What the...Idle Adjust? 2015 KW T680 Cummins ISX

While waiting around for my trailer to be loaded in my 2015 KW T680 with the Cummins ISX, I started beating (lightly) on my cruise control buttons to the beat of the song I was listening to and the idle raised a little. About 40 RPM. I thought something just broke on the engine or an injector stuck or something. I knew that if I turn the cruise control to on, and hold set, I can raise the RPM which is what I figured I had done but my cruise was off so I began pushing the Resume-Set button and found that I could raise or lower the idling RPMs. Ever since I bought the truck it idled at 700 RPM even after I had it deleted and tuned. It always sounded like it was too high but it was like that since forever so I didn't bother with it. I rarely idle because I use my APU but I have a freon leak so I've been idling a bit lately. Well I noticed when I dropped it down to 620 RPM it was a very very very smooth idle, much smoother than I've ever felt this truck idle and the idle fuel usage went from 0.8mpg to 0.6mpg. Now I realize that low idle is not good but how low is too low? I don't have a DPF or any of the emissions equipment so could lowering the idle to 620 do damage? I'd like to lower it to that because of the fuel saving and the lack of idle vibration but obviously I don't want to do this if it's going to do damage. Thoughts?

 

I drive a lot in the South and Southwest where it's 24/7 above 80 except in the winter so I always used my APU only. And I shut that off if it gets cool enough at night but it rarely does. I'm an owner operator so burning 8 gallons idling per night will not work with me but won't be a problem when I get my APU's A/C fixed. Right now it's 91 degrees in Jenks OK so I'd melt if it shut it down.

 

That would end up being $720 a month if it's the summer and hot all day and night which is pretty bad. Thankfully I have an APU, when it works which is 90% of the time I don't burn much. It supposedly burns 0.1 gph which saves me a lot of money but yes it does have high maintenance only if it's abused. I service mine everytime I take my truck in for a PM. I swap all the filters and change the oil. Recently had to put a new fuel pump on which cost $180 and took me 20 minutes. Not a big deal for the quality of life it provides.

Right now, bumped up to 1000 RPM it says it's burning 1.3 gal/hr and when the fan comes on it goes up to 1.7 gal/hr which over a 10 hour break under a 90 degree night would end up being 15 gallons a night which would be ($3.09/gal) $46 a night which would be $324 a week which would be over $1200 a month. Hardly an expense I want. Of course I hardly want to spend $8k+ on an in frame from excessive low idling.

Under normal idle it says it burns 0.7 gal/hr which would be half the above. Saves a lot of money to normal idle instead of high idle. I guess it would depend on how long it takes for your motor to give up to determine which costs more. Unfortunately some of us can't sleep in 90 degree ambient temperature and must leave the truck on basically 24/7 especially those of us with pets like me. Even when I go into the office to get papers and things for shipping/receiving my truck must remain on because in 90 degrees, it takes 5 minutes for the inside temperature of my truck to reach 90+ degrees. So literally my truck would be running 24/7 without my APU. APU is well worth the investment if you ask me. I'd say I have to spend $150 a month average servicing the APU. Right now I get by on recharging the freon that leaks out to empty in about a month.

But I don't plan on running the idle down at 620 RPMs from the damage it could do as stated. So until my APU is recharged...which is whenever I stop for my next 10, Normal idle is probably where I'll leave it.

 

Looks like at 700 RPM I'm at about 35 psi. Lowered to 620 RPM it puts it right on the line at 30 psi.