may have been configured to raise the idle when the PTO is engaged, this would have been set up by the PTO control switch wired to the computer.

So, if you have a PTO the ground wire to the computer could be grounding even when the PTO is disengaged, making the computer think the PTO is always engaged and keeping the idle high.

Or, if you don't have a PTO but the truck did at one time, and you repurposed the old PTO switch to do something else; the use of that switch would still be telling the computer to fast idle.

If the former or the latter is the case you can correct the idle by disconnecting the wire that sends a ground from the switch to the computer.

If you have a PTO this "fix" will disable the fast idle feature, if you want that then it is best to just replace the failing switch; if it is a repurposed switch then no worries.