idle loss

Do you swear by your statement? Honestly, what kind of post is that? You didn't ask any questions or even tell us what you're driving. How do you expect someone to help?

 

Some trucks with a Cummins you can increase or decrease the idle speed with the set/resume cruise switch. What type of engine do you have, year and model of truck? Freightlinerman is right, every truck and engine works differently and we need to know what you have or really no one can do much to help.

 

exhaust back pressure sensor could be bad causing the engine to run at a higher engine rpm

 

I am an International tech. Cummins idle speeds can be easily changed by simply hitting the set/accel or resume/dec switch, ( I can't remember which one). You do not turn on the cruise, leave it off. With the engine idling you should be able to change it up to 700 or 750 by hitting the switch. There is a parameter in the ECM for idle adjustment that is normally enabled at the factory. If you can't adjust the idle speed it is because that parameter has been disabled for reason.

 

Well, sometimes the ECM does need software updates to improve the engine, EGR, and other electronic operation and reliability issues. If your ECM did need the reflash that is money well spent. Probably the ECM memory setting for idle speed adjustment just got erased during the reflash. Glad to hear it is back where you like it! Sometimes that little change in idle rpm changes the resonance frequency enough so the mirrors don't shake or those dash panels don't rattle.