letting a diesel idle to long is not a good thing it lets carbon build up on the pistons and also makes the engine run cold which makes the piston rings wear out i am pretty sure that all powerstrokes 99 and later have the program from the factory which will make them idle 900 to 1200 rpms depending on ambient temperature outside and also has the exhaust back pressure valve helping it keep warm(tap the brake pedal this will also disable that also) when u tap the brake pedal it will drop back own to normal idling rpms