My brother's HO cummins 2003 idles normally around 500-600rpm, but when it starts it jumps to over a 1000rpm, which i think is normal for warming up. Here's the problem, after the truck has been working, and is warmed up, it still switches from a low idle to the high 1100rpm periodically. Is this normal? Is there something wrong with the injectors? The truck is a heavy use farm machine, has seen a lot of heavy towing.

Normal.

Keeps the engine from cooling off or overheating when idling for longer periods.

If the outside air is below a certain tempature the truck will ramp up to 1000 rpm's after 2 minutes of idling regardless of engine tempature. This idle up feature is designed to keep the engine from running cold and causing cylinder wash from raw unburned fuel. On extremly cold days my '04 will immedietly jump to 1000 rpm's upon starting.

Also, on my '04 I had the dealer reflash the computer which enables me to control the high idle myself when desired. This is controlled using the cruise control buttons and it allows me to set my idle anywhere from 1000-1500 rpm. However, for some reason this manual high idle is only available on '04 and later trucks so '03 owners are out of luck.

My 04.5 600 cummins has been re-flashed last week. How do you idle up using the cruise?