I am a relatively new diesel owner of a relatively stock 02 24 valve 4x4 with a 6spd manual. The only mods are a hybrid turbo with a slightly bigger spool and 4 inch exhaust. Lately, after some freeway driving the idle goes from 850 to 1200. Are there some common reasons for this?
Loren

 

Is it surging between the RPM's you specified, or just occasionally going up to 1200 and staying there?

 

In as little as 1 hour drive the idle rpm can creep up to 1200 and stay there. I turned the ignition off for 60 seconds and it goes back down to 850. I previously endured the surge and barely limped it to the shop. That time it was a camshaft position sensor which they replaced. I am not sure if this is the same thing again or not...How can you tell?
Loren

 

Today the idle crept up to 1000 on the way to work. I shut it down for 30 seconds and back to normal... Then almost home about 30 miles into the trip, I noted 1300 rpm. I thought WOW that is pretty high. 2 blocks from the light, the truck goes into limp mode and has very little power and throws an engine code. I turned it off again and was able to drive it home normally with code still displayed. The tach isn't jumping around and the problem is only evident when I come to a stop light or when it gets so high that the computer feels the need to take control.
Loren

 

It ended up being the pedal position sensor. Can be had on line for fairly cheap...

 

Look into the Timbo Apps. Under $200 I believe